alpha-blend

Alpha blending and compositing in (optionally) zero-dependency Rust.

There are three pieces:

• A CompositeMode decides how much of the source and destination survive. These are the Porter-Duff operators.
• A BlendMode decides how their colors are mixed. These are the W3C separable blend modes.
• A Composite pairs the two. Either half can be used on its own, in which case the other takes its default.

use alpha_blend::{BlendMode, Composite, CompositeMode, RgbaBlend, rgba::U8x4Rgba};

let src = U8x4Rgba::new(255, 128, 0, 128);
let dst = U8x4Rgba::new(0, 64, 255, 255);

// Plain source-over.
let out = CompositeMode::SourceOver.apply(src, dst);

// Multiply, composited source-over.
let out = Composite::new(BlendMode::Multiply, CompositeMode::SourceOver).apply(src, dst);

Everything is generic over the Channel trait, so u8 and f32 pixels go through the same code at their own precision. Integer channels round to nearest, once per operation, from a widened intermediate: scale(255, 255) == 255, and repeated compositing does not drift darker.

Examples

cargo run --example porter-duff --features bytemuck

A blue square and a red square, each 50% transparent, overlapping in the middle.

Compositing operators

Clear	Source	Destination
SourceOver	DestinationOver	SourceIn
DestinationIn	SourceOut	DestinationOut
SourceAtop	DestinationAtop	Xor
Plus

Blend modes

Normal	Multiply	Screen
Overlay	Darken	Lighten
ColorDodge	ColorBurn	HardLight
SoftLight	Difference	Exclusion

The four non-separable modes (Hue, Saturation, Color, Luminosity) are not implemented; they operate on the whole RGB triple rather than one channel at a time.

Contributing

This project uses just to run commands the same way as the CI:

• cargo just check to check formatting and lints.
• cargo just coverage to generate and preview code coverage.
• cargo just doc to generate and preview docs.
• cargo just test to run tests.

For a full list of commands, see the Justfile.