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Fix semi-transparent colors appearing too bright (#5824)

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The bug was in `Color32::from_rgba_unmultiplied` and by extension
affects:

* `Color32::from_rgba_unmultiplied`
* `hex_color!`
* `HexColor`
* `ColorImage::from_rgba_unmultiplied`
* All images with transparency (png, webp, …)
* `Color32::from_white_alpha`

The bug caused translucent colors to appear too bright.

## More
Color is hard.

When I started out egui I thought "linear space is objectively better,
for everything!" and then I've been slowly walking that back for various
reasons:

* sRGB textures not available everywhere
* gamma-space is more _perceptually_ even, so it makes sense to use for
anti-aliasing
* other applications do everything in gamma space, so that's what people
expect (this PR)

Similarly, pre-multiplied alpha _makes sense_ for blending colors. It
also enables additive colors, which is nice. But it does complicate
things. Especially when mixed with sRGB/gamma (As @karhu [points
out](https://github.com/emilk/egui/pull/5824#issuecomment-2738099254)).

## Related
* Closes https://github.com/emilk/egui/issues/5751
* Closes https://github.com/emilk/egui/issues/5771 ? (probably; hard to
tell without a repro)
* But not https://github.com/emilk/egui/issues/5810

## TODO
* [x] I broke the RGBA u8 color picker. Fix it

---------

Co-authored-by: Andreas Reich <andreas@rerun.io>
This commit is contained in:
Emil Ernerfeldt
2025-03-21 10:45:25 +01:00
committed by GitHub
parent d54e29d375
commit 3f731ec794
19 changed files with 399 additions and 103 deletions
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33
crates/egui_demo_lib/src/rendering_test.rs
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@@ -92,8 +92,6 @@ impl ColorTest {
ui.label("Test that vertex color times texture color is done in gamma space:");
ui.scope(|ui| {
ui.spacing_mut().item_spacing.y = 0.0; // No spacing between gradients
let tex_color = Color32::from_rgb(64, 128, 255);
let vertex_color = Color32::from_rgb(128, 196, 196);
let ground_truth = mul_color_gamma(tex_color, vertex_color);
@@ -106,6 +104,9 @@ impl ColorTest {
show_color(ui, vertex_color, color_size);
ui.label(" vertex color =");
});
ui.spacing_mut().item_spacing.y = 0.0; // No spacing between gradients
{
let g = Gradient::one_color(ground_truth);
self.vertex_gradient(ui, "Ground truth (vertices)", WHITE, &g);
@@ -129,6 +130,34 @@ impl ColorTest {
ui.separator();
ui.label("Test that blending is done in gamma space:");
ui.scope(|ui| {
let background = Color32::from_rgb(200, 60, 10);
let foreground = Color32::from_rgba_unmultiplied(108, 65, 200, 82);
let ground_truth = background.blend(foreground);
ui.horizontal(|ui| {
let color_size = ui.spacing().interact_size;
ui.label("Background:");
show_color(ui, background, color_size);
ui.label(", foreground: ");
show_color(ui, foreground, color_size);
});
ui.spacing_mut().item_spacing.y = 0.0; // No spacing between gradients
{
let g = Gradient::one_color(ground_truth);
self.vertex_gradient(ui, "Ground truth (vertices)", WHITE, &g);
self.tex_gradient(ui, "Ground truth (texture)", WHITE, &g);
}
{
let g = Gradient::one_color(foreground);
self.vertex_gradient(ui, "Vertex blending", background, &g);
self.tex_gradient(ui, "Texture blending", background, &g);
}
});
ui.separator();
// TODO(emilk): test color multiplication (image tint),
// to make sure vertex and texture color multiplication is done in linear space.

4
crates/egui_demo_lib/tests/snapshots/demos/Bézier Curve.png
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