Marey.Backend.Skia 0.7.0

Marey.Backend.Skia

The PNG and PDF backend for Marey, a dimensionally-typed visualization system for F#. netstandard2.1.

#r "nuget: Marey.Plot"
#r "nuget: Marey.Backend.Skia"

open Marey
open Marey.UnitsOfMeasure

let figure = Chart.line U.second U.metre t x

// Bytes, or straight to a file. `toFileBytes` rather than `toFile`: writing text has an
// encoding and writing bytes does not, and a PNG that picked up a byte-order mark on the way
// to disk would not open.
let png = figure |> Chart.render SkiaBackend.renderPng (Chart.size 800.0 500.0)
figure |> Chart.toFileBytes SkiaBackend.renderPdf "fall.pdf" (Chart.size 800.0 500.0)

The first backend that measures text

Marey.Core ships an approximate text measurer — a Helvetica advance-width table, no kerning, no ligatures — because a zero-dependency core cannot do better. This backend shapes with HarfBuzz and is the only one for which Capabilities.TextShaping is true.

The consequence is worth knowing before it looks like a bug: two backends of this library place an axis title at different coordinates for the same figure. Layout is a measure–arrange pass against ITextMeasurer, so a real measurer moves the gutters. At 12 px, AVATAR is 48.01 px estimated and 44.51 px shaped in Arial, and a real face reserves about a quarter more vertical room than the estimate. Both reach the panel rectangle.

Which font, and reproducibility

SVG names a family and lets the viewer resolve it. Skia has to pick a real face from what is installed, so the same figure is a slightly different PNG on two machines. That is reported rather than hidden — SkiaBackend.ResolvedFaces names, for every run in the scene, the entry in the theme's stack that matched and the family that was opened, with Matched = None meaning the whole stack missed and Skia's default was used.

For output that must be identical everywhere, SkiaOptions.FontFile states a font file and resolution is skipped.

Font fallback

A face holds the scripts it holds, and one face is not enough for a CJK axis title against a Latin theme. The measurer splits a run by coverage — per codepoint, the first face that has it, CSS font matching's rule — shapes each piece in its own face and sums the advances. ITextMeasurer is unchanged: it still answers a width, and a run one face covers is one piece.

It reports. ResolvedFace.Fallbacks names the faces that covered what the primary could not, with the codepoints each covered; ResolvedFace.Missing lists the codepoints nothing covered, which are drawn as .notdef. Both are empty for a Latin figure.

SkiaOptions.FontFiles states the whole chain and is the reproducible answer for a mixed-script figure. FontFile states one face and does not fall back — one face is the whole promise, and a codepoint it lacks is a reported box. Setting both is an error.

PDF

Vector, despite the package being the raster one: paths stay paths and glyphs stay glyphs. The page is the requested extent converted exactly — a 400 × 250 px figure is a 300 × 187.5 pt page — and the document carries no timestamp, so two renders of one figure are byte-identical. SkiaOptions.Pdf sets the title, author, subject and keywords.

Native assets

This is the one Marey package with third-party dependencies, and they include native libraries: SkiaSharp and SkiaSharp.HarfBuzz, with the Win32, macOS and Linux asset packages. On a slim Linux container libfontconfig1 and libfreetype6 have to be installed as well. If a native library is missing, the entry points raise a message naming the package to add rather than a DllNotFoundException naming a file.

Marey.Core, Marey.Units, Marey.Plot, Marey.Backend.Svg and Marey.Backend.Tikz have no dependency on this package or on SkiaSharp, and a test asserts it against the built assembly graph.

• The guide
• Examples