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

* Fix gamma issue by applying tone mapping in the shader

Vello renders it's 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.

* Add comment explaining why we do this to the shader

* Rename variables
This commit is contained in:
Dennis Kobert 2025-08-05 12:08:14 +02:00 committed by GitHub
parent 2e1396462c
commit 8b4adf4330
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1 changed files with 19 additions and 7 deletions
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26
desktop/src/render/composite_shader.wgsl
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@ -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> {