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Fix translucent image colors by correcting alpha premultiplication in image decoding and GPU texture transfers #4517

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Fix translucent image colors by correcting alpha premultiplication in image decoding and GPU texture transfers #4517

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4 changes: 2 additions & 2 deletions node-graph/libraries/no-std-types/src/color/color_types.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -360,8 +360,8 @@ impl Pixel for Color {

fn from_bytes(bytes: &[u8]) -> Self {
// `Image<Color>` pixel convention is linear-light with associated (premultiplied) alpha.
let srgba = SRGBA8::new(bytes[0], bytes[1], bytes[2], bytes[3]);
Color::from(srgba).apply_opacity(bytes[3] as f32 / 255.)
let color = Color::from(SRGBA8::new(bytes[0], bytes[1], bytes[2], bytes[3]));

@cubic-dev-ai cubic-dev-ai Bot Sep 11, 2026

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P3: This change duplicates the SRGBA8-to-premultiplied-Color conversion already used by image-data decoding and GPU readback. Extract the conversion into a shared helper and call it from all paths so future color or alpha fixes cannot leave these implementations inconsistent.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At node-graph/libraries/no-std-types/src/color/color_types.rs, line 363:

<comment>This change duplicates the SRGBA8-to-premultiplied-`Color` conversion already used by image-data decoding and GPU readback. Extract the conversion into a shared helper and call it from all paths so future color or alpha fixes cannot leave these implementations inconsistent.</comment>

<file context>
@@ -360,8 +360,8 @@ impl Pixel for Color {
 		// `Image<Color>` pixel convention is linear-light with associated (premultiplied) alpha.
-		let srgba = SRGBA8::new(bytes[0], bytes[1], bytes[2], bytes[3]);
-		Color::from(srgba).apply_opacity(bytes[3] as f32 / 255.)
+		let color = Color::from(SRGBA8::new(bytes[0], bytes[1], bytes[2], bytes[3]));
+		color.map_rgb(|channel| channel * color.a())
 	}
</file context>

color.map_rgb(|channel| channel * color.a())
}
fn byte_size() -> usize {
4
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4 changes: 2 additions & 2 deletions node-graph/libraries/raster-types/src/image.rs
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Expand Up @@ -148,8 +148,8 @@ impl Image<Color> {
.chunks_exact(4)
.map(|v| {
// `Image<Color>` pixels are stored linear-light with premultiplied alpha
let srgba = SRGBA8::new(v[0], v[1], v[2], v[3]);
Color::from(srgba).apply_opacity(v[3] as f32 / 255.)
let color = Color::from(SRGBA8::new(v[0], v[1], v[2], v[3]));
color.map_rgb(|channel| channel * color.a())

@cubic-dev-ai cubic-dev-ai Bot Sep 11, 2026

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P3: This premultiplication fix changes stored alpha semantics (no longer alpha-squared) and is the kind of subtle color-space regression that can silently reappear. Add a round-trip test that runs from_image_data on a known semi-transparent RGBA pixel and asserts the stored Color has alpha == v[3]/255 with premultiplied linear RGB (e.g. white at alpha 128 → rgb ≈ 0.502, alpha ≈ 0.502), and that to_png()/to_flat_u8 recovers the original u8 values.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At node-graph/libraries/raster-types/src/image.rs, line 152:

<comment>This premultiplication fix changes stored alpha semantics (no longer alpha-squared) and is the kind of subtle color-space regression that can silently reappear. Add a round-trip test that runs from_image_data on a known semi-transparent RGBA pixel and asserts the stored Color has alpha == v[3]/255 with premultiplied linear RGB (e.g. white at alpha 128 → rgb ≈ 0.502, alpha ≈ 0.502), and that to_png()/to_flat_u8 recovers the original u8 values.</comment>

<file context>
@@ -148,8 +148,8 @@ impl Image<Color> {
-				let srgba = SRGBA8::new(v[0], v[1], v[2], v[3]);
-				Color::from(srgba).apply_opacity(v[3] as f32 / 255.)
+				let color = Color::from(SRGBA8::new(v[0], v[1], v[2], v[3]));
+				color.map_rgb(|channel| channel * color.a())
 			})
 			.collect();
</file context>

})
.collect();
Image {
Expand Down
10 changes: 5 additions & 5 deletions node-graph/libraries/wgpu-executor/src/texture_conversion.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,14 +8,14 @@ use raster_types::Image;
use raster_types::{CPU, GPU, Raster, Texture};
use wgpu::{Extent3d, TextureFormat};

/// Uploads CPU image data to a GPU texture
/// Uploads CPU image data to a GPU texture as gamma sRGB with unassociated alpha, the convention for all GPU raster textures.
fn upload_to_texture(executor: &WgpuExecutor, queue: &wgpu::Queue, image: &Raster<CPU>) -> Texture {
let rgba8_data: Vec<SRGBA8> = image.data.iter().map(|x| (*x).into()).collect();
let rgba8_data = image.to_flat_u8().0;

let texture = executor.request_texture_with_format(glam::UVec2::new(image.width, image.height), TextureFormat::Rgba8UnormSrgb);
queue.write_texture(
texture.as_image_copy(),
bytemuck::cast_slice(rgba8_data.as_slice()),
&rgba8_data,
wgpu::TexelCopyBufferLayout {
offset: 0,
bytes_per_row: Some(4 * image.width),
Expand Down Expand Up @@ -116,8 +116,8 @@ impl RasterGpuToRasterCpuConverter {
let row_slice = &view[start..start + row_bytes];
for px in row_slice.chunks_exact(4) {
// `Image<Color>` pixels are stored linear-light with associated (premultiplied) alpha
let srgba = SRGBA8::new(px[0], px[1], px[2], px[3]);
cpu_data.push(Color::from(srgba).apply_opacity(px[3] as f32 / 255.));
let color = Color::from(SRGBA8::new(px[0], px[1], px[2], px[3]));
cpu_data.push(color.map_rgb(|channel| channel * color.a()));
}
}

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8 changes: 6 additions & 2 deletions node-graph/node-macro/src/shader_nodes/per_pixel_adjust.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -149,7 +149,11 @@ impl PerPixelAdjustCodegen<'_> {
.iter()
.map(|Param { ident, param_type, item_wrapped, .. }| {
let bare_value = match param_type {
ParamType::Image { .. } => quote!(Color::from_vec4(#ident.fetch_with(texel_coord, lod(0)))),
// Textures hold unassociated alpha but node functions take premultiplied `Color`
ParamType::Image { .. } => quote!({
let texel = Color::from_vec4(#ident.fetch_with(texel_coord, lod(0)));
texel.map_rgb(|channel| channel * texel.a())
}),
ParamType::Uniform => quote!(uniform.#ident),
};
if *item_wrapped { quote!(Item::new_from_element(#bare_value)) } else { bare_value }
Expand Down Expand Up @@ -184,7 +188,7 @@ impl PerPixelAdjustCodegen<'_> {
let uniform = <Uniform as #gcore_shaders::shaders::buffer_struct::BufferStruct>::read(*uniform);
let texel_coord = frag_coord.xy().as_uvec2();
let color: Color = #fn_name(#context, #(#call_args),*)#unwrap_result;
*color_out = color.to_vec4();
*color_out = color.to_unassociated_alpha().to_vec4();
}
}
})
Expand Down
6 changes: 3 additions & 3 deletions node-graph/nodes/gstd/src/platform_application_io.rs
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Expand Up @@ -174,9 +174,9 @@ fn decode_image(_: impl Ctx, data: Item<Resource>) -> Item<Raster<CPU>> {
data: image
.chunks(4)
.map(|pixel| {
// Decoded bytes are unassociated gamma sRGB; premultiply in gamma then lift to linear
let a = pixel[3];
Color::from_gamma_srgb_channels(pixel[0] * a, pixel[1] * a, pixel[2] * a, a)
// Decoded bytes are unassociated gamma sRGB, so lift to linear before premultiplying
let color = Color::from_gamma_srgb_channels(pixel[0], pixel[1], pixel[2], pixel[3]);
color.map_rgb(|channel| channel * color.a())
})
.collect(),
width: image.width(),
Expand Down
4 changes: 2 additions & 2 deletions node-graph/nodes/raster/src/std_nodes.rs
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Expand Up @@ -260,8 +260,8 @@ pub fn image<'a: 'n>(_: impl Ctx, resource: Item<Resource>) -> Item<Raster<CPU>>
data: image
.chunks(4)
.map(|pixel| {
let alpha = pixel[3];
Color::from_gamma_srgb_channels(pixel[0] * alpha, pixel[1] * alpha, pixel[2] * alpha, alpha)
let color = Color::from_gamma_srgb_channels(pixel[0], pixel[1], pixel[2], pixel[3]);
color.map_rgb(|channel| channel * color.a())
})
.collect(),
width: image.width(),
Expand Down
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