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* Add comment about avoiding NaNs to inverse alpha divisions
* Change inverse alpha computations to use max(a, epsilon) rather than +
* Emit alpha component in blit fragment shader (enables support for partially transparent windows)
This commit is contained in:
Chad Brokaw 2022-11-28 14:07:26 -05:00
parent 4fea305883
commit 923765aff9
3 changed files with 7 additions and 6 deletions
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5
piet-wgsl/shader/fine.wgsl
View file

@ -282,8 +282,9 @@ fn main(
let coords = xy_uint + vec2(i, 0u);
if coords.x < config.target_width && coords.y < config.target_height {
let fg = rgba[i];
let a_inv = 1.0 / (fg.a + 1e-6);
let rgba_sep = vec4(fg.r * a_inv, fg.g * a_inv, fg.b * a_inv, fg.a);
// Max with a small epsilon to avoid NaNs
let a_inv = 1.0 / max(fg.a, 1e-6);
let rgba_sep = vec4(fg.rgb * a_inv, fg.a);
textureStore(output, vec2<i32>(coords), rgba_sep);
}
}

6
piet-wgsl/shader/shared/blend.wgsl
View file

@ -318,10 +318,10 @@ fn blend_mix_compose(backdrop: vec4<f32>, src: vec4<f32>, mode: u32) -> vec4<f32
// Both normal+src_over blend and clip case
return backdrop * (1.0 - src.a) + src;
}
// Un-premultiply colors for blending
let inv_src_a = 1.0 / (src.a + EPSILON);
// Un-premultiply colors for blending. Max with a small epsilon to avoid NaNs.
let inv_src_a = 1.0 / max(src.a, EPSILON);
var cs = src.rgb * inv_src_a;
let inv_backdrop_a = 1.0 / (backdrop.a + EPSILON);
let inv_backdrop_a = 1.0 / max(backdrop.a, EPSILON);
let cb = backdrop.rgb * inv_backdrop_a;
let mix_mode = mode >> 8u;
let mixed = blend_mix(cb, cs, mix_mode);

2
piet-wgsl/src/lib.rs
View file

@ -205,7 +205,7 @@ impl BlitPipeline {
@fragment
fn fs_main(@builtin(position) pos: vec4<f32>) -> @location(0) vec4<f32> {
let rgba_sep = textureLoad(fine_output, vec2<i32>(pos.xy), 0);
return vec4(rgba_sep.rgb * rgba_sep.a, 1.0);
return vec4(rgba_sep.rgb * rgba_sep.a, rgba_sep.a);
}
"#;