Writing a Custom Rasterizer Backend

KernSmith's rasterizer system is pluggable. Any library that implements two interfaces — IRasterizer and IRasterizerCapabilities — and registers with RasterizerFactory works as a backend. This guide shows you how.

Quick Start

The fastest way to get started is to copy the example project:

cp -r samples/KernSmith.Rasterizer.Example MyRasterizer

This gives you a skeleton project with all required interfaces stubbed out. See the example's README for a step-by-step customization guide.

The Plugin Contract

Your backend implements two interfaces from KernSmith.Rasterizer:

IRasterizer

Required members (must implement):

Member	Purpose
`Capabilities`	Returns your `IRasterizerCapabilities` instance
`LoadFont(ReadOnlyMemory<byte>, int)`	Loads a font from raw file bytes
`RasterizeGlyph(int, RasterOptions)`	Renders one glyph to a bitmap. Return `null` for missing glyphs
`RasterizeAll(IEnumerable<int>, RasterOptions)`	Renders multiple glyphs, skipping missing ones
`Dispose()`	Releases native resources

Optional members (have default implementations — override only if your engine provides native support):

Member	Default	Purpose
`LoadSystemFont(string)`	Throws `NotSupportedException`	Load a system font by family name
`GetGlyphMetrics(int, RasterOptions)`	Returns `null`	Get metrics without rasterizing
`GetFontMetrics(RasterOptions)`	Returns `null`	Provide native font-wide metrics (ascent, descent, line height)
`GetKerningPairs(RasterOptions)`	Returns `null`	Provide pre-scaled kerning pairs
`SetVariationAxes(IReadOnlyList<VariationAxis>, Dictionary<string, float>)`	No-op	Apply variable font axis values
`SelectColorPalette(int)`	No-op	Select a CPAL color palette

When optional methods return null, KernSmith falls back to its built-in TTF table parsers.

IRasterizerCapabilities

Required members:

Member	Type	Purpose
`SupportsColorFonts`	`bool`	Can render COLR/CPAL/sbix/CBDT color glyphs
`SupportsVariableFonts`	`bool`	Can apply fvar axis coordinates
`SupportsSdf`	`bool`	Can produce signed distance field output
`SupportsOutlineStroke`	`bool`	Can stroke glyph outlines
`SupportedAntiAliasModes`	`IReadOnlyList<AntiAliasMode>`	Which AA modes are supported (`None`, `Grayscale`, `Light`, `Lcd`)

Optional members (default to false):

Member	Purpose
`HandlesOwnSizing`	Set to `true` if your engine handles point-to-pixel conversion internally
`SupportsSystemFonts`	Set to `true` if you implement `LoadSystemFont()`

RasterizedGlyph Output

Each rasterized glyph must provide all 8 properties:

new RasterizedGlyph
{
    Codepoint = codepoint,
    GlyphIndex = glyphIndex,       // Internal font glyph index
    BitmapData = pixelBytes,       // Grayscale (1 bpp) or RGBA (4 bpp)
    Width = bitmapWidth,
    Height = bitmapHeight,
    Pitch = bytesPerRow,           // May include row padding
    Metrics = new GlyphMetrics(bearingX, bearingY, advance, width, height),
    Format = PixelFormat.Grayscale8  // or PixelFormat.Rgba32
};

Registration

Backends register with RasterizerFactory via a [ModuleInitializer]. This runs automatically when your assembly loads — no manual wiring needed.

Since RasterizerBackend is an enum in the core assembly, third-party backends use a numeric cast. Pick a value of 100 or higher to avoid collisions with built-in backends (FreeType=0, Gdi=1, DirectWrite=2):

using System.Runtime.CompilerServices;
using KernSmith;
using KernSmith.Rasterizer;

namespace KernSmith.Rasterizers.MyRasterizer;

internal static class MyRegistration
{
#pragma warning disable CA2255
    [ModuleInitializer]
    internal static void Register()
    {
        RasterizerFactory.Register((RasterizerBackend)100, () => new MyRasterizer());
    }
#pragma warning restore CA2255
}

Built-in backends are auto-discovered by RasterizerFactory on first access. For third-party backends, the [ModuleInitializer] fires when your assembly loads. If your assembly isn't loaded automatically (e.g., nothing in the app references a type from it), add a direct type reference or call RuntimeHelpers.RunModuleConstructor():

// Only needed if .NET doesn't load your assembly automatically
System.Runtime.CompilerServices.RuntimeHelpers.RunModuleConstructor(
    typeof(KernSmith.Rasterizers.MyRasterizer.MyRasterizer).Module.ModuleHandle);

Packaging

Structure your NuGet package like this:

Package ID: KernSmith.Rasterizers.YourName
Dependency: KernSmith (the core library)
TFM: net10.0 for cross-platform, or net10.0-windows / net10.0-linux if platform-specific

No special metapackage or framework reference is needed.

Testing

Validate your backend against FreeType reference output:

Load a test font (e.g., Roboto-Regular.ttf) with both FreeType and your backend
Rasterize the same character set at the same size
Compare glyph metrics (advance, bearings) — small rounding differences (±1px) are normal
Compare bitmap output visually or via pixel diff

Use RasterizerFactory.IsRegistered() to verify your module initializer fires correctly:

// Force assembly load (needed for third-party backends not in the auto-discovery list)
RuntimeHelpers.RunModuleConstructor(typeof(MyRasterizer).Module.ModuleHandle);

// Verify registration
Assert.True(RasterizerFactory.IsRegistered((RasterizerBackend)100));

// Create and use
using var rasterizer = RasterizerFactory.Create((RasterizerBackend)100);
Assert.NotNull(rasterizer.Capabilities);

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