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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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60
crates/ecolor/src/lib.rs
View File

@@ -1,9 +1,20 @@
//! Color conversions and types.
//!
//! This crate is built for the wants and needs of [`egui`](https://github.com/emilk/egui/).
//!
//! If you want an actual _good_ color crate, use [`color`](https://crates.io/crates/color) instead.
//!
//! If you want a compact color representation, use [`Color32`].
//! If you want to manipulate RGBA colors use [`Rgba`].
//! If you want to manipulate RGBA colors in linear space use [`Rgba`].
//! If you want to manipulate colors in a way closer to how humans think about colors, use [`HsvaGamma`].
//!
//! ## Conventions
//! The word "gamma" or "srgb" is used to refer to values in the non-linear space defined by
//! [the sRGB transfer function](https://en.wikipedia.org/wiki/SRGB).
//! We use `u8` for anything in the "gamma" space.
//!
//! We use `f32` in 0-1 range for anything in the linear space.
//!
//! ## Feature flags
#![cfg_attr(feature = "document-features", doc = document_features::document_features!())]
//!
@@ -39,23 +50,46 @@ pub use hex_color_runtime::*;
impl From<Color32> for Rgba {
fn from(srgba: Color32) -> Self {
Self([
linear_f32_from_gamma_u8(srgba.0[0]),
linear_f32_from_gamma_u8(srgba.0[1]),
linear_f32_from_gamma_u8(srgba.0[2]),
linear_f32_from_linear_u8(srgba.0[3]),
])
let [r, g, b, a] = srgba.to_array();
if a == 0 {
// Additive, or completely transparent
Self([
linear_f32_from_gamma_u8(r),
linear_f32_from_gamma_u8(g),
linear_f32_from_gamma_u8(b),
0.0,
])
} else {
let a = linear_f32_from_linear_u8(a);
Self([
linear_from_gamma(r as f32 / (255.0 * a)) * a,
linear_from_gamma(g as f32 / (255.0 * a)) * a,
linear_from_gamma(b as f32 / (255.0 * a)) * a,
a,
])
}
}
}
impl From<Rgba> for Color32 {
fn from(rgba: Rgba) -> Self {
Self([
gamma_u8_from_linear_f32(rgba.0[0]),
gamma_u8_from_linear_f32(rgba.0[1]),
gamma_u8_from_linear_f32(rgba.0[2]),
linear_u8_from_linear_f32(rgba.0[3]),
])
let [r, g, b, a] = rgba.to_array();
if a == 0.0 {
// Additive, or completely transparent
Self([
gamma_u8_from_linear_f32(r),
gamma_u8_from_linear_f32(g),
gamma_u8_from_linear_f32(b),
0,
])
} else {
Self([
fast_round(gamma_u8_from_linear_f32(r / a) as f32 * a),
fast_round(gamma_u8_from_linear_f32(g / a) as f32 * a),
fast_round(gamma_u8_from_linear_f32(b / a) as f32 * a),
linear_u8_from_linear_f32(a),
])
}
}
}