Skip to content

skia-source-alpha-contract.md

Latest commit

 

History

History
47 lines (37 loc) · 3.44 KB

skia-source-alpha-contract.md

File metadata and controls

47 lines (37 loc) · 3.44 KB
Raw

Skia source-alpha contract

Status: implemented guardrail for SKIA-119

XNA BlendState contains factors and functions, not a flag saying whether a caller's texture bytes are straight or already premultiplied. Skia, however, evaluates image shaders and blenders in premultiplied working colour. The renderer therefore declares the conversion for every accepted route; it never infers one from the numeric relationship between RGB and alpha.

Storage-to-working-colour table

Source Owned/public storage Requested route Bytes received by the Skia blender
Texture2D Canonical top-row-first RGBA bytes; GetData is exact. Premultiplied Preserve RGBA components. Invalid-premultiplied values such as RGB greater than alpha are intentionally not “fixed.”
Texture2D Same canonical bytes. Straight Multiply RGB by source alpha exactly once; preserve alpha.
RenderTarget2D Premultiplied SkSurface; public readback unpremultiplies. Either Reuse the one premultiplied snapshot. Relabelling it as straight would multiply alpha twice.
Tagged CNA_SKIA_SKSL_V1 effect Premultiplied child values and reflected uniforms. Explicit ABI main returns one premultiplied colour. The effect cannot silently change the following blender's source contract.

SkiaSourceStorageAlpha, ResolveSkiaWorkingSourceRoute, and each image renderer's StorageAlphaEXT() encode this table. Texture2D::SnapshotImage() selects its two labelled views; RenderTarget2D::SnapshotImage() proves both requested routes resolve to its existing surface.

Tint is part of the same conversion. For a premultiplied-labelled source, tint RGB and alpha remain independent (source.rgb * tint.rgb, source.a * tint.a). For a straight-labelled texture, image evaluation has already supplied the source-alpha RGB factor, so MakeSkiaTintScale() folds tint alpha into working RGB exactly once. The same four scale values are passed to the custom-effect cnaTint uniform.

Accepted blend routes

Mapping Source label Working interpretation
Opaque Premultiplied Preserve the source vector; replace destination.
AlphaBlend Premultiplied Preserve source RGB and apply inverse source alpha to destination.
NonPremultiplied Straight Texture evaluation supplies the RGB source-alpha factor; the runtime blender computes independent output alpha.
Additive Straight Texture evaluation supplies the RGB source-alpha factor; the runtime blender adds destination and computes independent alpha.
DestinationColorPrototype Premultiplied Preserve source components for the one opaque pixel-proven custom tuple.
Generated Premultiplied Preserve Texture2D/effect working components, reuse target surface components, evaluate every selected factor explicitly, then encode the logical result for SkSurface.

The five optimized entries carry SkiaSourceAlphaConvention in kSkiaBlendMappings; the generic route declares kSkiaGeneratedBlendSourceAlphaConvention. SKIA-120's scalar oracle and SKIA-122's all-mode Texture2D/RenderTarget2D/tagged-effect pixels prove that boundary. Invalid raw selectors still reject rather than guessing a nearby preset.

Skia_SourceAlpha_Policy is display-free. It reads raw premultiplied Skia bytes for discriminating translucent Texture2D, RenderTarget2D and custom-effect sources, verifies tint scales, and audits that every accepted mapping names exactly one convention.