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Skia generated raster blender

Status: promoted arbitrary raster blend surface complete for SKIA-120–124

SkiaGeneratedBlender compiles one generic SkRuntimeEffect::MakeForBlender program per process. Six integer uniforms select the independent RGB/alpha source factor, destination factor and function, while two float4 uniforms carry the blend constant and output write mask. A state change instantiates only a small uniform block and SkBlender; it does not generate source text or grow an unbounded cache.

Selector semantics

  • All 13 Blend ordinals have a closed branch: One, Zero, source/inverse source colour, source/inverse source alpha, destination/inverse destination colour, destination/inverse destination alpha, constant/inverse constant, and SourceAlphaSaturation.
  • SourceAlphaSaturation is min(sourceAlpha, 1-destinationAlpha) for RGB and One for alpha, matching the EasyGL/OpenGL factor.
  • Add, Subtract and ReverseSubtract operate on independently weighted source/destination terms and clamp to the normalized target range.
  • EasyGL maps Max and Min to OpenGL GL_MAX/GL_MIN, for which source/destination factors are ignored. The generated route matches that actual renderer behavior rather than applying the stale weighted wording formerly present in the planning matrix.
  • The destination supplied by Skia is premultiplied surface storage. The generator recovers logical RGB, evaluates the EasyGL formula, then re-encodes the result for SkSurface. Generated routes use SKIA-119's Premultiplied label so source/shader components are preserved and every factor is applied explicitly.

Invalid factor/function ordinals and non-finite or out-of-range constant channels return null with a deterministic field-specific error before blender construction. A pinned-Skia compilation failure is cached with its diagnostic, so repeated calls cannot trigger a compile storm.

Public state integration

SkiaRenderer::ApplyBlendState keeps the five established preset/runtime mappings on their pixel-proven routes. Every other valid six-selector tuple constructs the generic blender with the current GraphicsDevice.BlendFactor. SetBlendFactor rebuilds that fixed uniform block transactionally, so a live A→B→A change cannot reuse stale constants or grow a selector cache.

The write-mask uniform is applied after the independent RGB/alpha equation and SkSurface encoding. Each disabled bit selects the original premultiplied destination component, including the no-write mask. Disabling blending selects source replacement; a partial mask uses the established Opaque masked route, and applying another state while disabled refreshes that mask. Re-enabling restores the configured generated blender. The one-target raster policy still rejects non-default masks for targets 1–3 and every non-default MultiSampleMask before state mutation.

Skia_GeneratedBlender_Raster is display-free. Its independent C++ scalar oracle exercises all 13 factor branches in RGB and alpha positions, all five functions independently in RGB and alpha, constant/inverse constant, SourceAlphaSaturation, reverse subtract and a mixed separate-alpha tuple. It also proves one compilation attempt and deterministic validation failures.

Skia_GeneratedBlendState_Policy is the public SKIA-121 boundary: baked and live constants, all 16 target-0 masks, disabled replacement and re-enable restoration have exact opaque pixels.

Skia_GeneratedBlend_BatchModes closes SKIA-122. The common paint path produces one equivalent translucent pixel from Texture2D components, an equivalent premultiplied RenderTarget2D snapshot, and an identity explicit SkSL effect in Deferred, Immediate, Texture, BackToFront, and FrontToBack. Texture mode sorts two distinct sources, and stock Opaque remains exact after successful effect use or malformed-effect rejection.

ClassifySkiaBlendSelectors is used by both ApplyBlendState and the exhaustive display-free audit. It assigns all 714,025 valid tuples to five established routes or 714,020 generic routes; out-of-range raw values are Invalid. Skia_GeneratedBlend_PublicCorpus then renders the minimized 62-scene public set against an independent EasyGL/OpenGL scalar oracle, covering every factor in all four positions and every function in both equations.

Promoted boundary

SKIA-124 promotes exactly that selector surface for 2D raster draws: every tuple formed from the 13 public Blend values and five public BlendFunction values is accepted, including independent RGB/alpha equations and live blend constants. All 16 target-0 colour-write masks compose with those tuples. The five established mappings retain their specialized routes; the remaining 714,020 tuples use the generated blender.

This is not a claim for invalid raw enum values, target-1/2/3 masks, a non-default multisample mask, MRT, MSAA, arbitrary EasyGL GLSL, or 3D/cube/volume sampling. Those inputs still refuse before drawing. The release audit pins this distinction to the startup diagnostic, parity ledger, and feature matrix so a future edit cannot silently widen or narrow the advertised surface.