ProGPU.Transpiler 0.1.0-preview.13

ProGPU Substrate Framework

ProGPU is a high-performance, GPU-first UI framework and composition substrate for .NET, built on top of Silk.NET and WebGPU (wgpu-native). It provides a lightweight, low-allocation alternative to traditional heavyweight UI frameworks by routing all vector graphics, text layout, and composition operations directly to the GPU using native WebGPU draw pipelines.

NuGet Packages

ProGPU release packages are built from eng/progpu-package-list.sh by the Release GitHub Actions workflow. Samples, tests, diagnostics, and framework shim projects are intentionally not packed.

Package	Purpose	NuGet
ProGPU.Backend	WebGPU device, swapchain, Silk.NET windowing, and platform backend services.
ProGPU.DirectX	DirectX-compatible facade and shader-oriented API surface implemented on ProGPU/WebGPU.
ProGPU.Transpiler	Shader/source transformation helpers used by generated GPU pipelines.
ProGPU.Compute	Compute pipeline helpers for GPU-side effects, acceleration, and future hit-test indexes.
ProGPU.Vector	Vector primitives, paths, geometry, brushes, pens, and rasterization data models.
ProGPU.Text	Text layout, glyph metrics, and GPU-ready text rendering helpers.
ProGPU.Scene	Scene graph, compositor commands, retained visuals, effects, and presentation primitives.
ProGPU.Layout	Measure/arrange layout substrate shared by higher-level UI adapters.
ProGPU.Virtualization	Virtualization helpers for large retained visual and item surfaces.
ProGPU.WinUI	WinUI-shaped controls and app model implemented on ProGPU.
ProGPU.WinUI.Charts	Chart controls and chart rendering primitives for the WinUI-shaped layer.
ProGPU.WinUI.Designer	Designer/editor controls and diagnostics for ProGPU WinUI surfaces.
ProGPU.Avalonia	Avalonia integration and compositor backend adapter.
ProGPU.Uno	Uno/WinUI integration and compositor backend adapter.
ProGPU.Dxf	DXF import/rendering support for ProGPU vector scenes.
ProGPU.SkiaSharp	ProGPU-backed portable SkiaSharp compatibility shim used by drawing and imaging adapters.
ProGPU.System.Drawing.Common	ProGPU-backed portable System.Drawing.Common compatibility shim for LibreWinForms and GDI-style callers.
LibreWPF.Interop	LibreWPF portable interop contracts consumed by the ProGPU/Silk.NET SDK lane.

Local package build:

PROGPU_PACKAGE_VERSION=0.1.0-preview.13 ./eng/progpu-pack.sh

Local publishing reads the API key from NUGET_API_KEY without storing it in the repository:

PROGPU_PACKAGE_VERSION=0.1.0-preview.13 ./eng/progpu-publish.sh

The release workflow validates docs, restores, builds, tests, packs .nupkg/.snupkg artifacts, and can publish to NuGet.org when NUGET_API_KEY is configured. See docs/release.md.

Projects Using ProGPU

LibreWPF

LibreWPF ports the managed WPF runtime and SDK to the ProGPU/Silk.NET platform. Applications can switch to the custom SDK while retaining familiar WPF source, XAML, controls, and Windows-shaped APIs on supported non-Windows hosts.

Package	Purpose	NuGet
LibreWPF.Sdk	MSBuild SDK that redirects WPF applications to the portable ProGPU/Silk.NET platform.
LibreWPF.ProGPU	WPF host, retained/source replay bridge, input integration, and ProGPU compositor adapter.
LibreWPF.Transport	Managed WPF assemblies, reference assemblies, themes, XAML build tasks, and runtime metadata.

LibreWinForms

LibreWinForms provides portable WinForms-shaped APIs hosted by the ProGPU/LibreWPF stack. It preserves the common System.Windows.Forms development model while replacing native GDI rendering with the ProGPU-backed compatibility layer.

Package	Purpose	NuGet
LibreWinForms.Sdk	MSBuild SDK that configures applications for the portable LibreWinForms package set.
LibreWinForms.System.Windows.Forms	Portable System.Windows.Forms API and runtime surface.
LibreWinForms.WindowsFormsIntegration	Portable bridge for hosting WinForms content in LibreWPF applications.

Avalonia ProGPU Backend

The Avalonia ProGPU backend replaces the Skia renderer with a GPU-first WebGPU implementation while preserving Avalonia's rendering contracts. It also exposes an API lease for issuing custom ProGPU vector operations and WebGPU shaders inside an Avalonia frame.

Package	Purpose	NuGet
ProGPU.Avalonia.Rendering	ProGPU, Silk.NET, and WebGPU rendering platform for Avalonia.

Silk.NET Avalonia Backend

The Silk.NET Avalonia backend supplies cross-platform desktop windowing, input, surfaces, and WebGPU integration. It is designed to pair with the ProGPU renderer but can host another compatible Avalonia renderer.

Package	Purpose	NuGet
ProGPU.Avalonia.SilkNet	Cross-platform Silk.NET windowing platform for Avalonia.

SkiaSharp Compatibility Shim

The shim provides a managed SkiaSharp 4.148-shaped API over ProGPU vector, text, imaging, path, and compositing primitives without loading native Skia binaries. Compatibility consumers such as Svg.Skia can use the ProGPU renderer while CPU-only metadata and geometry operations remain independent of WebGPU initialization.

Detailed API coverage, rendering behavior, algorithms, and complexity guarantees are maintained in the SkiaSharp compatibility log.

Package	Purpose	NuGet
ProGPU.SkiaSharp	ProGPU-backed SkiaSharp API compatibility layer.

Svg.Skia

Svg.Skia renders SVG 1.1 documents and its supported static SVG 2 subset through a SkiaSharp-shaped canvas. Its W3C and resvg test lanes also exercise ProGPU.SkiaSharp, providing broad compatibility and rendering-parity coverage for the shim.

The Svg.Skia parity workflow pins the exact Svg.Skia source commit and runs separate native-SkiaSharp and ProGPU-shim checkouts on macOS. The native W3C lane must remain 530 passes with 3 skips; resvg and the non-W3C remainder must stay fully green through the shim. The ProGPU W3C lane validates its exact reviewed difference inventory and uploads both TRX files plus actual PNGs. A resolved row, a new failure, or a changed failure set intentionally fails the inventory gate until the images are reviewed and eng/svg-skia-w3c-known-differences.txt is updated in the same change.

Package	Purpose	NuGet
Svg.Skia	Core SVG-to-SkiaSharp renderer.
ShimSkiaSharp	Backend-neutral SkiaSharp API abstraction used by Svg.Skia integrations.
Svg.Skia.JavaScript	Optional JavaScript execution support for SVG documents.
Svg.Controls.Skia.Avalonia	Avalonia control integration for the Svg.Skia renderer.
Svg.SourceGenerator.Skia	Source generator for compiling SVG resources into SkiaSharp drawing code.

---

Architectural Hierarchy

The ProGPU framework is built in a modular, layered stack that bridges native graphics APIs and system windowing up to a modern, declarative WinUI-compatible user interface layer.

graph TD
    subgraph L6 ["Layer 6: Application Layer"]
        App["Gallery Dashboard / LOL/s & MotionMark Benchmarks"]
    end

    subgraph L5 ["Layer 5: WinUI Framework Layer"]
        Controls["Grid, StackPanel, ScrollViewer, Border, Pivot, RichTextBlock"]
        FE["FrameworkElement"]
        LN["LayoutNode - Measure & Arrange Sizing Negotiation"]
    end

    subgraph L4 ["Layer 4: Scene Graph & Effects Layer"]
        CV["ContainerVisual / DrawingVisual / Visual with ChangeVersion"]
        ILN["ILayoutNode Interface - Layout and Scene Invalidation"]
        FX["GPGPU Multi-Pass Effects Pipeline - Blur & DropShadow"]
    end

    subgraph L3 ["Layer 3: Compositor, Text & GPGPU Rasterizer"]
        Cache["Compiled Scene Cache - Versions, Targets, Atlases, Layers"]
        Comp["Compositor - Z-Ordered Draw Lists and GPU Buffer Compiler"]
        Text["TTF Layout, Rich Text Command Cache, Glyph Atlas"]
        Rast["Compute-Bound 4x SSAA Glyph and Path Rasterizers"]
    end

    subgraph L2 ["Layer 2: Graphics Infrastructure"]
        Wgpu["WgpuContext - WebGPU Adapter/Device & Swapchain Management"]
    end

    subgraph L1 ["Layer 1: System & Windowing"]
        Silk["Silk.NET Windowing & GLFW OS Event Loop"]
    end

    App --> Controls
    Controls --> FE
    FE --> LN
    LN --> CV
    CV --> ILN
    CV --> FX
    ILN --> Cache
    Cache --> Comp
    FX --> Rast
    Comp --> Rast
    Rast --> Wgpu
    Wgpu --> Silk

Layer Description

1. System & Windowing (Layer 1): Interacts with the operating system event queue and monitors display boundaries via Silk.NET and GLFW. It handles window load, resize, rendering loops, and low-level mouse and keyboard input events.
2. Graphics Infrastructure (Layer 2): Manages physical GPU adapter querying, logical device creation, graphics command queues, and swapchain surface configuration.
3. Compositor, Text & GPGPU Rasterizer (Layer 3): Validates and reuses compiled scenes when their visual versions, target configuration, atlas generations, overlays, and cached layers are unchanged. Cache misses compile high-level commands into ordered draw lists and reusable GPU buffers. Framework adapters retain shaped glyph indices and positions as one glyph-run command; the compositor caches font feature availability and rasterizes glyph and vector outlines analytically in WGSL at physical-pixel resolution.
4. Scene Graph & Effects Layer (Layer 4): Establishes the retained ContainerVisual, DrawingVisual, and Visual hierarchy. Mutations propagate ChangeVersion and dirty state so layout, compiled-scene, and CacheAsLayer reuse remain correct. Mask and effect passes use offscreen textures and intentionally stay on the dynamic compilation path.
5. WinUI Framework Layer (Layer 5): Implements cached Measure and Arrange, controls, input, and CPU visual-tree hit testing. The WinUI host disables the compositor's duplicate GPU hit-test index while direct compositor consumers retain it by default.
6. Application Layer (Layer 6): Hosts gallery pages, diagnostics, and opt-in performance workloads. Sample animation and status updates invalidate only the visuals that actually changed.

Shader Source and Startup Contract

Fixed GPU programs live as individual .wgsl, .glsl, or .hlsl files under the owning project's Shaders/ directory. Directory.Build.props embeds these resources into each assembly, and ShaderResource decodes each source once into a process-wide cache. Pipeline owners retain the returned reference in static readonly fields, so rendering and compute hot paths perform no shader file I/O, manifest lookup, UTF-8 decoding, helper concatenation, or per-frame source allocation.

Each resource documents its algorithm, time complexity, and storage or bandwidth complexity at the top of the file. Final render and compute modules are self-contained, including analytic curve helpers that were previously concatenated from C# strings. Dynamic systems keep only input-dependent generation in C#: the DirectX HLSL translator emits WGSL from caller programs, WPF effects generate active sampler declarations, and ShaderToy appends user code. Their fixed headers and fragment wrappers are still resource-backed and cached.

ShaderResourceTests verifies that every source file is present in its assembly, loaded text matches the checked-in resource, required cost-model documentation exists, and fixed production stage modules do not reappear as C# literals. This keeps shader reuse and performance properties enforceable as the renderer evolves.

---

Current Frame Architecture and Performance Baseline

Microsoft.UI.Xaml.Window.RenderFrameCore records each host phase independently: dispatcher work, rendering callbacks, framebuffer/DPI setup, animation, layout, surface acquisition, compositor work, and presentation. Compositor.RenderScene then chooses between a retained fast path and a dynamic compile path:

flowchart TD
    Frame["Window frame"] --> Dispatch["Drain bounded UI work"]
    Dispatch --> Update["Rendering callbacks, animation, cached layout"]
    Update --> Acquire["Acquire physical-pixel surface"]
    Acquire --> Validate{"Compiled scene still valid?"}
    Validate -- Yes --> Reuse["Reuse draw lists, GPU buffers, brushes, hit index, and atlas entries"]
    Validate -- No --> Compile["Compile visual tree and external layers"]
    Compile --> AtlasStable{"PathAtlas coordinates stable?"}
    AtlasStable -- Yes --> Upload["Upload changed geometry, brushes, and uniforms"]
    AtlasStable -- "No, first failure" --> ResetAtlas["Reset once and discard CPU compilation"]
    ResetAtlas --> Compile
    AtlasStable -- "No after retry" --> FailFrame["Fail explicitly; live paths exceed capacity"]
    Upload --> Raster["Batch pending glyph and path rasterization"]
    Raster --> Capture{"Scene safe to retain?"}
    Capture -- Yes --> Remember["Capture versions, target, atlas generations, and layer textures"]
    Capture -- No --> Dynamic["Keep dynamic path for effects, masks, diagnostics, or DrawingVisual"]
    Remember --> Render["Execute ordered WebGPU render pass"]
    Dynamic --> Render
    Reuse --> Render
    Render --> Present["Present"]

The compiled scene cache is enabled by default with CompositorOptions.EnableCompiledSceneCache. A hit requires the same root identity and ChangeVersion, logical and physical target, viewport, DPI scale, glyph/path atlas generations, tooltip, external layer versions, and valid CacheAsLayer textures. Dynamic diagnostics force a miss. Mutable DrawingVisual content, masks, and effects are deliberately not retained because their output can change without a stable immutable command contract.

CacheAsLayer and compiled-scene reuse solve different costs. CacheAsLayer turns a stable subtree into one texture draw while the rest of a scene may still compile. Whole-scene reuse preserves the already compiled draw lists and GPU buffers for a stable frame. Atlas Generation values make both paths safe when a glyph/path atlas is cleared or repacked.

The WinUI host sets EnableGpuHitTesting = false because InputSystem already performs CPU visual-tree hit testing. Direct scene consumers keep the compositor GPU hit-test index enabled by default. This avoids building two indexes for every WinUI frame without changing input behavior.

Reference Performance

The opt-in sample harness reports wall-clock FPS, per-phase timings, allocation rate, scene-cache decisions, draw counts, and workload throughput. A July 2026 macOS 120 Hz reference run of the current architecture produced the following results; hardware, window size, and page state affect absolute values.

Workload	VSync	Wall FPS	Workload throughput	Scene cache
LOL/s Benchmark	On	120.21	11,996 LOL/s	Dynamic, 0/480 hits
LOL/s Benchmark	Off	245.04	48,905 LOL/s	Dynamic, 0/480 hits
Markdown Playground	Off	519.02	Static after warmup	299/300 hits
DXF CAD Viewer	Off	535.55	Static after warmup	299/300 hits

Run the same deterministic workload from the repository root:

dotnet build src/ProGPU.Samples/ProGPU.Samples.csproj -c Release

PROGPU_SAMPLE_BENCHMARK_PAGE='LOL/s Benchmark' \
PROGPU_SAMPLE_BENCHMARK_WARMUP_FRAMES=240 \
PROGPU_SAMPLE_BENCHMARK_MEASURE_FRAMES=480 \
PROGPU_SAMPLE_BENCHMARK_VSYNC=true \
dotnet run --project src/ProGPU.Samples/ProGPU.Samples.csproj -c Release --no-build

Set PROGPU_SAMPLE_BENCHMARK_VSYNC=false for uncapped throughput, or change the page to Markdown Playground or DXF CAD Viewer to verify static-scene reuse. The first measured static frame may populate the cache; subsequent frames should report hits unless the page intentionally animates or invalidates.

Rendering quality remains part of the performance contract. The optimized text path retains the glyph index chosen during layout, hoists transform/raster invariants out of glyph loops, and skips color/bitmap table probes only when the parsed font has no such tables. Avalonia solid outline text records one retained glyph run instead of one path per glyph: shaped indices are retained, Vector2 positions are converted once when the platform glyph run is created, and redraws reuse both arrays. Recording is O(1) with no glyph-count-dependent allocation; compositor compilation is O(G) for G glyphs. Gradient brushes and color/bitmap fonts keep their path or texture fallbacks.

Vector glyphs keep 8x8 path-atlas coverage and use a device-pixel-size transfer calibrated against native Skia: small axis-aligned text preserves fine edge detail, large text receives the slightly stronger coverage needed to match Skia's visual weight, and rotated/reflected text keeps its separately calibrated branch. The physical-size classification includes display DPI, transform scale, and static-buffer zoom, and is computed once per text command for reuse by every glyph. Glyph geometry, subpixel placement, physical DPI rasterization, winding rules, brush opacity, and blend behavior remain unchanged.

Texture resampling follows the same contract. SKCubicResampler is an immutable two-coefficient SkiaSharp value with native float equality, hashing, and named Mitchell (1/3, 1/3) and Catmull-Rom (0, 1/2) kernels. SKSamplingOptions is the matching immutable discriminated value for nearest/linear filtering, mipmapping, cubic coefficients, and requested anisotropy. Cubic draws retain B/C through the recorded command and texture vertices, and the WGSL texture shader evaluates the full Mitchell-Netravali kernel. Named and custom kernels therefore remain distinct. Anisotropic draws retain their requested value through recording, clamp only at the portable WebGPU boundary to [1, 16], generate mipmaps, and reuse one lazily created sampler and persistent texture bind group per effective value; ordinary draws do not enter this cache. Value construction, reads, comparison, and hashing are allocation-free O(1) CPU operations. Sampler lookup is amortized O(1) with at most 15 anisotropic sampler objects, and the common Catmull-Rom render path keeps its original compact polynomial and pixel output.

---

Technical Specifications: Performance Optimizations

The sections below describe the cooperating layout, scene, text, atlas, batching, effects, and platform optimizations. They share one invariant: cached work is reused only while every input that can affect pixels remains valid.

1. WinUI-Compatible High-Performance Layout Caching & Invalidation

Sizing Negotiation Lifecycle

Traditional layout systems recursively traverse the entire scene graph every frame to negotiate sizing, causing massive $O(N)$ CPU overhead on complex visual trees even when the UI is static.

ProGPU introduces a cached sizing negotiation model that short-circuits measurements using layout dirty flags and cached input boundaries:

flowchart TD
    Start["Measure Pass availableSize"] --> Cached{"_isMeasureValid and availableSize == _previousAvailableSize?"}
    Cached -- Yes --> O1Exit["O1 Early Exit - Return Cached DesiredSize"]
    Cached -- No --> Calc["Calculate Margin Insets & Bounds Constraints"]
    Calc --> Override["Execute MeasureOverride child passes recursively"]
    Override --> CacheResult["Store DesiredSize, _previousAvailableSize & set _isMeasureValid = true"]
    
    CacheResult --> ArrangeStart["Arrange Pass finalRect"]
    ArrangeStart --> CachedArr{"_isArrangeValid and _isMeasureValid and finalRect == _previousFinalRect?"}
    CachedArr -- Yes --> O1ExitArr["O1 Early Exit - Return Immediately"]
    CachedArr -- No --> Align["Calculate Offset Coordinates & Horizontal/Vertical Alignments"]
    Align --> OverrideArr["Execute ArrangeOverride child placements recursively"]
    OverrideArr --> CacheResultArr["Store Offset/Size, _previousFinalRect & set _isArrangeValid = true"]

• Measure Cache: Inside LayoutNode.Measure(), if _isMeasureValid is true and the incoming availableSize matches _previousAvailableSize, the pass returns immediately. MeasureOverride and recursive child traversals are fully bypassed in $O(1)$ time.
• Arrange Cache: Inside LayoutNode.Arrange(), if _isArrangeValid and _isMeasureValid are true and the incoming finalRect matches _previousFinalRect, the pass short-circuits. Children offsets are not recalculated, and recursive child arrangements are bypassed.
• Parent Bubble-Up Invalidation: When layout-affecting properties (such as Margin, Padding, WidthConstraint, HeightConstraint, alignments, or child mutations) are changed, they invoke InvalidateMeasure() or InvalidateArrange(). These clear local flags and bubble up the invalidation recursively to parent nodes, forcing only the dirty subtrees to be re-evaluated during the next frame's deferred layout pass.

Decoupled Visual Invalidation

To prevent circular dependencies between the ProGPU.Scene assembly (base visual layer) and the ProGPU.Layout assembly (WinUI framework layer), the ILayoutNode interface is defined in ProGPU.Scene:

public interface ILayoutNode
{
    void InvalidateMeasure();
}

Visual tree mutation methods (ContainerVisual.AddChild, RemoveChild, ClearChildren) check if this implements ILayoutNode. If so, they invoke InvalidateMeasure(), ensuring that any changes in visual tree structure automatically mark the layout path dirty without explicit parent layout references.

Retained-Scene Invalidation and Text-Page Responsiveness

Layout validity and pixel validity are separate. A layout property can alter clipping, alignment, padding, or descendant placement even when the final Size and Offset happen to compare equal. The LayoutNode setters therefore invalidate measure or arrange and call visual Invalidate(), which advances ChangeVersion to the compiled-scene root. This prevents a retained frame from displaying stale text or geometry after a layout-only mutation.

Text interaction uses the narrowest valid invalidation path. RichTextBlock observes every retained TextElement in its inline tree, so changing Run.Text, font, size, or foreground advances the owning visual version and invalidates layout without waiting for pointer activity. Hyperlink hover, caret movement, and selection changes dirty only render commands and pixels; they retain positioned characters and do not discard document layout. Markdown uses one immutable process-wide Markdig pipeline, warmed asynchronously during application startup. Single-column Markdown measured with infinite height performs one engine layout instead of a measurement layout followed by an identical second pass, while multi-column measurement reuses scratch lists. Width and multi-column-height changes explicitly invalidate layout, and empty content transactionally clears positioned characters, embedded children, and retained commands.

Page navigation keeps stable ownership boundaries:

• Replacing NavigationView.Content changes only the SplitView content child. The pane and all menu-item visuals remain parented, preserving their layout, theme resources, and cached layer.
• SplitView removes and inserts only the child that changed instead of clearing and rebuilding both children.
• Reparenting a dependency-object subtree recursively reapplies theme state only when the resolved theme or theme family actually changes. Moving a cached page between parents with the same theme therefore performs an O(H) ancestor-context comparison for tree height H instead of O(N) invalidation over the page subtree.
• System-font discovery and TextMate grammar loading use one shared background warm-up. Font menu controls are created only when the dropdown is opened. A code editor renders the same source as plain themed runs while grammar loading is pending, then retokenizes on the UI dispatcher with the shared theme grammar. Page activation therefore performs neither a synchronous filesystem font scan nor per-editor grammar initialization.

Large indexed pages remain virtual from source to pixels. ItemsControl retains an IList source rather than eagerly copying and boxing every item. Attaching a 65,535-glyph source is O(1) time and storage; UniformVirtualizingGridPanel realizes and binds only V visible/overscan items in O(V), reuses its recycler-index buffer, and does not allocate a binding closure on each viewport update. Selection and syntax-highlight changes rebind the V active containers in place instead of recycling, reparenting, and remeasuring them. Scrollbar z-order uses an in-parent reorder that invalidates pixels but not layout. The glyph browser represents indices through a read-only computed list, so it allocates neither a 65,535-entry source array nor an internal duplicate. FontIcon records the cached raw outline with a public DrawPath transform, preserving line, quadratic, cubic, and arc segments without allocating a transformed path per cell or render.

---

2. High-Performance Struct Equality and Comparison

Layout caching relies heavily on comparing boundary structs (Thickness and Rect) on every node. Standard C# struct comparison utilizes generic ValueType.Equals, which triggers CPU reflection, runtime boxing, and high memory allocations.

To eliminate this bottleneck, we implemented type-safe, non-boxing, custom equality overloads for both structs:

• Thickness (Margins and Paddings)
• Rect (Layout arrangements and clipping boundaries)

Each struct now overrides Equals(Thickness/Rect other), Equals(object? obj), GetHashCode(), and provides high-speed operators:

public bool Equals(Rect other)
{
    return X == other.X && Y == other.Y && Width == other.Width && Height == other.Height;
}

public static bool operator ==(Rect left, Rect right)
{
    return left.Equals(right);
}

public static bool operator !=(Rect left, Rect right)
{
    return !left.Equals(right);
}

These overloads compile down to direct float comparison instructions, achieving zero-allocation, ultra-fast boundary checks.

---

3. VSync-Off Graphics Pipeline Swapchain

To allow graphics and layout benchmarks to be evaluated at their true physical limit, we disabled vertical synchronization (VSync) throttling across all layers of the GPU pipeline:

• Windowing Layer: Window options in the main, developer tools, and dynamic window controllers explicitly configure VSync to be disabled:

  options.VSync = false;
  

• WebGPU Swapchain: Inside WgpuContext.ConfigureSwapChain(), the surface capabilities of the GPU adapter are queried. If PresentMode.Immediate is supported, the swapchain present configuration bypasses synchronization lockups:

  PresentMode presentMode = PresentMode.Fifo; // Fallback VSync
  for (uint i = 0; i < capabilities.PresentModeCount; i++)
  {
      if (capabilities.PresentModes[i] == PresentMode.Immediate)
      {
          presentMode = PresentMode.Immediate; // VSync Off
          break;
      }
  }
  

This enables the graphics swapchain to present frames as quickly as the GPU queue is filled, releasing the 60 FPS constraint and allowing framerates to soar into the hundreds or thousands of FPS.

---

4. Dynamic Backpressure-Throttled Event Dispatcher

The LOL/s benchmark stresses the visual framework by constantly removing and adding hundreds of poolable text controls to a canvas using a background thread loop.

• The Livelock Risk: If a background thread pushes UI events (like AddChild or property changes) to the main thread's dispatcher loop as fast as possible without throttling, it will quickly overflow the main thread's event queue. The main thread then spends entire frame cycles acquiring queue locks to process actions, creating massive lock contention that completely starves the UI thread and freezes the application.
• The Backpressure Solution: We introduced a thread-safe PendingCount property to the main UIThread queue. The background benchmark thread loops continuously without fixed sleep periods, but monitors queue occupancy:

  flowchart TD
      Start["Background Task Loop"] --> CheckBackpressure{"UIThread.PendingCount > 100?"}
      CheckBackpressure -- Yes --> Sleep["Thread.Sleep 1ms / Release Monitor Locks"]
      Sleep --> Start
      CheckBackpressure -- No --> Post["Post Action immediately / No Sleep"]
      Post --> UIThread["UIThread.RunPending - Main Thread drains queue"]
      UIThread --> AddChild["AddChild/RemoveChild visual tree mutation"]
  

  • Backpressure Active (>100): The background thread sleeps for exactly 1ms. This releases the queue monitor lock completely and relinquishes the CPU slice, allowing the main UI thread to drain the event queue with zero lock contention. The application remains 100% responsive and immune to livelocks.
  • Backpressure Inactive (<=100): The background thread runs with zero sleep, dispatching new visual mutations to the UI thread continuously to maximize throughput.

---

5. Compositor Mesh Compilation via Span-Based Direct Writes

In real-time GPU-based vector rendering, compiling high-level primitives (such as Rectangles, Ellipses, Rounded Rectangles, Paths, Lines, and Bezier curves) into dynamic vertex and index buffers is a major CPU bottleneck. Standard implementation using sequential .Add(...) calls on List<T> invokes continuous bounds checks, potential array resizing/reallocations, and element copying overhead.

To maximize throughput, the Compositor is optimized using high-performance Span<T> memory writes:

• Pre-Allocation Throttling: Instead of building meshes incrementally, the compositor determines the exact number of vertices and indices required for a primitive beforehand.
• Backing Buffer SetCount: The internal list count is directly resized using CollectionsMarshal.SetCount(list, newCount) to avoid iterative dynamic reallocation/growth logic inside .NET's List<T>.
• Direct Memory Access: The internal backing array is extracted as a type-safe memory slice via CollectionsMarshal.AsSpan(list).Slice(offset, count).
• Fast Assembly Assignment: Vertices and indices are written directly to indices in the returned Span<T> or pre-filled using Span.Fill(defaultValue) for uniform values.
• Bulk Memory Clipping: Clamping vector coordinates to active clipping boundaries is performed in a single linear pass over the direct Span<VectorVertex> reference, bypassing indexed list getters.

int originalVertexCount = _vectorVerticesList.Count;
int vertexToAdd = 2 * (N + 1);
CollectionsMarshal.SetCount(_vectorVerticesList, originalVertexCount + vertexToAdd);
var vertexSpan = CollectionsMarshal.AsSpan(_vectorVerticesList).Slice(originalVertexCount, vertexToAdd);
vertexSpan.Fill(baseVertex);

This ensures that the mesh compiler achieves zero-allocation dynamic buffer construction, minimal instruction-level overhead, and runs at near-native C-speed.

---

6. Retained MotionMark Geometry and Frame Scheduling

In traditional UI and vector engines, every active visual element in an animation loop is modeled as a heap-allocated class object. During high-count stress tests (such as the MotionMark benchmark rendering thousands of dynamically moving curves), these allocations put immense pressure on the .NET Garbage Collector (GC), leading to periodic micro-stutters and frame drops.

ProGPU eliminates this overhead using densely stored value types, retained geometry, and explicit pre-render scheduling:

• Dense Elements: Animated shapes are modeled using compact Element and GridPoint value types, avoiding one managed object allocation per segment:

  public struct Element
  {
      public SegmentKind Kind;
      public GridPoint Start;
      public GridPoint Control1;
      public GridPoint Control2;
      public GridPoint End;
      public Vector4 Color;
      public float Width;
      public bool Split;
      public SolidColorBrush CachedBrush;
      public Pen CachedPen;
      public PathGeometry CachedPath;
  }
  

• Retained Official Paths: Logical 80x40 grid points are mapped when elements are generated or the viewport changes. Each segment owns one retained PathGeometry; steady frames submit the same geometry references through the public DrawingContext.DrawPath API instead of allocating paths and segment objects in OnRender.
• Two Public-API Modes: Individual mode emits one retained path command per segment. The default grouped mode reuses pooled PathGeometry/PathFigure containers and retained segment references for each split-delimited group, reducing command compilation without bypassing the renderer.
• Pre-Render Animation Scheduling: The visual implements IAnimatedElement, so the normal sample-tree update traversal advances it every frame while it is attached. Update(delta) mutates split state and invalidates before compositor compilation; OnRender is side-effect free. Pointer movement is no longer needed to keep the benchmark active.
• Time-Normalized Split Work: The original 0.5% split probability at 60 Hz is accumulated as 0.3 * N * delta toggles. Selecting K due toggles is O(K); rebuilding split-delimited retained groups before rendering is O(N + G) for N segments and G groups, and OnRender then records only O(G) grouped commands without mutating geometry. Persistent geometry storage is O(N); the retained group pool and end-index table are O(Gmax), bounded by N, after warmup. Pens, brushes, HUD strings, and path containers are refreshed only when their owning settings, theme, geometry, or viewport change.
• Measured Result: On the 120 Hz reference macOS machine, the 1,000-element uncapped benchmark improved from 146.9 to 190.4 wall FPS and reduced visual compilation from 3.71 to 2.24 ms per frame. With VSync enabled it sustains 120 FPS with a deliberate Root version changed cache miss on every animated frame.

---

7. GPGPU Real-Time Multi-Pass Effects Pipeline

Standard graphics engines struggle to apply dynamic blurred effects (such as Gaussian backdrop blurs, soft ambient drop shadows, and neon glowing halos) to standard layout elements in real-time due to high composition and memory transfer overhead. ProGPU overcomes this with a multi-pass offscreen composition and compute processing system.

graph TD
    Subtree["Subtree Render Pass"] -->|Draw Elements 1x MSAA| Src["Source Offscreen Texture"]
    Src -->|Horiz. Dispatch| HCompute["Gaussian Blur Compute Shader Pass 1"]
    HCompute -->|Vert. Dispatch| VCompute["Gaussian Blur Compute Shader Pass 2"]
    VCompute -->|Output Framebuffer| Dest["Destination Blurs/Shadows Texture"]
    Dest -->|Matrix Align and Z-Order Bind| Framebuffer["Primary Swapchain Framebuffer"]

• Dynamic Texture Caching: Textures (Source, Temp, and Destination buffers) are cached per-element in a specialized dictionary (_effectTextures). They are dynamically resized only when the element's actual visual bounds mutate, eliminating frame-by-frame allocation/deallocation thrashing.
• Offscreen Redirection: Standard scene-graph rendering in ProGPU uses 4x MSAA for vector geometry. Since WebGPU compute shaders cannot directly read or sample multisampled textures, ProGPU compiles a specialized 1x MSAA offscreen rendering pipeline (_vectorPipelineOffscreen, _textPipelineOffscreen, _texturePipelineOffscreen). When an element has an active effect:
  1. The compositor preserves the active vector batch state and clips.
  2. It redirects all rendering of the element and its entire visual child subtree into the 1x MSAA offscreen Source texture using an isolated orthographic projection matrix.
  3. Restores the main batch state after capture.
• Two-Pass Compute Acceleration: The compute pass binds the Source texture and executes a horizontal-pass WGSL compute shader, writing intermediate results to the Temp texture. It then binds the Temp texture to execute a vertical-pass compute shader, outputting the final blurred mask to the Destination texture.
• High-Performance Compositing: The final blurred texture is drawn back onto the main screen swapchain as a textured quad. For drop shadows, the texture is drawn with configurable offsets, blending colors, and alpha multipliers, and the original Source texture is composited cleanly on top, maintaining crisp bounds.

---

8. GPU-Bound Analytical Vector Path Rasterization

To bypass CPU bottlenecks (e.g. flattening Bezier curves into thousands of lines and performing heavy triangulation), ProGPU integrates a pure GPU-bound vector path rasterizer. The engine computes vector fills analytically directly inside custom WebGPU WGSL compute shaders.

Sequential 16-Byte Aligned Struct Layouts

To satisfy WebGPU/WGSL uniform and storage buffer packing requirements, layout metrics are organized into sequentially packed structs matching exact 16-byte memory alignments:

[StructLayout(LayoutKind.Sequential, Pack = 16)]
public struct PathUniforms
{
    public float XStart;   public float YStart;
    public float Scale;    public uint PathIndex;
    public uint AtlasX;    public uint AtlasY;
    public uint Width;     public uint Height;
}

[StructLayout(LayoutKind.Sequential, Pack = 16)]
public struct GpuPathRecord
{
    public uint StartSegment;  public uint SegmentCount;
    public float MinX;         public float MinY;
    public float MaxX;         public float MaxY;
    public uint Pad0;          public uint Pad1;
}

[StructLayout(LayoutKind.Sequential, Pack = 16)]
public struct GpuPathSegment
{
    public Vector2 P0;         public Vector2 P1;
    public Vector2 P2;         public Vector2 P3;
    public uint SegmentType;   public uint Pad0;
    public uint Pad1;          public uint Pad2;
}

Analytical Non-Zero Winding Number WGSL Shaders

The rasterizer counts curve intersections analytically using a horizontal ray casting winding-number algorithm directly in WGSL:

• Line Intersection: Evaluates linear roots analytically: $$t = \frac{p_y - A_y}{B_y - A_y}$$
• Quadratic Bezier Intersection: Solves quadratic equation $(1-t)^2 A_y + 2(1-t)t B_y + t^2 C_y - p_y = 0$ for $t \in [0, 1]$. Valid intersections are checked against the ray, and winding adjustments are updated based on the tangent derivative: $$P'_y(t) = 2(1-t)(B_y - A_y) + 2t(C_y - B_y)$$
• Cubic Bezier Intersection: Expands the cubic Bezier equation into $a t^3 + b t^2 + c t + d = 0$. The compute shader executes Cardano's formula (solve_cubic helper in WGSL) to find up to 3 real roots, updating the winding number according to the cubic tangent derivative: $$P'_y(t) = 3 a t^2 + 2 b t + c$$

Performance Enhancements & Quality Correctness

• CPU Path Cache (_pathGeometryCache): Compiled segment arrays and pre-calculated local bounds are cached for each unique PathGeometry. Dynamic frames skip CPU figures traversal, and copy segment spans directly, reducing CPU path compilation times to 0.30ms for 100,000 shapes.
• Pixel-Level Bounding Box Shader Skip: To eliminate GPU rasterization bottlenecks, the fine-rasterization pixel loop performs a screen-space bounding box check:

  if (px < inst.screenMinX || px > inst.screenMaxX || py < inst.screenMinY || py > inst.screenMaxY) {
      continue;
  }
  

  Pixels outside the shape boundaries immediately bypass local coordinate transforms, 4-sample subpixel loops, and expensive winding calculations. This discards ~95% of active operations per pixel, resulting in a 15x rendering speedup.
• 4x SSAA Quality Correctness: Replaced screen coordinates with transformed local coordinates in the Sample 2 containment checks of the PathRasterizerShader. This ensures that under high multisampling/supersampling, anti-aliased edge pixels align perfectly, delivering sharp, hardware-accurate vector strokes and fills.

---

9. High-Quality Anti-Aliasing & Expanded-Quad Render Padding

Standard Signed Distance Field (SDF) rendering often clips the outer half of strokes or the edges of anti-aliasing gradients because the generated quad boundaries are drawn exactly at the shape's mathematical dimensions. This limits pixel operations outside the bounding box, resulting in a rough, aliased border.

To achieve state-of-the-art vector quality with zero performance degradation, we implemented a dual-stage quad inflation and pixel-distance anti-aliasing framework:

• Separated-Pass Quad Expansion: During shape compilation in Compositor.cs, drawing of Rectangles, Ellipses, and Rounded Rectangles is divided into independent Brush (fill) and Pen (stroke) passes.
  • Fill Pass (Brush): Inflates bounding quad vertices and texCoord offset variables outwards by a padding of 1.5 pixels.
  • Stroke Pass (Pen): Inflates bounding quad vertices and texCoord offsets by thickness / 2.0 + 1.5 pixels. This expansion guarantees that the outer half of a stroke of width $T$, as well as its smooth anti-aliasing gradient, are fully rendered without quad boundary clipping.
• Pixel-Distance WGSL Stroke Anti-Aliasing: For GPU-expanded Lines, Quadratic Beziers, Cubic Beziers, and elliptical Arcs, the vertex shader computes the exact signed pixel distance from the center spline to the expanded vertex boundaries, passing it to the fragment shader via gridIndex. The fragment shader evaluates anti-aliasing dynamically using:

  let d_pixels = abs(input.gridIndex);
  let d_shape = d_pixels - input.strokeThickness * 0.5;
  shapeAlpha = 1.0 - smoothstep(-0.5, 0.5, d_shape);
  

  This calculates a crisp, subpixel-accurate smoothstep edge transition directly in screen-space pixel coordinates, eliminating aliased jagged edges on all lines and splines.

---

10. High-Performance Theming, Styling & Templating Engine

ProGPU implements a lightweight, high-performance, and memory-safe theming, styling, and templating engine designed to emulate the logical capabilities of WinUI 3 but operating with minimal CPU and memory overhead.

flowchart TD
    Reg["DependencyProperty.Register"] -->|Sequential Indexing| DP["Index-Based Property Mapped Arrays"]
    DP -->|Precedence Resolution| GetVal["O1 GetValue Precedence Sweep"]
    Theme["ThemeManager.ThemeChanged"] -->|Lazy Invalidation| Dirty["Set IsThemeDirty = true"]
    Dirty -->|On-Demand Query| GetVal
    
    subgraph Storage ["O(1) Parallel Contiguous Value and Theme Arrays"]
        Local["_localValues"]
        Style["_styleValues"]
        DStyle["_defaultStyleValues"]
        LocalTheme["_localThemeResources"]
        StyleTheme["_styleThemeResources"]
        DStyleTheme["_defaultStyleThemeResources"]
    end

$O(1)$ Sequential Flat-Array Property Storage

Traditional XAML frameworks store DependencyObject property values in heavy dictionaries (Dictionary<DependencyProperty, object>), which trigger expensive hash calculation, collisions, and lookup overhead inside tight render or layout loops. ProGPU bypasses dictionaries entirely by introducing sequential indexing:

• Sequential Indexing: Every registered DependencyProperty is assigned a unique, sequential, zero-based Index from a thread-safe static list during bootstrap.
• Direct Array Access: DependencyObject stores properties in a set of parallel contiguous flat arrays (_localValues, _styleValues, _defaultStyleValues, _effectiveValues, and _valueSources) matching the index sizes.
• Precedence Resolution: Property value resolution (GetValue(dp)) is simplified to direct index checks on these arrays in $O(1)$ time, resolving values via native priority precedence: $$\text{Local} \succ \text{Style} \succ \text{Default Style} \succ \text{Inherited} \succ \text{Default}$$

Lazy, Invalidation-Tracked Dynamic Theming

Eagerly traversing and updating dynamic brushes across the entire visual tree on every theme change triggers substantial CPU frame stutters. ProGPU bypasses this via a lazy evaluation pipeline:

• Visual Tree Invalidation: When a theme toggle is triggered, ThemeManager.ThemeChanged fires. The system recursively propagates a cheap IsThemeDirty = true flag down the scene graph (NotifyThemeChanged), avoiding immediate value updates.
• Parallel Flat Theme Mappings: Dynamic references are stored in parallel arrays (_localThemeResources, _styleThemeResources, and _defaultStyleThemeResources). During subsequent property reads (GetValue(dp)), if the dirty flag is set, the system sweeps these parallel arrays, re-evaluates active key lookups against the theme palette, and rebuilds only the affected elements' effective values in a single sequential linear pass.

Reflection-Free, Weak Callback Template Bindings

To support lightweight control customization without the heavy reflection, expression compilation, or string-matching of traditional bindings:

• Index-Based Callbacks: DependencyObject maintains an index-sequential list of callbacks registered via RegisterPropertyChangedCallback(dp, callback).
• WinUI-Compliant Tokens: Registration returns a unique long token, allowing surgical unregistration via UnregisterPropertyChangedCallback(dp, token).
• Weak, Self-Cleaning Template Binding: TemplateBinding coordinates bindings between controls and template roots using weak references (WeakReference<DependencyObject>). On every callback trigger, if it detects that the target control has been garbage-collected, the binding automatically unregisters itself from the source object, completely preventing memory leaks.

Decoupled Multi-Window & Popup Inspector

To support robust diagnostic capabilities:

• Multi-Window Visual Inspector: Refactored the DevTools visual tree population (RefreshVisualTree) to dynamically traverse all active windows registered in WindowManager.ActiveWindows (filtering out the inspector itself), and automatically falling back to the thread-static InputSystem.Root for raw Silk.NET window bindings.
• Popup & Dialog Hierarchies: Merges active floating popups and dialogs from PopupService.ActivePopups as a dedicated branch in the visual tree, making overlay dialogs fully inspectable.
• Global Invalidation Hub: Replaced thread-local repaints with a public InvalidateAllMainWindows() hub in DevToolsService, ensuring hover overlays, inspection borders, and property changes instantly refresh across all active window compositors.

---

11. High-Fidelity GPU Text & Retina Rendering (macOS High-DPI Quality)

Traditional GPU engines suffer from low-resolution stretch blurriness on macOS high-DPI (Retina) screens because they configure the SwapChain to match logical coordinates, letting the operating system scale the output. ProGPU achieves true macOS Retina rendering quality while maintaining high performance through four main pillars:

• Physical-Pixel Backing Store SwapChain: The WebGPU swapchain and render pipelines are driven directly by the window's physical FramebufferSize instead of logical size (e.g. 2560x1600 instead of 1280x800). This aligns all vector and rasterization outputs exactly 1:1 with hardware pixels, eliminating OS-level linear stretching blur.
• DPI-Aware Physical Glyph Caching: Computes the high-DPI scaling factor dynamically (dpiScale = FramebufferSize.X / Size.X) and pre-rasterizes glyphs in the GlyphAtlas at their actual physical pixel font size (cmd.FontSize * dpiScale), ensuring that the atlas contains the high-resolution 2x textures.
• 4x Physical Subpixel Snapping: Snippets the screen-transformed baseline cursor position to physical device pixels (transPos * dpiScale) and snaps the horizontal coordinate to the nearest 1/4th physical pixel, completely eliminating subpixel blur on the screen.
• Retina Snap-Back logical mapping: Snapped physical coordinates of the drawing quad are divided by dpiScale before writing them to the vertex buffer, mapping them back to logical space for the compositor's orthographic projection matrix. The GPU hardware then renders the logical quad exactly 1-to-1 with screen physical pixels!
• Direction-Aware Winding Curve Crossing Corrections: Replaced the static, direction-agnostic interval checks in both the quadratic and cubic Bezier crossing solvers with Precise Direction-Aware Half-Open Winding Intervals based on the vertical derivative sign (deriv_y):
  • Upward Crossing (deriv_y > 0.0): Valid range is [0.0, 1.0) (inclusive of start, exclusive of end).
  • Downward Crossing (deriv_y < 0.0): Valid range is (0.0, 1.0] (exclusive of start, inclusive of end). This eliminates boundary vertex double-counting and zero-counting across all transition types (line-to-curve, curve-to-line, curve-to-curve) in both GlyphRasterizer and PathRasterizer shaders, completely preventing horizontal seam and drop-out artifacts at curve joins (such as on letters like G/g).

Text Compilation Fast Path and Rich Text Command Reuse

Text-heavy pages avoid repeated work without changing raster quality:

• TextLayout stores the resolved GlyphIndex beside each positioned glyph. The compositor uses that index directly instead of repeating character-map lookup during every compile.
• TtfFont resolves HasColorGlyphs and HasBitmapGlyphs once after parsing the table directory. Normal outline fonts therefore avoid per-glyph COLR/CPAL/SVG/bitmap probes, while fonts that contain those tables still use the full color or bitmap path.
• DPI/raster size, transform scale, rotation state, basis vectors, synthetic-bold parameters, and Skia font stretch/shear are computed once per text command or glyph run rather than once per glyph. Explicit positions remain in shaped logical coordinates; only glyph-local outlines or atlas quads are transformed, and vector cache keys include the local stretch and shear.
• CFF and explicit vector glyphs preserve the established baked fractional-position coverage model required by Skia/Svg.Skia parity, but canonicalize each local axis to 128 phases. The residual position stays in the draw transform, so the final quad is not snapped; local coverage quantization is at most 1/256 coordinate per axis. After the parent transform, vector text uses a separate four-phase device-translation key per axis, matching quarter-pixel glyph coverage with at most 1/8 device-pixel coverage error; ordinary paths retain their 64-phase keys. Only vector-text scale keys retain ten binary mantissa bits, with at most 1/2048 relative scale error. This bounds continuously changing float keys without lowering the existing 8x8 high-precision winding coverage or changing ordinary path behavior. Compilation is O(G) for G glyph instances. A glyph/style/size has at most 128² local phase variants and 16 device-phase variants, while the process-wide transformed-outline cache is capped at 4,096 entries and PathAtlas residency remains physically bounded by atlas capacity. The rotating 512-position regression settles at 129 resident paths instead of 512 exact-position paths; a separate 256-position parent-transform regression settles at 16 coverage entries. Both verify visible coverage for every glyph.
• RichTextBlock and MarkdownTextBlock retain generated drawing commands until layout, theme, selection, or hyperlink-hover state changes. Replaying C stable text/table commands is O(C) reference-copy work with no parsing, glyph positioning, or command-text allocation. A content or width change reparses only when text changed and performs the required O(B + G) block/glyph layout for B source nodes and G positioned glyphs.

---

12. Layered High-DPI Visual Caching (CacheAsLayer)

In high-performance GPU-bound UI frameworks, recursively traversing large, static visual subtrees (such as complex sidebar menus, navigation drawers, and presentation panels) every frame at double physical coordinates (FramebufferSize) on macOS Retina screens incurs heavy CPU-to-GPU overhead (layout traversal, vertex mesh generation, matrix multiplications, draw call issuance, and constant buffer uploads).

ProGPU introduces Layered High-DPI Visual Caching (CacheAsLayer) to completely eliminate redundant rendering loops for static or rarely modified subtrees:

flowchart TD
    Compile["CompileVisualTree node"] --> CacheChecked{"node.CacheAsLayer and Compositor.IsCacheAsLayerEnabled?"}
    CacheChecked -- No --> NormalPass["Standard Pass: Recurse Visual Subtree and Compile Primitives"]
    CacheChecked -- Yes --> DirtyCheck{"node.IsDirty or node.LayerTexture == null?"}
    
    DirtyCheck -- Yes --> RenderOff["Execute RenderOffscreen centered in node.LayerTexture"]
    RenderOff --> MarkClean["Set node.IsDirty = false"]
    MarkClean --> DrawTexture["Compile single DrawTexture command onto Swapchain"]
    
    DirtyCheck -- No --> DrawTexture

• Offscreen Physical Buffering: When CacheAsLayer = true is set on a static visual (like the NavigationView's sidebar pane), the compositor redirects rendering of the node and its entire subtree into an isolated offscreen texture (LayerTexture) allocated at exact physical pixel dimensions: $$w = \text{logicalWidth} \cdot \text{dpiScale}, \quad h = \text{logicalHeight} \cdot \text{dpiScale}$$
• O(1) Render Bypass: On subsequent frames, if node.IsDirty == false and the cache is valid, the compositor completely skips visual tree traversal, geometry generation, and command decoding for the entire subtree. Instead, it issues exactly 1 Texture draw call (rendering the pre-compiled LayerTexture back onto the swapchain), achieving an instant 1.77x rendering acceleration.
• Razor-Sharp Typography & 1:1 Pixel Alignment: During offscreen rendering, the projection matrix uses logical boundaries, but text glyphs are snapped and rasterized at the physical dpiScale inside CompileTextCommand. Drawing this cached layer texture back onto the physical swapchain guarantees perfect 1:1 physical pixel alignment and native-sharp typography on macOS Retina displays without bilinear filtering blur.
• Lazy Dirty-State Propagation: When any child element inside the cached subtree changes (e.g. hovered, clicked, or typed into), invalidation sets IsDirty = true and bubbles up to the cached parent node. The compositor automatically detects this dirty state on the next frame, re-runs RenderOffscreen to update the cache in a single frame, and marks it clean again.
• Global Settings Switch: The caching system can be enabled or disabled completely at runtime globally:
  • Individual Control: Visual.CacheAsLayer = true;
  • Global Override: Compositor.IsCacheAsLayerEnabled = true / false; (Toggleable via the Application Settings panel).

---

13. Dynamic Z-Ordered Draw Call Batching

In retained scene graphs with interleaved primitive types (such as vector geometries, offscreen computer-generated textures, and rich text visual elements), simple bulk-draw grouping causes Z-order overlap bugs. If all textures or all texts are batched and drawn at the very end of layer compilation, solid backgrounds or overlay vectors can draw on top of pre-rendered textures, resulting in black or empty areas.

ProGPU implements a Dynamic Z-Ordered Draw Call Batching mechanism within Compositor.cs to achieve optimal batching performance while strictly preserving visual Z-order:

• Pending Batch Tracking: Instead of immediate submission, consecutive vector shape and text draw commands are accumulated into contiguous ranges tracked via _pendingVectorStart and _pendingTextStart pointers.
• Ordered Flush Commits (CommitPendingDrawCalls): Whenever a boundary-crossing operation is encountered (such as an offscreen compiled texture draw call or layer bounds transition), the compositor flushes accumulated vector and text batches using CommitPendingDrawCalls(). This groups consecutive visual primitives into single drawing calls while guaranteeing they are submitted to the GPU command encoder in the exact Z-order depth traversed by the visual tree.
• Zero-Allocation Dynamic Offsets: The batched ranges directly index into GPU-mapped vertex and index backing buffers, avoiding CPU copy operations and preserving near-native rendering speeds.

Retained lattice and nine-patch batching

Skia-compatible lattice and nine-patch image draws use the same ordered texture path without multiplying submission overhead:

• The CPU iterator follows Skia's alternating fixed/scalable segment model. With enough destination space, fixed source segments keep their pixel length and scalable segments divide the remainder proportionally. When the destination is smaller than all fixed segments, scalable segments collapse to zero and fixed segments shrink proportionally.
• A call records one RenderCommand containing one contiguous TexturePatch[]. Transparent and collapsed cells are removed during layout. Fixed-color cells retain their filtered RGBA value beside ordinary source/destination texture cells.
• CompileTextureCommand reserves one contiguous vertex/index range, emits four vertices and six indices per visible cell, and creates one CompositorDrawCall. Fixed-color cells use the same texture pipeline with a flat per-vertex discriminator, so they avoid texture sampling without causing a pipeline switch or an extra draw.
• For X and Y div counts and C visible cells, layout costs O(X + Y + C) time and storage, compositor expansion costs O(C), and GPU submission remains one draw call per lattice operation. This prevents a conventional 9-patch from becoming nine retained commands or nine GPU submissions.

Retained vertex meshes

SKVertices and DrawingContext.DrawVertexMesh provide the matching batched path for arbitrary colored triangle meshes:

• Positions, optional texture coordinates, vertex colors, and 16-bit indices are copied once into an immutable VertexMesh2D. Triangle lists, strips, and fans retain their original topology instead of being converted into one retained path per face.
• The compositor transforms each vertex once, normalizes valid faces into one contiguous index range, and leaves invalid indexed faces out of the final count. The entire mesh remains part of the surrounding vector batch and therefore does not add one draw call per triangle.
• Vertex colors travel premultiplied and are combined with the paint brush by the vector shader. The WGSL path implements Skia's Porter-Duff, arithmetic, separable, and non-separable vertex-color blend modes before the ordinary mask and framebuffer blend stages.
• GPU hit testing traverses the same normalized triangles. For V vertices, I input indices, and T output triangles, retained construction costs O(V + I) time and storage, compilation costs O(V + T), and hit-index construction costs O(T).

Coons patches use this same mesh path. The tessellator evaluates clockwise top, right, reversed-bottom, and reversed-left cubic boundaries, combines the two ruled surfaces, and subtracts their bilinear corner surface. Device-space boundary lengths select a grid at roughly one partition per 10 pixels with an 8x8 minimum; extreme patches are proportionally limited to 60,000 indices. Corner colors interpolate in premultiplied space and texture coordinates interpolate bilinearly. An Lx by Ly patch costs O(Lx * Ly) CPU time/storage but still records one command and enters one vector batch.

Transformed sprite atlases

SKCanvas.DrawAtlas reuses the retained texture-patch batch for sprite-heavy scenes:

• Each non-empty source rectangle becomes one patch with an SKRotationScaleMatrix converted to a sprite-local scale/rotation/translation. Bounds are accumulated from the four transformed corners; an optional caller cull rectangle remains a conservative quick-reject/hit bound and does not clip individual sprites.
• The whole atlas call retains one patch array, one copied texture resource, one sampler choice, and one indexed texture draw. Nearest, linear, mipmapped, custom cubic, and anisotropic sampling metadata stays uniform across the sprite batch.
• Optional sprite colors are premultiplied once in vertex data. The texture shader treats the sampled sprite as source and the color as destination, implements all Skia blend modes, and then applies paint opacity, masks, the selected texture alpha representation, and the framebuffer blend mode.
• For S visible sprites, layout, bounds, and vertex/index generation cost O(S) time and storage. GPU submission remains one draw rather than S texture draws and does not create per-sprite bind groups.

---

14. Zero-Allocation Vector Drawing & Skia-like GpuPicture Caching

High-performance vector rendering loops are highly sensitive to Garbage Collection (GC) pressure. Passing coordinate arrays (such as Vector2[] for complex polylines, curves, or CAD structures) on every frame forces heap allocation and copying, resulting in massive GC thrashing.

ProGPU completely eliminates this overhead by introducing a zero-allocation vector drawing engine driven by ReadOnlySpan<T> and a Skia-like GpuPicture command caching architecture:

flowchart TD
    subgraph AllocPool ["Zero-Allocation Frame Draw (Pooling)"]
        DrawCall["DrawPolyline(Pen, ReadOnlySpan<Vector2> points)"] --> GetPool["Acquire continuous PointBuffer from DrawingContext"]
        GetPool --> CopySpan["Copy points data in bulk using high-speed Span.CopyTo"]
        CopySpan --> RecordCmd["Record RenderCommand with PointBufferOffset and PointBufferCount"]
    end

    subgraph CacheSystem ["Pre-Recorded Caching Loop (GpuPicture)"]
        RecStart["GpuPictureRecorder.BeginRecording(bounds)"] --> RecDraw["Record vector commands into local buffers once"]
        RecDraw --> RecEnd["EndRecording() compiles into immutable GpuPicture"]
        RecEnd --> DrawCache["context.DrawPicture(picture, cameraViewMatrix)"]
        DrawCache --> CompositorPlay["Compositor compiles and plays back directly in-place (Zero-Copy)"]
    end

Pre-Allocated Continuous Memory Pools

Since ReadOnlySpan<T> is a stack-only ref struct, it cannot be stored on the heap or inside standard lists. To allow zero-allocation span-based rendering, DrawingContext maintains internal pre-allocated continuous memory lists:

• PointBuffer (List<Vector2>)
• DoubleBuffer (List<double>)
• Line3DBuffer (List<Line3D>)
• FloatBuffer (List<float>)

On every frame refresh, calling .Clear() on these buffers resets their logical Count to 0 but retains their internal backing array capacity. Drawing coordinates are copied into these pre-allocated pools using high-speed bulk Span<T>.CopyTo operations. As long as capacity is sufficient, frame-by-frame rendering runs at near-native speed with absolutely zero heap allocations.

Unified IRenderDataProvider Interface

To support both real-time dynamic rendering (where coordinates live in the active DrawingContext pools) and cached playback (where coordinates live in static arrays), we introduce the IRenderDataProvider interface:

public interface IRenderDataProvider
{
    ReadOnlySpan<Vector2> GetPoints(int offset, int count);
    ReadOnlySpan<double> GetDoubles(int offset, int count);
    ReadOnlySpan<Line3D> GetLines3D(int offset, int count);
    ReadOnlySpan<float> GetFloats(int offset, int count);
}

Both DrawingContext and GpuPicture implement IRenderDataProvider. Inside WebGPU mesh compilation, the compositor queries coordinate spans directly from the active provider using the offsets and counts recorded in the RenderCommand.

Skia-like GpuPicture and GpuPictureRecorder

• Recording: Call GpuPictureRecorder.BeginRecording(bounds) to retrieve a recording DrawingContext. Commands are recorded normally using the zero-allocation span APIs. Call recorder.EndRecording() to compile the active lists into an immutable GpuPicture object (which allocates static arrays only once during compile time).
• Playback: Render a pre-recorded picture via context.DrawPicture(picture) or apply dynamic camera transitions in GPU-space via context.DrawPicture(picture, cameraViewMatrix).
• Zero-Copy Playback: At the compositor level, when a DrawPicture command is encountered, it recursively plays back the pre-compiled picture commands directly in-place using the picture itself as the IRenderDataProvider, completely avoiding CPU copying or allocation during rendering.

Core API Specification

1. High-Performance Zero-Allocation Span Signatures

// Draws polylines or polygon outlines directly from stack memory
public void DrawPolyline(Pen pen, ReadOnlySpan<Vector2> points, bool isClosed = false);

// Draws quadratic or cubic B-Spline curves
public void DrawSpline(Pen pen, ReadOnlySpan<Vector2> controlPoints, ReadOnlySpan<double> knots, int degree);

// Draws rational, weighted NURBS curves
public void DrawSpline(Pen pen, ReadOnlySpan<Vector2> controlPoints, ReadOnlySpan<double> knots, ReadOnlySpan<double> weights, int degree, bool isClosed);

// Draws 3D ACIS solids or wireframe boundaries
public void DrawAcisSolid(Pen pen, ReadOnlySpan<Line3D> edges, Matrix4x4 modelTransform);

// Hardware-accelerated dynamic chart line series
public void DrawGpuLineSeries(ReadOnlySpan<float> interleavedCoords, int pointsCount, float thickness, Brush brush);

// Hardware-accelerated dynamic chart scatter series
public void DrawGpuScatterSeries(ReadOnlySpan<float> interleavedCoords, int pointsCount, float radius, Brush brush);

2. Backward-Compatible Array-Based Signatures (WinUI Parity)

Wraps standard heap-allocated arrays into ReadOnlySpan<T> using new ReadOnlySpan<T>(array) and forwards to the high-performance pipeline. Assigns legacy fields (SplineWeights, Edges3D) on the created RenderCommand structures to preserve 100% test compatibility and visual tree diagnostics:

public void DrawPolyline(Pen pen, Vector2[] points, bool isClosed = false);
public void DrawSpline(Pen pen, Vector2[] controlPoints, double[] knots, int degree);
public void DrawSpline(Pen pen, Vector2[] controlPoints, double[] knots, double[]? weights, int degree, bool isClosed);
public void DrawAcisSolid(Pen pen, List<Line3D> edges, Matrix4x4 modelTransform);

---

15. WinUI-Style Cooperating Scroll Virtualization

High-performance viewport virtualization is highly sensitive to coordinate math re-calculation and z-order sorting. To guarantee flawless macOS Retina-quality scrollbar overlay Z-order depth, precise boundary clipping, and locked 60 FPS scrolling speeds, ProGPU implements a WinUI-Style Cooperating Scroll Virtualization architecture:

flowchart TD
    subgraph Parent ["ItemsControl (Templated Control)"]
        Border["Border (Chrome Background)"] --> ScrollViewer["ScrollViewer (Viewport Clipping)"]
    end

    subgraph Child ["VirtualizingPanel (Cooperating Child)"]
        Panel["UniformVirtualizingGridPanel / VirtualizingStackPanel"]
    end

    ScrollViewer -->|Hosts Panel inside Content| Panel
    Panel -->|Traverses Visual Tree| ParentQuery{"Parent ScrollViewer found?"}
    ParentQuery -- Yes --> Cooperate["Cooperating Mode: Dynamic Offset Bindings"]
    ParentQuery -- No --> Standalone["Standalone Mode: Fallback ScrollBarOverlay child"]

    Cooperate -->|MeasurePass: DesiredSize.Y = TotalVirtualHeight| ScrollViewer
    ScrollViewer -->|Updates scrollbars and sets VerticalOffset| Cooperate
    ScrollViewer -->|Physically translates panel by -VerticalOffset| Panel
    Cooperate -->|UpdateViewport: Render cells at absolute position row*ItemHeight| Panel

Dual-Mode Sizing & Viewport Cooperation

• Cooperating Mode: When hosted inside a parent ScrollViewer, VirtualizingPanel dynamically traverses up the visual parent chain (ScrollViewerOwner) to establish a direct binding link:
  • Unified Offsets: Reading and writing ScrollOffset binds directly to ScrollViewer.VerticalOffset.
  • Adaptive Viewport: The layout viewport bounds (ViewportWidth / ViewportHeight) scale automatically with the parent ScrollViewer window boundaries.
  • Extent Reporting: During the measure pass (MeasureOverride), the panel computes the total height of all items (TotalVirtualHeight) and returns it as its desired size. This informs the ScrollViewer of the total scroll extent, sizing the capsule scrollbar perfectly.
  • Z-Order Supremacy: The panel's local scrollbar overlay visual is removed, allowing the ScrollViewer to draw its native glassmorphic capsule scrollbar in its own OnRender pass. Because the scrollbar is rendered after all visual children (including the panel and its cell cards) are painted, the scrollbar remains perfectly on top of all item cards and intercepts clicks first.
• Standalone Mode: If a ScrollViewer is not found, the panel falls back to Standalone Mode, drawing its own internal ScrollBarOverlay child visual and intercepting pointer wheel events directly, ensuring full backward compatibility.

Absolute Coordinate Mapping (Anti-Drift)

To eliminate floating-point coordinate drift and keep layout compilation cycles fast:

• In cooperating mode, the ScrollViewer physically translates its Content container by -_verticalOffset and -_horizontalOffset during the arrange pass.
• The virtualizing panel detects this physical shift and places the active visible cell visuals at their absolute virtual coordinate coordinates (e.g., row * ItemHeight for grids or i * ItemHeight for stack panels) relative to the panel, letting the parent graphics pipeline translate them onto the screen. This reduces layout calculations to simple, zero-copy integer multiplication.

---

16. Hardware-Accelerated Static DXF Rendering & Crisp Static Text Buffers

CAD drawings (like DXF files) contain hundreds of thousands or millions of vector elements (lines, circles, polyline arcs, splines, and complex hatches). Recursively compiling these vector primitives from a dynamic visual tree every frame on camera changes (zoom/pan) is CPU-prohibitive.

ProGPU introduces Hardware-Accelerated Static WebGPU Buffers (Option B) which compiles all vector primitives once into a static, GPU-mapped vertex/index store (DxfStaticBuffer). Panning and zooming are executed entirely on the GPU via updates to the viewport uniforms, maintaining a locked 60+ FPS on massive, million-entity CAD models.

The Blurry Text Dilemma

While static geometry scales infinitely on the GPU, TrueType Font (TTF) text is drawn as textured quads pointing to a bitmap-cached GlyphAtlas. Zooming in stretches these pre-rendered quads, causing bilinear texture blur because the glyph atlas texture was rasterized at a static zoom scale.

ProGPU resolves this by implementing Crisp Static Text Buffers via Dynamic Re-compilation:

flowchart TD
    ZoomChange{"Context.Zoom != _lastZoom?"}
    ZoomChange -- No --> DrawStatic["Draw Static Dxf Buffer - 100% GPU Bound (Panning Free)"]
    ZoomChange -- Yes --> Recompile["Trigger RecompileStaticText on CPU"]
    
    Recompile --> ScaleDPI["Scale effective dpiScale = _currentDpiScale * Context.Zoom"]
    ScaleDPI --> RasterGlyph["Rasterize Glyph at physical FontSize * dpiScale * Zoom inside Atlas"]
    RasterGlyph --> ModelSpace["Divide quad vertex coords by effective dpiScale (cancel out Zoom)"]
    ModelSpace --> WriteGPU["Dynamic Copy-on-Write vertex/index re-upload to GpuBuffer"]
    WriteGPU --> DrawStatic

• Panning is Completely Free: Since panning does not affect font size or rasterization dimensions, panning a static drawing remains 100% GPU-bound and runs with zero CPU overhead.
• Retina-Sharp Snapping: On camera zoom changes, the compositor triggers a surgical, sub-millisecond re-compilation of ONLY the text commands using the new zoom factor: $$\text{effectiveDpiScale} = \text{dpiScale} \cdot \text{Zoom}$$
• Glyph Sizing: Glyphs are rasterized into the shared GlyphAtlas at their exact, high-resolution physical size (FontSize * effectiveDpiScale), ensuring pixel-perfect Retina snapping.
• Automatic Scaling Cancelation: The compiled quad vertex positions ($v_0, v_1, v_2, v_3$) are divided by effectiveDpiScale to map them back to base model/world coordinates. When the vertex shader multiplies them by the custom model-to-screen MVP matrix (which scales by Zoom), the zoom factor is mathematically canceled out, mapping the quad 1-to-1 to physical screen pixels with zero texture stretching or blur!

High-Performance Zoom & Scaling Optimizations

To support instantaneous zoom transitions on massive CAD models containing thousands of text elements (such as Schemat IOS Karvina CZ.dxf), ProGPU integrates three advanced graphics-pipeline optimizations:

1. $O(\text{TextCount})$ Pre-Filtered Text Records Cache:

   • Problem: Scanning millions of drawing commands recursively on the CPU during zoomed snapping steps to filter out text elements introduced noticeable interface stutters.
   • Solution: During the initial compilation of the static buffer, the compositor captures the exact DrawText commands and their parent block transformations into a flat TextRecords array in the DxfStaticBuffer:

     public struct StaticTextRecord
     {
         public RenderCommand Command;
         public Matrix4x4 Transform;
     }
     

     Subsequent snapped zoom changes bypass the drawing hierarchy entirely and recompile only the text records, reducing complexity from $O(\text{TotalElements})$ to a highly efficient $O(\text{TextElements})$ execution.

2. Discrete Font Snapping & Quad Scaling:

   • Problem: As the camera zoom levels increase, font sizes become extremely large (up to 128f), which rapidly bloats and thrashes the shared GlyphAtlas texture ($2048 \times 2048$), triggering frequent cache evictions. Computing 4-way subpixel snap coordinates for huge fonts also increases memory area consumption by $4\times$.
   • Solution:
     • Clamping: Caps the maximum physical font size rasterized into the atlas to 64f (instead of 128f). GPU bilinear filtering scales these large high-resolution sources up without visual quality loss, using $4\times$ less atlas area.
     • Size Snapping: Snaps rasterFontSize to discrete steps (0.5px steps below 24px, 2px steps above 24px) for perfect cache hit ratios. Quad quad boundaries are scaled proportionally by scaleRatio = physicalFontSize / rasterFontSize to ensure mathematical size precision on screen remains 100% exact.
     • Subpixel Bypassing: Disables subpixel snapping for font sizes larger than 24f (since subpixel shifts are visually imperceptible on large characters), saving an additional $4\times$ in atlas footprint.

3. WebGPU Queue & Driver Submission Batching:

   • Problem: Previously, rasterizing each new glyph synchronously created a temporary uniform buffer, constructed a WebGPU bind group, instantiated a command encoder, and immediately executed a sequential queue submission (QueueSubmit). For drawings with thousands of characters, this sequential driver loop caused severe CPU/GPU Metal synchronization bottlenecks on macOS.
   • Solution: Implemented nestable batching APIs (BeginBatch / EndBatch) in GlyphAtlas.cs to pool and combine glyph compute dispatches. A normal scene records all new glyphs into one CommandEncoder and executes one QueueSubmit. The 256 KB uniform ring stores 1,024 256-byte-aligned dispatch records; exceptionally large batches flush before wrap and continue with a fresh encoder, so an unsubmitted dispatch can never observe overwritten uniforms. Submission complexity is $O(\lceil G / 1024 \rceil)$ for $G$ new glyphs, while raster work remains $O(P)$ for $P$ covered glyph pixels.

4. Stable Atlas Coordinates and Capacity Fallback:

   • Problem: Clearing a full atlas during compilation relocates UVs that earlier text vertices and static buffers still reference. Clearing proactively near capacity can also turn every changing frame into a full glyph re-rasterization cycle.
   • Solution: Atlas allocation is transactional: a failed shelf placement does not mutate packing state, cached coordinates, or Generation. Existing glyphs remain reusable and the new glyph is rendered from its outline through the high-quality vector path. This avoids missing letters and cache thrash while preserving the same geometry and coverage policy. Capacity probing is O(1); an uncached fallback is O(S + P), where S is outline segment count and P is covered path pixels.

---

17. Batched Uniform Storage (Glyph & Path Atlases)

To eliminate one temporary GPU buffer per rasterized item, GlyphAtlas uses a fixed aligned ring while PathAtlas packs each pending batch into shared uniform, record, and segment buffers:

• Single Bulk Pre-allocation: Allocates a 256KB glyph uniform GpuBuffer once at startup. At WebGPU's 256-byte binding alignment this stores 1,024 dispatch records. Path batches use shared packed storage/record/segment uploads sized to their pending work rather than one buffer per path.
• 256-Byte Alignment Compliance: Follows the WebGPU standard (minUniformBufferOffsetAlignment boundary constraint of 256 bytes) by rounding up structural uniform offsets with a fast bitwise operation: $$\text{alignedSize} = (\text{SizeOf<Uniforms>} + 255) & \sim 255$$
• Fast Queue Copy-on-Write: Inside glyph batch rasterization, parameters are written directly to the pre-allocated ring buffer at the current _ringOffset using QueueWriteBuffer, avoiding one buffer allocation per glyph:

  _context.Wgpu.QueueWriteBuffer(_context.Queue, _uniformRingBuffer.BufferPtr, _ringOffset, &uniforms, (uint)Marshal.SizeOf<GlyphUniforms>());
  

• Binding Slice Offsets: Dynamic bind groups point to exact ring slices using Offset = _ringOffset and Size = Marshal.SizeOf<Uniforms>(). The batch is submitted before the next aligned slice would wrap, then continues from offset zero in a new encoder. Normal scenes still require one submission and the hot loop creates no temporary uniform buffers.
• Generation-Tracked Reuse: GlyphAtlas.Generation changes only on an explicit clear, and PathAtlas.Generation changes on clear or repack. The compiled-scene cache records both values so it never reuses UVs after atlas contents move. Glyph capacity exhaustion preserves coordinates and therefore does not increment the generation.
• Demand-Driven Path Capacity Recovery: Advancing a frame never treats render-target dimensions as future path dimensions and never clears valid PathAtlas entries speculatively. A recoverable allocation failure aborts only CPU scene compilation, clears the atlas once, increments Generation, and retries the same onscreen or offscreen frame from the retained scene. Existing UVs are never moved underneath submitted draw calls, and one-shot surfaces do not lose paths while waiting for another frame. Normal frame advancement is O(1), stable scenes keep their atlas and compiled-scene hits indefinitely, and rare recovery is O(C + A + P) for C retained commands recompiled, atlas clear area A, and rerasterized covered pixels P. A frame whose live path set cannot fit after the reset fails explicitly instead of looping or silently dropping geometry.

---

18. Double-Buffered Geometry Swapchains (DxfStaticBuffer)

Updating dense vector meshes and text quads during snapped zoom events can cause severe CPU-GPU hardware execution stalls. If the CPU disposes and recreates vertex/index buffers while the GPU command queue is actively reading from them, the graphics driver is forced to block CPU execution to synchronize hardware lifecycles.

To prevent these stalls and achieve perfectly fluid rendering, we implemented a Double-Buffering Swapchain pattern:

• Asynchronous Back-Buffering: Maintains dual buffer sets in DxfStaticBuffer:
  • Front-Buffers (TextVertexBuffer, TextIndexBuffer, TextIndexCount) currently being drawn by the compositor.
  • Back-Buffers (_textVertexBufferBack, _textIndexBufferBack, _textIndexCountBack) dedicated to accommodating the next camera layout recalculation.
• Non-Blocking Dynamic Copy: When UpdateTextBuffer is invoked during snapped zooms, it resizes and writes to the back-buffers asynchronously.
• Zero-Allocation Swapping: Swaps the front and back buffer references instantly using cheap variable re-assignment on the CPU:

  var tempVertexBuffer = TextVertexBuffer;
  TextVertexBuffer = _textVertexBufferBack;
  _textVertexBufferBack = tempVertexBuffer;
  

• Static Bind-Group Stability: Because vertex and index buffer mappings are bound directly via render encoder draw commands rather than static composition bind groups, swapping front/back buffers bypasses bind-group recreation or layout invalidations entirely, ensuring stutter-free, instant zoom actions.

---

19. Snapped Blur Radii & Stable Effect Pipelines

Offscreen Gaussian blur and drop shadow dispatches are highly sensitive to parameter fluctuations during keyframe animations or hover transitions. Smooth float radius adjustments (e.g. transitioning from 1.0f to 3.0f) dynamically modify the computed iteration count: $$\text{iterations} = \text{Clamp}(\text{Round}(\text{radius} / 2.5), 1, 8)$$ This causes the rendering loop to alter command-buffer layouts and recreate dynamic bind groups frame-by-frame, creating noticeable micro-stutters.

To stabilize effect execution, we implemented a Snapped Radii Pipeline:

• Discrete Increments: Symmetrically snaps incoming radius and blurRadius parameters to discrete 0.5f pixel boundaries at the entry points of ApplyGaussianBlur and ApplyDropShadow:

  float snappedRadius = MathF.Round(radius * 2f) / 2f;
  

• Pipeline and Bind-Group Lock: Snapping ensures that the computed iteration count remains perfectly locked and stable during intermediate keyframes. WGSL shader binding entries, textures, and command layouts remain identical across frame transitions, delivering extremely fluid hover animations and eliminating transient render delays.

---

Module & Project Architecture Breakdown

The ProGPU solution is partitioned into modular, highly specialized C# projects. Each project governs a specific layer of the UI, vector, or graphics compilation loops:

Project	Assembly Name	Core Architectural Responsibility	Key Components & Classes
ProGPU.Backend	ProGPU.Backend.dll	Low-level hardware infrastructure and WebGPU swapchain orchestration.	WgpuContext, Window, Shaders, RenderPipelineCache
ProGPU.Compute	ProGPU.Compute.dll	Orchestration of WebGPU GPGPU compute pipelines and parallel filter dispatches.	ComputeAccelerator, ComputeShaders
ProGPU.Vector	ProGPU.Vector.dll	Mathematical primitives, Bezier models, path segment parsing, and atlas mapping.	PathGeometry, PathFigure, GpuPathSegment, PathAtlas
ProGPU.Text	ProGPU.Text.dll	TrueType parsing, retained glyph identity, word wrapping, line layout, and generation-tracked glyph atlas storage.	TtfFont, GlyphAtlas, TextLayout, TextRunGlyph
ProGPU.Scene	ProGPU.Scene.dll	Retained visual tree, compiled-scene validation, ordered draw-list/GPU-buffer compilation, optional GPU hit testing, and effects.	Compositor, CompositorOptions, CompositorMetrics, ContainerVisual, DrawingVisual
ProGPU.Layout	ProGPU.Layout.dll	XAML-compatible sizing negotiation lifecycle (Measure / Arrange) and layout panels.	LayoutNode, StackPanel, GridPanel, CanvasPanel
ProGPU.WinUI	ProGPU.WinUI.dll	Interactive controls, CPU input/hit testing, frame-phase instrumentation, and command-cached rich documents.	Window, WindowFrameMetrics, RichTextBlock, ScrollViewer, SplitView
ProGPU.Virtualization	ProGPU.Virtualization.dll	Dynamic scrolling viewport orchestration and UI virtualization controllers.	VirtualizingPanel, ViewportInfo
ProGPU.Samples	ProGPU.Samples.dll	Showcase bootstrap, bounded UI scheduling, animation drivers, diagnostics, and repeatable stress/performance workloads.	MainWindowController, SamplePerformanceBenchmark, LolsPage, UIThread

---

WebGPU WGSL Shader Specifications & Implementations

ProGPU routes all graphics and compute tasks directly to the GPU using specialized WGSL (WebGPU Shading Language) shaders. The following sections detail their purpose, execution pipelines, and exact implementations.

1. VectorShader (Rasterization Graphics Pipeline)

• Role: Primary graphics pipeline shader for standard UI rendering. Responsible for rasterizing vector shapes (rectangles, ellipses, rounded rectangles) and evaluating Bezier curves and elliptical arcs on the GPU.
• Why It is Used: Avoids uploading dense pre-tessellated mesh structures. Instead, it utilizes cheap mathematical Signed Distance Fields (SDFs) and GPU vertex expansion to draw vector primitives with zero CPU overhead.
• Implementation Mechanics:
  • GPU Stroke Expansion & Miter Scaling (sType == 3u): Expands lines dynamically in the vertex shader. Computes normal vectors ($miterN$) at segment junctions, scales them by $1/\cos(\theta)$ ($miterScale$), and offsets vertices to form precise, variable-thickness miter joints. Passes the signed pixel distance from the center line to the fragment shader via gridIndex for zero-cost edge anti-aliasing.
  • Dynamic Bezier Evaluation (sType == 5u & 6u): Replaces CPU Bezier flattening. For Quadratics and Cubics, the vertex shader interpolates coordinates directly based on the thread's vertexIndex and parametric factor $t \in [0, 1]$, calculating curve positions and tangents to offset vertices outward along normal vectors on the fly, storing signed pixel distances in gridIndex.
  • Dynamic Arc Evaluation (sType == 11u): Replaces CPU arc flattening for valid path strokes. The compositor sends the transformed ellipse center plus two axis vectors, and the vertex shader evaluates arc positions and tangents parametrically before stroke expansion.
  • Analytical SDF Fragment Evaluation (sType < 3u): Computes Signed Distance Fields for Rectangles, Ellipses, and Rounded Rectangles. Anti-aliases boundaries dynamically using screen-space partial derivatives: $$\text{fw} = \max(\text{fwidth}(d), 0.0001)$$ $$\alpha = 1.0 - \text{smoothstep}(-0.5 \cdot \text{fw}, 0.5 \cdot \text{fw}, d)$$
  • Pixel-Distance Stroke Anti-Aliasing (sType == 3u \|\| 5u \|\| 6u \|\| 11u): Resolves aliasing for lines, curves, and arcs by evaluating screen-space smoothstep transitions using the interpolated gridIndex pixel distance to the stroke boundary: $$d_{\text{shape}} = \text{abs}(\text{gridIndex}) - \text{strokeThickness} \cdot 0.5$$ $$\alpha = 1.0 - \text{smoothstep}(-0.5, 0.5, d_{\text{shape}})$$
  • Gradient Interpolation: Evaluates Linear (brushType == 1u) and Radial (brushType == 2u) gradients dynamically for up to 4 stop colors by calculating projection coordinates and interpolating between bounds using stop offsets.
  • Batched vertex meshes (sType == 18u): Interpolates premultiplied vertex colors across triangle lists, strips, and fans, then combines that color with the evaluated paint brush using the requested Skia blend mode. One mesh contributes one retained command and one contiguous vector index range rather than one command per triangle.

2. TextureShader (Image, lattice, and sampling pipeline)

• Role: Draws ordinary image quads, batched lattice/nine-patch cells, and transformed sprite atlases through one indexed texture pipeline.
• Why It is Used: Preserves image Z-order while keeping each retained image, lattice, or atlas operation to one GPU draw, regardless of patch or sprite count.
• Implementation Mechanics:
  • Normal cells interpolate source UVs and use nearest, linear, mipmapped, or a bounded 4x4 Mitchell-Netravali cubic sample footprint.
  • Lattice fixed-color cells carry a flat patchKind discriminator and return their vertex RGBA directly, performing no image sample. Separate straight and premultiplied encodings preserve blend correctness for both texture alpha modes.
  • Atlas cells carry a flat color-blend mode and paint-opacity value. Sampled sprite color is the source and premultiplied per-sprite color is the destination; bounded Porter-Duff, separable, and non-separable functions match Skia's atlas blend ordering.
  • Every fragment samples the active opacity mask once. Premultiplied image and fixed-color paths scale RGB with coverage; straight-alpha paths retain straight RGB and scale alpha.
  • CPU layout and vertex generation are O(C) for C visible cells, fragment work is O(1), and one indexed draw stores four vertices and six indices per cell.

3. TextShader (SDF Glyph Render Pipeline)

• Role: Specialized graphics shader for high-speed, sharp text display.
• Why It is Used: Traditional text rasterization blurs heavily under scaling. The TextShader samples high-precision SDF textures and applies dilation offsets and power-based sharpness filters to ensure text remains crisp at any display size or zoom level.
• Implementation Mechanics:
  • Samples the single-channel glyph atlas: let alpha = textureSample(atlasTexture, atlasSampler, input.texCoord).r;
  • Applies a dilation scale based on the requested stroke thickness: let dilated = clamp(alpha * input.strokeThickness, 0.0, 1.0);
  • Filters sharpness using a power curve driven by the corner radius: let finalAlpha = pow(dilated, input.cornerRadius);

4. GlyphRasterizerShader (GPGPU Analytical Glyph Rasterizer)

• Role: WebGPU compute shader tasked with pre-rasterizing vector glyph outlines into the glyph atlas texture.
• Why It is Used: Bypasses slow CPU-based glyph rasterizers entirely, using parallel GPU threads to rasterize outlines directly on the GPU.
• Implementation Mechanics:
  • Operates on a $16 \times 16$ thread group.
  • Calculates intersections using a 16x supersampled (SSAA) analytical winding-number raycaster.
  • Solves quadratic equations directly inside the WGSL shader (solve_quadratic) to evaluate Bezier curve boundaries, updating winding directions according to the curve's vertical tangent derivative.
  • Writes the calculated coverage mask directly to the storage texture: textureStore(atlasTexture, writeCoord, vec4<f32>(coverage, 0.0, 0.0, 0.0));

5. PathRasterizerShader (GPGPU Analytical Vector Path Rasterizer)

• Role: Advanced WebGPU compute shader that computes analytical non-zero winding fills for arbitrary paths.
• Why It is Used: Bypasses CPU segment flattening and triangulation completely, allowing the GPU to raycast complex Bezier geometry directly.
• Implementation Mechanics:
  • Computes intersections of horizontal rays with Line, Quadratic Bezier, and Cubic Bezier segments.
  • Features an analytical Cardano's formula solver (solve_cubic inside WGSL) to evaluate cubic Bezier roots: $$p = b - \frac{a^2}{3}, \quad q = c - \frac{ab}{3} + \frac{2a^3}{27}, \quad D = \frac{q^2}{4} + \frac{p^3}{27}$$ If $D \leq 0$, it extracts up to 3 real roots using trigonometric cosine angles, updating the winding number according to the tangent derivative $P'_y(t) = 3 a t^2 + 2 b t + c$.
  • Executes 4-point supersampling (SSAA) using subpixel sampling coordinate offsets (+0.25, +0.75) in local space (fp2), achieving hardware-accurate anti-aliased edge coverage.

6. GaussianBlur (Horizontal & Vertical Compute Filters)

• Role: Parallel compute shaders for high-performance backdrop and glass blurs.
• Why It is Used: Bypasses slow pixel shader convolution passes by executing parallel thread blocks directly on texture buffers.
• Implementation Mechanics:
  • Operates in two consecutive passes (Horizontal, then Vertical) to split rendering complexity from $O(K^2)$ to $O(K)$ instructions per pixel.
  • Executes an unrolled 5-tap Gaussian kernel using hardcoded weights to avoid memory fetch latency: $$\text{color} = 0.0625 \cdot T[-2] + 0.25 \cdot T[-1] + 0.375 \cdot T[0] + 0.25 \cdot T[1] + 0.0625 \cdot T[2]$$
  • Clamps texture coordinate bounds inside textureLoad to eliminate edge bleed artifacts.

7. DropShadow (Ambient Shadow & Neon Glow Compute Filter)

• Role: WebGPU compute shader calculating soft drop shadows and glowing neon halos for layout elements.
• Why It is Used: Evaluates dynamic blurring and translation offsets over element boundaries in a single dispatch pass.
• Implementation Mechanics:
  • Operates on a $16 \times 16$ thread block.
  • Takes a Params uniform block specifying translating offset, shadow color, and blurRadius.
  • Loops over a sliding window of size [-blurRadius, blurRadius].
  • Extracts the source offscreen texture's alpha channel, averages the coverage, and outputs the shifted, blurred, and color-multiplied mask back to the destination buffer: $$\text{shadowColor} = \vec{C}{\text{params}} \cdot (A{\text{sum}} / \text{count})$$

---

Development & Diagnostic Tools

ProGPU includes rendering diagnostics and a repeatable in-process frame benchmark:

1. TrueType Font Outline Diagnostic Tool (TtfDiag)

Located in tools/TtfDiag/, this is a generic console tool designed to inspect outline structures, endpoint coordinates, and control points of TrueType fonts. It is especially useful for diagnosing text rendering quality, drop-out artifacts, or glyph parsing inconsistencies.

• Usage:

  # Run using the system's Arial font (supplemental) fallback to inspect specific glyphs (e.g. 'G' and 'g')
  dotnet run --project tools/TtfDiag -- Arial Gg
  
  # Run with an absolute path to a custom font and custom character sequence
  dotnet run --project tools/TtfDiag -- /System/Library/Fonts/Supplemental/Georgia.ttf ABC
  

• Output: Dumps the exact TrueType outline geometry, closed/filled figure status, segment types (Lines/Quadratic Beziers), and precise coordinates using standard invariant decimal formatting.

2. DXF Vector CAD Diagnostic Tool (DxfDiag)

Located in tools/DxfDiag/, this is a standalone command-line utility to inspect DXF vector files. It lists all available layouts and layers, prints active layout geometric bounds, recursive block hierarchies, nested insert attributes (tags/values), and detects coordinate outliers exceeding absolute limits ($> 1,000,000$). The complete diagnostic trace is saved to outliers.txt in the local directory.

• Usage:

  # Run on a target DXF drawing file to inspect the default active space layout
  dotnet run --project tools/DxfDiag -- <path-to-dxf-file>
  
  # Run on a target DXF drawing file and explicitly target a specific layout space (e.g. 'A0')
  dotnet run --project tools/DxfDiag -- <path-to-dxf-file> --layout A0
  

• Output: Generates a detailed audit of entity counts, viewport settings, block trees, and coordinates, saving the report to outliers.txt and logging a summary to the console.

3. Sample Frame Benchmark

SamplePerformanceBenchmark is disabled during normal sample use and activates only when PROGPU_SAMPLE_BENCHMARK_PAGE is set. It selects the requested page, applies the requested VSync mode, warms the renderer, measures a fixed frame count, prints one [SampleBenchmark] RESULT line, and closes the app.

The result separates host layout/animation/surface phases from compositor compile/upload/render phases and includes allocated bytes per frame, cache hits and miss reason, draw/vertex counts, and LOL/s workload counters. Use it for before/after comparisons on the same machine, configuration, window state, and page. Do not compare a VSync-limited result with an uncapped run.

---

Platform Integration & Host Control Embedding (Avalonia & Uno Platform)

ProGPU is designed to act as an embedded high-performance graphics substrate inside standard host XAML frameworks. We provide native integration packages for both Avalonia (ProGPU.Avalonia) and Uno Platform (ProGPU.Uno), allowing developers to overlay low-allocation WebGPU rendering canvases directly inside standard desktop applications.

1. Hybrid Rendering Architecture

The integration layer hosts a headless, offscreen WgpuContext and Compositor instance inside a custom control subclass (Control in Avalonia, ContentControl in Uno). WebGPU renders all visual tree and CAD vectors offscreen, which are then blitted directly to the host's screen.

graph TD
    subgraph UIThread ["Host UI Thread (Input & Sizing)"]
        Size[Sizing Negotiation: Measure & Arrange] --> Input[Pointer Event Capture & Translation]
    end
    
    subgraph GPUThread ["GPU & WebGPU Staging Loop"]
        Input -->|InputSystem.Inject| WG[WebGPU Core Offscreen Render]
        Size -->|Logical Bounds| WG
        WG -->|CommandEncoderCopy| ST[Staging Buffer VRAM]
        ST -->|Sync MapRead| MP[Mapped CPU Pointer]
        MP -->|Direct Pointer Blit| WB[WriteableBitmap 96 DPI]
        WB -->|Invalidate / DrawImage| SCR[High-DPI Retina Screen]
    end

---

2. High-Performance Direct Bitmap Blitting Pipeline

Due to standard platform-agnostic FFI limitations in wgpu-native, raw WGPUTexture pointers cannot be shared directly with the compositor's graphics context (Metal/D3D) as IOSurfaceRef or id<MTLTexture> handles without writing custom native Rust/C++ bridging wrappers.

To bypass these FFI opaque struct constraints and deliver 100% stable, platform-independent rendering, ProGPU implements a highly optimized Direct Bitmap Blitting pipeline:

• Aligned GPU Staging Buffers: WebGPU allocates a staging buffer backed by BufferUsage.MapRead | BufferUsage.CopyDst. The row pitch (BytesPerRow) is aligned to the nearest 256 bytes per WebGPU specifications to satisfy FFI layout requirements: $$\text{BytesPerRow} = (\text{width} \cdot \text{bytesPerPixel} + 255) \ & \ \sim 255$$
• Synchronous MapRead Polling: Each frame, a command encoder executes CopyTextureToBuffer from the offscreen target to the staging buffer. The buffer is mapped via BufferMapAsync, and the UI thread polls wgpuDevicePoll in a light spin loop until mapping completes.
• Direct Row Pointer Blitting: Once mapped, the raw VRAM memory address is extracted. The control performs a high-speed pointer-based copy utilizing native System.Buffer.MemoryCopy straight into the locked buffer address of the host's high-DPI WriteableBitmap:

  using (var locked = _writeableBitmap.Lock())
  {
      byte* srcBytes = (byte*)mappedPtr;
      byte* dstBytes = (byte*)locked.Address;
      uint rowBytes = _renderWidth * bytesPerPixel;
  
      for (uint y = 0; y < _renderHeight; y++)
      {
          byte* srcRow = srcBytes + (y * _bytesPerRow);
          byte* dstRow = dstBytes + (y * (uint)locked.RowBytes);
          System.Buffer.MemoryCopy(srcRow, dstRow, rowBytes, rowBytes);
      }
  }
  

  This row-by-row blitting executes in microseconds on the CPU, achieving near-zero visual overhead and bypassing bilinear filtering blur.

---

3. High-DPI Retina Calibration & Anti-Double-Scaling

On macOS Retina displays (e.g. DpiScale = 2.0), standard platform-specific graphics renderers often apply the display's scaling factor twice when drawing a high-DPI bitmap, blowing up the layout and creating blurry graphics.

ProGPU resolves this double-scaling bug through strict physical-to-logical coordination:

• 96 DPI Isolation: The host WriteableBitmap is instantiated at a constant 96 DPI (new Vector(96, 96)), making its logical size match its physical size.
• Logical-Bounds Offscreen Rendering: Viewport dimensions passed to Compositor.RenderOffscreen are strictly mapped in logical coordinates, while the internal WebGPU pipeline multiplies them by DpiScale to align the physical viewport.
• Clean Down-Scaling: During the draw pass, the physical staging bitmap is scaled down into the host control's logical bounds using a standard 1-to-1 stretch layout (Stretch.Fill in Uno, context.DrawImage in Avalonia). The physical pixels map precisely 1:1 with screen hardware coordinates, yielding absolute razor-sharp text and graphics.

---

4. Symmetrical Input Routing & Event Translation

The integration libraries bridge the event-handling loop symmetrically:

• Coordinate Translation: Pointer event handlers (OnPointerMoved, OnPointerPressed, etc.) intercept native positions, translate them into logical Vector2 boundaries, and route them to ProGPU's input engine:

  InputSystem.InjectMouseMove(new Vector2((float)pos.X, (float)pos.Y));
  

• Input State Invalidation: Input events mark the active WinUI input state dirty, forcing immediate layouts hit-testing and scheduling dynamic repaint requests to update hover overlays and cursors instantly.

---

5. locked High-Refresh Rate VSync Loops (120 FPS+)

To allow embedded graphics and animation benches to run at their physical display limit, standard timer loops are replaced by self-scheduling graphics dispatchers:

• Avalonia: Hooks directly into the system's VSync loop using:

  TopLevel.RequestAnimationFrame(OnAnimationTick);
  

  This self-scheduling tick fires callbacks exactly aligned with the physical monitor's refresh rate, unlocking 120 FPS / 144 FPS rendering without frame tearing.
• Uno Platform: Subscribes directly to CompositionTarget.Rendering to drive the WebGPU command submissions and refresh statistics exactly aligned with each compositor pass.

---

III. Path 2: Zero-Copy Shared Texture Rendering Pipeline

To bypass the overhead of copying pixels from VRAM to CPU staging buffers and back to VRAM (double-copy blitting), ProGPU implements a cutting-edge Zero-Copy Shared Texture Rendering Pipeline. This architecture achieves direct GPU-to-GPU memory sharing between the offscreen WebGPU rendering engine and the host UI composition tree.

sequenceDiagram
    participant WebGPU as WebGPU Engine
    participant OS as OS Shared Resource (IOSurface / D3D11)
    participant Avalonia as Avalonia Compositor Tree
    participant GPU as physical GPU VRAM

    WebGPU->>OS: 1. Render directly to Shared Handle (Zero CPU Copy)
    OS->>GPU: 2. Texture contents persist in VRAM
    Avalonia->>OS: 3. Import Shared Handle via ICompositionGpuInterop
    Avalonia->>GPU: 4. Draw directly from VRAM (Zero Copy / 120 FPS+)

1. Architectural Overview & Memory Sharing Mechanics

The Zero-Copy pipeline eliminates host CPU copies entirely by allocating a hardware-backed shared OS memory handle directly in C#, wrapping it inside WebGPU as a render target, and importing it into the host visual tree:

Operating System	Shared Resource Type	Native Handle Reference	Allocation Strategy
macOS	Apple IOSurface	IOSurfaceRef (global handle)	CoreFoundation/AppKit unmanaged dictionary creation
Windows	Direct3D11 Shared Texture	DXGI HANDLE (global shared key)	Standalone ID3D11Device with D3D11_RESOURCE_MISC_SHARED

2. C# Hardware-Backed Allocation Details

A. macOS IOSurface Allocation

CoreFoundation and Objective-C runtime P/Invokes are used to construct the surface configuration plist:

• IOSurfaceWidth & IOSurfaceHeight: Target dimensions.
• IOSurfaceBytesPerElement: 4 bytes per pixel.
• IOSurfacePixelFormat: 'BGRA' (packed 32-bit integer 1111970369).
• IOSurfaceBytesPerRow: Aligned to 256 bytes.
• IOSurfaceAllocSize: Total byte size.

B. Windows D3D11 Shared Handle Allocation

Direct COM VTable indexing is utilized to create resources dynamically:

• D3D11CreateDevice: Instantiates a standalone hardware D3D11 device.
• CreateTexture2D: Allocates the texture with D3D11_BIND_RENDER_TARGET | D3D11_BIND_SHADER_RESOURCE bind flags and the D3D11_RESOURCE_MISC_SHARED misc flag.
• QueryInterface: Extracts the IDXGIResource COM pointer.
• GetSharedHandle: Obtains the global shared handle pointer.

3. Integrating with Avalonia's ICompositionGpuInterop

The host control hooks into Avalonia's composition engine during initialization:

1. Query Interop Interface:

   var interop = await compositor.TryGetCompositionGpuInterop();
   

2. Verify Compatibility: Verify that the compositor's graphics backend supports the active platform's handle type (IOSurfaceRef on macOS, D3D11TextureGlobalSharedHandle on Windows).
3. Import Image: Create a PlatformHandle from the allocated raw pointer and import it:

   var platformHandle = new PlatformHandle(_sharedHandle, _gpuHandleType);
   _importedGpuImage = _gpuInterop.ImportImage(platformHandle, properties);
   

4. Present via Composition Surface: Create a standard CompositionSurfaceVisual and assign its Surface to a CompositionDrawingSurface. On every tick, simply call:

   _ = _drawingSurface.UpdateAsync(_importedGpuImage);
   

   This triggers a hardware-accelerated present, drawing the shared texture directly in the compositor loop without CPU copying.

4. The WebGPU FFI Bridge Boundary (Native Integration)

Standard cross-platform wgpu-native bindings do not export helper functions out-of-the-box to wrap arbitrary IOSurfaceRef or shared ID3D11Texture2D handles into WebGPU texture objects. To complete the zero-copy pipeline on the WebGPU side, a small custom native wrapper (written in Rust or C++) must bridge the HAL (Hardware Abstraction Layer) boundary:

// Custom native Rust crate bridging wgpu-core and OS handles
use wgpu_core::hub::Global;
use wgpu_hal::api::{Metal, Dx12};

#[no_mangle]
pub unsafe extern "C" fn wgpuDeviceCreateTextureFromMacIOSurface(
    device_ptr: *mut libc::c_void,
    iosurface_ptr: *mut libc::c_void,
    width: u32,
    height: u32
) -> *mut libc::c_void {
    let global = &*Global::default();
    // 1. Extract raw device representation
    let device_id = std::mem::transmute(device_ptr);
    
    // 2. Fetch the Metal device and wrap the IOSurface handle via wgpu_hal
    let surface: Metal::Texture = Metal::texture_from_raw(iosurface_ptr as *mut _);
    
    // 3. Register the newly created texture inside the wgpu-core context
    let texture_id = global.device_create_texture_from_hal::<Metal>(
        device_id,
        surface,
        width,
        height
    );
    
    std::mem::transmute(texture_id)
}

This bridge allows WebGPU command encoders to bind the texture as a standard RenderPassColorAttachment, completing the zero-copy pipeline.

5. Asynchronous Double-Buffered Update & Polling Architecture

To achieve VSync-locked rendering (120 FPS+) and completely eliminate UI-thread blocking or frame flickering, ProGPU utilizes a high-performance Asynchronous Double-Buffered Update Loop driven by a Dedicated Background Device Polling Thread.

This architecture guarantees 0% CPU blocking on the main UI thread and prevents read-write VRAM conflicts between the renderer and the host compositor.

sequenceDiagram
    participant UI as UI Thread (RenderFrameAsync)
    participant BG as Background Polling Thread
    participant WGPU as WebGPU Device / Queue
    participant Swap as SwapchainImage (Double Buffered)
    participant Comp as Avalonia Compositor Thread

    UI->>WGPU: 1. Render scene offscreen to WgpuTexture (Image A)
    UI->>WGPU: 2. Queue CopyTextureToStagingBuffer
    UI->>WGPU: 3. Invoke MapBufferAsync (non-blocking Task)
    Note over UI,BG: UI thread yields control immediately
    Loop Continuous Polling
        BG->>WGPU: 4. wgpuDevicePoll(Device, false) every 2ms
    End
    WGPU-->>BG: 5. Mapping complete! Trigger MapCallback
    BG-->>UI: 6. Complete TaskCompletionSource (Resume UI)
    UI->>Swap: 7. CopyMappedToSharedTexture (MemoryCopy / UpdateSubresource)
    UI->>WGPU: 8. BufferUnmap
    UI->>Comp: 9. UpdateAsync (Swapchain Image A)
    Note over UI,Comp: Image A is now bound to Compositor. Swap to Image B.

A. Double-Buffered Swapchain Image Model (SwapchainImage)

A dedicated SwapchainImage class encapsulates the graphics assets for a single frame. The host control manages a pool of two swapchain images (SwapchainImage[2]):

• Compositor Frame Lock: One image is locked by the Avalonia compositor for current presentation.
• Renderer Target: The other image is being written to asynchronously by the WebGPU rendering loop.
• Role Swap: Once rendering and memory copies are completed, the roles are swapped in an alternating cycle: _currentWriteImageIndex = (_currentWriteImageIndex + 1) % 2.

private class SwapchainImage : IDisposable
{
    public IntPtr SharedHandle;
    public ICompositionImportedGpuImage? ImportedImage;
    public GpuTexture? WgpuTexture;
    public IntPtr StagingBuffer;
    public uint StagingBufferSize;
    public uint BytesPerRow;

    // Windows Specific Direct3D 11 Resources
    public IntPtr WinD3DDevice;
    public IntPtr WinTexture2D;
}

B. Continuous Background Device Polling Thread

WebGPU asynchronous operations (such as staging buffer mapping) require the device queue event loop to be polled via wgpuDevicePoll. To keep the UI and Avalonia render threads completely unblocked, ProGPU runs a continuous, low-latency background polling thread that executes wgpuDevicePoll every 2 milliseconds:

private void StartPolling()
{
    _pollingThread = new Thread(() => {
        while (!_pollingCts.Token.IsCancellationRequested) {
            wgpuDevicePoll(_wgpuContext.Device, false, null);
            Thread.Sleep(2);
        }
    }) { IsBackground = true, Name = "ProGpuDevicePolling" };
    _pollingThread.Start();
}

C. Asynchronous Non-Blocking Map Pipeline

The buffer mapping callback is wrapped in a standard C# TaskCompletionSource<bool>. Calling await MapBufferAsync(...) suspends the rendering task without blocking any CPU execution context. The background polling thread completes the mapping asynchronously, waking up the rendering task instantly:

private Task MapBufferAsync(IntPtr buffer, MapMode mode, nuint size)
{
    unsafe {
        var tcs = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
        var handle = GCHandle.Alloc(tcs);
        var userData = (void*)GCHandle.ToIntPtr(handle);
        _wgpuContext.Wgpu.BufferMapAsync((GpuBuffer*)buffer, mode, 0, size, s_mapCallback, userData);
        return tcs.Task;
    }
}

D. Safe Pointer-Unsafe Segregation

To comply with the C# compiler constraints that prohibit await operations inside unsafe contexts, ProGPU segregates low-level pointer copying into two dedicated synchronous unsafe helper functions:

1. CopyTextureToStagingBuffer: Encodes the offscreen render-target texture copy to the staging buffer and submits the command buffer.
2. CopyMappedToSharedTexture: Retrieves the staging buffer's mapped range, locks the native OS texture, copies raw bytes row-by-row, unlocks the texture, and unmaps the buffer.

// macOS row-by-row IOSurface memory copy
GpuSharingInterop.IOSurfaceLock(image.SharedHandle, 0, null);
void* destPtr = GpuSharingInterop.IOSurfaceGetBaseAddress(image.SharedHandle);
System.Buffer.MemoryCopy(srcRow, destRow, rowBytes, rowBytes);
GpuSharingInterop.IOSurfaceUnlock(image.SharedHandle, 0, null);

// Windows D3D11 UpdateSubresource call via COM VTable index 49
GpuSharingInterop.COMHelper.CallUpdateSubresource(context, image.WinTexture2D, 0, IntPtr.Zero, mappedPtr, image.BytesPerRow, 0);

6. Graceful Runtime Fallback

If graphics interop is not supported by the environment (e.g. software rendering, missing drivers, or Linux configurations lacking Vulkan opaque handles), the control gracefully falls back to the Decoupled Render-Thread Blitting Pipeline (Phase 2). This ensures 100% functionality and visual parity across all host configurations!