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Desktop: Fix Vello rendering colors with doubly-applied gamma by converting to linear in the shader #2998

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Merged
TrueDoctor merged 3 commits into from
Aug 5, 2025
Merged

Desktop: Fix Vello rendering colors with doubly-applied gamma by converting to linear in the shader #2998

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c459950
Fix gamma issue by applying tone mapping in the shader
TrueDoctor Aug 5, 2025
9b6d308
Add comment explaining why we do this to the shader
TrueDoctor Aug 5, 2025
163052b
Rename variables
TrueDoctor Aug 5, 2025
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26 changes: 19 additions & 7 deletions desktop/src/render/composite_shader.wgsl
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Original file line number Diff line number Diff line change
Expand Up @@ -49,17 +49,29 @@ fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
return ui;
}

let vp_cord = (in.tex_coords - constants.viewport_offset) * constants.viewport_scale;
let viewport_coordinate = (in.tex_coords - constants.viewport_offset) * constants.viewport_scale;

let ov = textureSample(t_overlays, s_diffuse, vp_cord);
let vp = textureSample(t_viewport, s_diffuse, vp_cord);
// Vello renders its values to an `RgbaUnorm` texture, but if we try to use this in the main rendering pipeline
// which renders to an `Srgb` surface, gamma mapping is applied twice. This converts back to linear to compensate.
let overlay_raw = textureSample(t_overlays, s_diffuse, viewport_coordinate);
let overlay = vec4<f32>(srgb_to_linear(overlay_raw.rgb), overlay_raw.a);
let viewport_raw = textureSample(t_viewport, s_diffuse, viewport_coordinate);
let viewport = vec4<f32>(srgb_to_linear(viewport_raw.rgb), viewport_raw.a);

if (ov.a < 0.001) {
return blend(ui, vp);
if (overlay.a < 0.001) {
return blend(ui, viewport);
}

let comp = blend(ov, vp);
return blend(ui, comp);
let composite = blend(overlay, viewport);
return blend(ui, composite);
}

fn srgb_to_linear(srgb: vec3<f32>) -> vec3<f32> {
return select(
pow((srgb + 0.055) / 1.055, vec3<f32>(2.4)),
srgb / 12.92,
srgb <= vec3<f32>(0.04045)
);
}

fn blend(fg: vec4<f32>, bg: vec4<f32>) -> vec4<f32> {
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