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fix(viewport): use premultiplied blend on surface blit

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Geometry passes draw with standard `SrcAlpha / 1-SrcAlpha` into a
transparent MSAA target, so AA-edge fragments sit as `(rgb * a, a)` in
the resolve texture. The previous straight-alpha blit then multiplied
those by `a` a second time, darkening thin lines and curves over the
transparent background. `PREMULTIPLIED_ALPHA_BLENDING` matches what is
actually in the resolve texture and renders edges at full intensity
while still leaving fully-transparent pixels untouched on the surface.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
This commit is contained in:
Hakan Seven 2026-05-28 10:29:05 +03:00
commit dac835fe33
1 changed files with 11 additions and 6 deletions
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17
src/scene/pipeline/mod.rs
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@ -772,12 +772,17 @@ impl Pipeline {
entry_point: Some("fs_main"), entry_point: Some("fs_main"),
targets: &[Some(wgpu::ColorTargetState { targets: &[Some(wgpu::ColorTargetState {
format, format,
// Alpha-blend the resolve texture onto the surface so a // Premultiplied-alpha blend: the geometry passes already
// transparent shader clear (alpha=0 outside any drawn // wrote into a transparent MSAA target with standard
// geometry) leaves whatever the underlying widget // `SrcAlpha / 1-SrcAlpha` blending, so AA-edge fragments
// painted — model container bg in a model layout, // sit as `(rgb * a, a)` in the resolve texture. Treating
// PaperCanvas sheet in a paper layout — visible. // that as straight alpha during the surface blit would
blend: Some(wgpu::BlendState::ALPHA_BLENDING), // multiply by alpha a second time and darken thin lines
// / curves. `PREMULTIPLIED_ALPHA_BLENDING` uses `One` as
// the source colour factor and leaves the dst weighted
// by `1-SrcAlpha`, which is the correct compositing
// operator for already-premultiplied content.
blend: Some(wgpu::BlendState::PREMULTIPLIED_ALPHA_BLENDING),
write_mask: wgpu::ColorWrites::ALL, write_mask: wgpu::ColorWrites::ALL,
})], })],
compilation_options: wgpu::PipelineCompilationOptions::default(), compilation_options: wgpu::PipelineCompilationOptions::default(),