Sdcb.SimdCodecs.Gif 1.2.0

Sdcb.SimdCodecs.Gif

Pure-managed GIF (87a / 89a) decoding and encoding for .NET 10, derived from the GIF89a specification: global and local colour tables, interlacing, transparency, every disposal method and the NETSCAPE2.0 loop count. A frame is the whole canvas as a viewer shows it, every earlier frame composited in.

using Sdcb.SimdCodecs.Gif;

byte[] data = File.ReadAllBytes("image.gif");
GifInfo info = GifDecoder.ReadInfo(data);          // canvas, frame count, alpha, loop count
GifImage first = GifDecoder.Decode(data);          // the first frame, RGBA

Every frame, by index or in order, through the Sdcb.SimdCodecs frame reader:

using Sdcb.SimdCodecs;
using Sdcb.SimdCodecs.Abstractions;

if (SimdCodecs.TryOpenFrames(data, out IImageFrameReader? reader))
using (reader)
{
    var buffer = new byte[info.Width * info.Height * 4];
    foreach (FrameTiming frame in reader.EnumerateFramesInto(buffer, 0, ImagePixelFormat.Bgra32))
        Show(buffer, frame.Duration);
}

Encoding

byte[] gif = GifEncoder.Encode(rgba, width, height, channels: 4);

byte[] animation = GifEncoder.EncodeAnimation(
    [new GifAnimationFrame(frame0, TimeSpan.FromMilliseconds(100)), new GifAnimationFrame(frame1, TimeSpan.FromMilliseconds(100))],
    width, height, channels: 4, new GifEncodeOptions { LoopCount = 0, Dither = true });

GIF holds at most 256 colours and 1-bit transparency, so the encoder:

• treats alpha below 128 as transparent, the rest as opaque;
• uses one global palette for every frame: exact (lossless) when the opaque colours of all frames fit in 256, or 255 beside a transparent index; otherwise built by Wu's variance-minimising quantizer, with optional Floyd–Steinberg dithering (Dither);
• writes an opaque animation's frames as the rectangle that changed, unchanged pixels as the transparent index; an animation with transparency writes whole frames, each cleared before the next;
• takes LoopCount as plays (0 forever, 1 writes no loop extension), the same meaning the frame reader reports.

Input is 1 (grey), 3 (RGB) or 4 (RGBA) channels of 8 bits. The quantizer's quality and the encoder's speed are not tuned yet: a benchmark and quality gate come separately.

Decisions

The specification leaves these to the decoder; they are made as browsers make them.

Canvases and images above 2^26 pixels are refused, to bound what a decode allocates.

LINQPad samples

The package ships two runnable samples in linqpad-samples/:

• decode: reads a real animated GIF, displays its metadata and first frame, then composites frames one at a time into a reused RGBA buffer.
• encode: creates a small RGBA animation, compares palette quantization with and without dithering, and decodes the result back for display.