Module compositor 

Multi-layer GPU compositing: FrameLayer stack in, one texture out.

Alpha convention

The canvas starts transparent (its texture is zero-initialised) and each layer is composited onto it in z_order. Two rules, which shaders/blend.wgsl and nodes::BlendModeNode implement identically:

• Colour is composited against an opaque black backdrop, so the blend result is not reweighted by the backdrop alpha the way W3C’s Cs' = (1 - ab) * Cs + ab * B(Cb, Cs) would. That matches the CPU compositor’s color=c=#000000 canvas, which ADR-0007 keeps as the correctness reference.
• Alpha accumulates as src-over coverage, ao = as + ab * (1 - as): zero where no layer has drawn (letterbox bands, the transparent regions a transform leaves behind), rising toward one as opaque layers cover it (#1750).
• RGB is premultiplied by that alpha. A white layer composited at opacity 0.5 over the empty canvas reads back (128, 128, 128, 128), not (255, 255, 255, 128). A consumer that wants straight alpha divides by ao, guarding ao == 0.

Consumers that flatten to an opaque format (the export path converts rgba to yuv420p) ignore the alpha and read the premultiplied RGB directly, which is the same thing as compositing over black; consumers that composite further, or that need to know what a layer covered, read the alpha.

Structs

Compositor

Stateful high-level multi-layer GPU compositor.

FrameLayer

A single layer in the composition stack.

LayerTransform

2D affine transform parameters for a compositor layer.