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Fixes color picker hue drift at low alpha values (#8208)

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At low apha values, premultiplied colors lose precision.
This PR makes the color picker use unmultiplied colors internally.

Before:


https://github.com/user-attachments/assets/4617a355-daa9-4911-86e6-518ac6867014

After:


https://github.com/user-attachments/assets/d9681b01-50d8-418e-b5a5-79b4bd1bbddf
This commit is contained in:
Gábor Gyebnár
2026-06-05 15:58:36 +02:00
committed by GitHub
parent db7559368d
commit 71c4ff3c33
5 changed files with 39 additions and 12 deletions
Download Patch File Download Diff File Expand all files Collapse all files

35
crates/egui/src/widgets/color_picker.rs
View File

@@ -85,7 +85,29 @@ pub fn show_color_at(painter: &Painter, color: Color32, rect: Rect) {
}
}
fn color_button(ui: &mut Ui, color: Color32, open: bool) -> Response {
/// Show a color with background checkers to demonstrate transparency (if any).
fn show_srgba_unmultiplied(ui: &mut Ui, srgba: [u8; 4], desired_size: Vec2) -> Response {
let (rect, response) = ui.allocate_at_least(desired_size, Sense::hover());
if ui.is_rect_visible(rect) {
show_srgba_unmultiplied_at(ui.painter(), srgba, rect);
}
response
}
/// Show a color with background checkers to demonstrate transparency (if any).
fn show_srgba_unmultiplied_at(painter: &Painter, [r, g, b, a]: [u8; 4], rect: Rect) {
if a == 255 {
painter.rect_filled(rect, 0.0, Color32::from_rgb(r, g, b));
} else {
background_checkers(painter, rect);
let left = Rect::from_min_max(rect.left_top(), rect.center_bottom());
let right = Rect::from_min_max(rect.center_top(), rect.right_bottom());
painter.rect_filled(left, 0.0, Color32::from_rgba_unmultiplied(r, g, b, a));
painter.rect_filled(right, 0.0, Color32::from_rgb(r, g, b));
}
}
fn color_button(ui: &mut Ui, srgba: [u8; 4], open: bool) -> Response {
let size = ui.spacing().interact_size;
let (rect, response) = ui.allocate_exact_size(size, Sense::click());
response.widget_info(|| WidgetInfo::new(WidgetType::ColorButton));
@@ -99,7 +121,7 @@ fn color_button(ui: &mut Ui, color: Color32, open: bool) -> Response {
let rect = rect.expand(visuals.expansion);
let stroke_width = 1.0;
show_color_at(ui.painter(), color, rect.shrink(stroke_width));
show_srgba_unmultiplied_at(ui.painter(), srgba, rect.shrink(stroke_width));
let corner_radius = visuals.corner_radius.at_most(2); // Can't do more rounding because the background grid doesn't do any rounding
ui.painter().rect_stroke(
@@ -314,7 +336,12 @@ fn color_picker_hsvag_2d(ui: &mut Ui, hsvag: &mut HsvaGamma, alpha: Alpha) {
}
let current_color_size = vec2(ui.spacing().slider_width, ui.spacing().interact_size.y);
show_color(ui, *hsvag, current_color_size).on_hover_text("Selected color");
show_srgba_unmultiplied(
ui,
Hsva::from(*hsvag).to_srgba_unmultiplied(),
current_color_size,
)
.on_hover_text("Selected color");
if alpha == Alpha::BlendOrAdditive {
let a = &mut hsvag.a;
@@ -491,7 +518,7 @@ pub fn color_picker_color32(ui: &mut Ui, srgba: &mut Color32, alpha: Alpha) -> b
pub fn color_edit_button_hsva(ui: &mut Ui, hsva: &mut Hsva, alpha: Alpha) -> Response {
let popup_id = ui.auto_id_with("popup");
let open = Popup::is_id_open(ui.ctx(), popup_id);
let mut button_response = color_button(ui, (*hsva).into(), open);
let mut button_response = color_button(ui, hsva.to_srgba_unmultiplied(), open);
if ui.style().explanation_tooltips {
button_response = button_response.on_hover_text("Click to edit color");
}