WIP: Oklab color picker

egui/src/memory.rs

@@ -1,12 +1,8 @@
 use std::collections::{HashMap, HashSet};
 
 use crate::{
-    area, collapsing_header, menu,
-    paint::color::{Color32, Hsva},
-    resize, scroll_area,
-    util::Cache,
-    widgets::text_edit,
-    window, Id, LayerId, Pos2, Rect, Style,
+    area, collapsing_header, menu, paint::color::Color32, resize, scroll_area, util::Cache,
+    widgets::text_edit, window, Id, LayerId, Pos2, Rect, Style,
 };
 
 // ----------------------------------------------------------------------------
@@ -47,7 +43,7 @@ pub struct Memory {
 
     /// Used by color picker
     #[cfg_attr(feature = "persistence", serde(skip))]
-    pub(crate) color_cache: Cache<Color32, Hsva>,
+    pub(crate) color_cache: Cache<Color32, crate::color::Lcha>,
 
     /// Which popup-window is open (if any)?
     /// Could be a combo box, color picker, menu etc.

egui/src/ui.rs

@@ -799,6 +799,16 @@ impl Ui {
         color_picker::color_edit_button_srgba(self, srgba, color_picker::Alpha::BlendOrAdditive)
     }
 
+    /// Shows a button with the given color.
+    /// If the user clicks the button, a full color picker is shown.
+    pub fn color_edit_button_lcha(&mut self, lcha: &mut Lcha) -> Response {
+        widgets::color_picker::color_edit_button_lcha(
+            self,
+            lcha,
+            color_picker::Alpha::BlendOrAdditive,
+        )
+    }
+
     /// Shows a button with the given color.
     /// If the user clicks the button, a full color picker is shown.
     pub fn color_edit_button_hsva(&mut self, hsva: &mut Hsva) -> Response {
@@ -842,10 +852,10 @@ impl Ui {
     /// The given color is in `sRGBA` space without premultiplied alpha.
     /// If unsure, what "premultiplied alpha" is, then this is probably the function you want to use.
     pub fn color_edit_button_srgba_unmultiplied(&mut self, srgba: &mut [u8; 4]) -> Response {
-        let mut hsva = Hsva::from_srgba_unmultiplied(*srgba);
+        let mut lcha = Lcha::from_srgba_unmultiplied(*srgba);
         let response =
-            color_picker::color_edit_button_hsva(self, &mut hsva, color_picker::Alpha::OnlyBlend);
-        *srgba = hsva.to_srgba_unmultiplied();
+            color_picker::color_edit_button_lcha(self, &mut lcha, color_picker::Alpha::OnlyBlend);
+        *srgba = lcha.to_srgba_unmultiplied();
         response
     }
 
@@ -853,13 +863,13 @@ impl Ui {
     /// If the user clicks the button, a full color picker is shown.
     /// The given color is in linear RGBA space with premultiplied alpha
     pub fn color_edit_button_rgba_premultiplied(&mut self, rgba: &mut [f32; 4]) -> Response {
-        let mut hsva = Hsva::from_rgba_premultiplied(*rgba);
-        let response = color_picker::color_edit_button_hsva(
+        let mut lcha = Lcha::from_rgba_premultiplied(*rgba);
+        let response = color_picker::color_edit_button_lcha(
             self,
-            &mut hsva,
+            &mut lcha,
             color_picker::Alpha::BlendOrAdditive,
         );
-        *rgba = hsva.to_rgba_premultiplied();
+        *rgba = lcha.to_rgba_premultiplied();
         response
     }
 
@@ -868,10 +878,10 @@ impl Ui {
     /// The given color is in linear RGBA space without premultiplied alpha.
     /// If unsure, what "premultiplied alpha" is, then this is probably the function you want to use.
     pub fn color_edit_button_rgba_unmultiplied(&mut self, rgba: &mut [f32; 4]) -> Response {
-        let mut hsva = Hsva::from_rgba_unmultiplied(*rgba);
+        let mut lcha = Lcha::from_rgba_unmultiplied(*rgba);
         let response =
-            color_picker::color_edit_button_hsva(self, &mut hsva, color_picker::Alpha::OnlyBlend);
-        *rgba = hsva.to_rgba_unmultiplied();
+            color_picker::color_edit_button_lcha(self, &mut lcha, color_picker::Alpha::OnlyBlend);
+        *rgba = lcha.to_rgba_unmultiplied();
         response
     }
 }

egui/src/widgets/color_picker.rs

@@ -13,10 +13,19 @@ fn contrast_color(color: impl Into<Rgba>) -> Color32 {
     }
 }
 
+fn out_of_gamut_as_transparent(rgba: Rgba) -> Color32 {
+    let (r, g, b) = (rgba.r(), rgba.g(), rgba.b());
+    if r < 0.0 || g < 0.0 || b < 0.0 || r > 1.0 || g > 1.0 || b > 1.0 {
+        Color32::TRANSPARENT // out-of-gamut
+    } else {
+        Color32::from(rgba)
+    }
+}
+
 /// Number of vertices per dimension in the color sliders.
 /// We need at least 6 for hues, and more for smooth 2D areas.
 /// Should always be a multiple of 6 to hit the peak hues in HSV/HSL (every 60°).
-const N: u32 = 6 * 6;
+const N: u32 = 6 * 32;
 
 fn background_checkers(painter: &Painter, rect: Rect) {
     let rect = rect.shrink(0.5); // Small hack to avoid the checkers from peeking through the sides
@@ -85,7 +94,7 @@ fn color_button(ui: &mut Ui, color: Color32) -> Response {
     response
 }
 
-fn color_slider_1d(ui: &mut Ui, value: &mut f32, color_at: impl Fn(f32) -> Color32) -> Response {
+fn color_slider_1d(ui: &mut Ui, value: &mut f32, color_at: impl Fn(f32) -> Rgba) -> Response {
     #![allow(clippy::identity_op)]
 
     let desired_size = vec2(
@@ -107,7 +116,7 @@ fn color_slider_1d(ui: &mut Ui, value: &mut f32, color_at: impl Fn(f32) -> Color
         let mut mesh = Mesh::default();
         for i in 0..=N {
             let t = i as f32 / (N as f32);
-            let color = color_at(t);
+            let color = out_of_gamut_as_transparent(color_at(t));
             let x = lerp(rect.left()..=rect.right(), t);
             mesh.colored_vertex(pos2(x, rect.top()), color);
             mesh.colored_vertex(pos2(x, rect.bottom()), color);
@@ -144,7 +153,7 @@ fn color_slider_2d(
     ui: &mut Ui,
     x_value: &mut f32,
     y_value: &mut f32,
-    color_at: impl Fn(f32, f32) -> Color32,
+    color_at: impl Fn(f32, f32) -> Rgba,
 ) -> Response {
     let desired_size = Vec2::splat(ui.style().spacing.slider_width);
     let (rect, response) = ui.allocate_at_least(desired_size, Sense::click_and_drag());
@@ -161,7 +170,8 @@ fn color_slider_2d(
         for yi in 0..=N {
             let xt = xi as f32 / (N as f32);
             let yt = yi as f32 / (N as f32);
-            let color = color_at(xt, yt);
+            let color = out_of_gamut_as_transparent(color_at(xt, yt));
+
             let x = lerp(rect.left()..=rect.right(), xt);
             let y = lerp(rect.bottom()..=rect.top(), yt);
             mesh.colored_vertex(pos2(x, y), color);
@@ -186,7 +196,7 @@ fn color_slider_2d(
     ui.painter().add(Shape::Circle {
         center: pos2(x, y),
         radius: rect.width() / 12.0,
-        fill: picked_color,
+        fill: picked_color.into(),
         stroke: Stroke::new(visuals.fg_stroke.width, contrast_color(picked_color)),
     });
 
@@ -345,25 +355,149 @@ pub fn color_edit_button_hsva(ui: &mut Ui, hsva: &mut Hsva, alpha: Alpha) -> Res
     button_response
 }
 
+fn color_picker_lcha_2d(ui: &mut Ui, lcha: &mut Lcha, alpha: Alpha) {
+    let current_color_size = vec2(
+        ui.style().spacing.slider_width,
+        ui.style().spacing.interact_size.y * 2.0,
+    );
+
+    ui.horizontal(|ui| {
+        ui.vertical(|ui| {
+            let opaque = Lcha { a: 1.0, ..*lcha };
+
+            if alpha == Alpha::Opaque {
+                lcha.a = 1.0;
+                show_color(ui, *lcha, current_color_size).on_hover_text("Current color");
+            } else {
+                let a = &mut lcha.a;
+
+                // We signal additive blending by storing a negative alpha (a bit ironic).
+                let mut additive = *a < 0.0;
+
+                if alpha == Alpha::OnlyBlend {
+                    if additive {
+                        *a = 0.5;
+                    }
+
+                    color_slider_1d(ui, a, |a| Lcha { a, ..opaque }.into()).on_hover_text("Alpha");
+                } else {
+                    ui.horizontal(|ui| {
+                        ui.label("Blending:");
+                        ui.radio_value(&mut additive, false, "Normal");
+                        ui.radio_value(&mut additive, true, "Additive");
+
+                        if additive {
+                            *a = -a.abs();
+                        }
+
+                        if !additive {
+                            *a = a.abs();
+                        }
+                    });
+
+                    if !additive {
+                        color_slider_1d(ui, a, |a| Lcha { a, ..opaque }.into())
+                            .on_hover_text("Alpha");
+                    }
+                }
+
+                show_color(ui, *lcha, current_color_size).on_hover_text("Current color");
+                show_color(ui, opaque, current_color_size).on_hover_text("Current color (opaque)");
+            }
+
+            let opaque = Lcha { a: 1.0, ..*lcha };
+            let Lcha { l, c, h, a: _ } = lcha;
+
+            color_slider_1d(ui, h, |h| Lcha { h, ..opaque }.into()).on_hover_text("Hue");
+            color_slider_1d(ui, c, |c| Lcha { c, ..opaque }.into()).on_hover_text("Chroma");
+            color_slider_1d(ui, l, |l| Lcha { l, ..opaque }.into()).on_hover_text("Lightness");
+            color_slider_2d(ui, l, c, |l, c| Lcha { l, c, ..opaque }.into())
+                .on_hover_text("Lightness - Chroma");
+        });
+
+        ui.vertical(|ui| {
+            let opaque = Lcha { a: 1.0, ..*lcha };
+            let Lcha { l, c, h, a: _ } = lcha;
+            color_slider_2d(ui, h, l, |h, l| Lcha { h, l, ..opaque }.into())
+                .on_hover_text("Hue - Lightness");
+            color_slider_2d(ui, h, c, |h, c| Lcha { h, c, ..opaque }.into())
+                .on_hover_text("Hue - Chroma");
+        });
+    });
+}
+
+pub fn color_edit_button_lcha(ui: &mut Ui, lcha: &mut Lcha, alpha: Alpha) -> Response {
+    let pupup_id = ui.auto_id_with("popup");
+    let button_response = color_button(ui, (*lcha).into()).on_hover_text("Click to edit color");
+
+    if button_response.clicked() {
+        ui.memory().toggle_popup(pupup_id);
+    }
+    // TODO: make it easier to show a temporary popup that closes when you click outside it
+    if ui.memory().is_popup_open(pupup_id) {
+        let area_response = Area::new(pupup_id)
+            .order(Order::Foreground)
+            .default_pos(button_response.rect.max)
+            .show(ui.ctx(), |ui| {
+                Frame::popup(ui.style()).show(ui, |ui| {
+                    ui.style_mut().spacing.slider_width = 256.0;
+                    color_picker_lcha_2d(ui, lcha, alpha);
+                })
+            });
+
+        if !button_response.clicked() {
+            let clicked_outside = ui.input().pointer.any_click() && !area_response.hovered;
+            if clicked_outside || ui.input().key_pressed(Key::Escape) {
+                ui.memory().close_popup();
+            }
+        }
+    }
+
+    button_response
+}
+
+// /// Shows a button with the given color.
+// /// If the user clicks the button, a full color picker is shown.
+// pub fn color_edit_button_srgba(ui: &mut Ui, srgba: &mut Color32) -> Response {
+//     // To ensure we keep hue slider when `srgba` is grey we store the
+//     // full `Hsva` in a cache:
+
+//     let mut hsva = ui
+//         .ctx()
+//         .memory()
+//         .color_cache
+//         .get(srgba)
+//         .cloned()
+//         .unwrap_or_else(|| Hsva::from(*srgba));
+
+//     let response = color_edit_button_hsva(ui, &mut hsva);
+
+//     *srgba = Color32::from(hsva);
+
+//     ui.ctx().memory().color_cache.set(*srgba, hsva);
+
+//     response
+// }
+
 /// Shows a button with the given color.
 /// If the user clicks the button, a full color picker is shown.
 pub fn color_edit_button_srgba(ui: &mut Ui, srgba: &mut Color32, alpha: Alpha) -> Response {
     // To ensure we keep hue slider when `srgba` is grey we store the
-    // full `Hsva` in a cache:
+    // full `Lcha` in a cache:
 
-    let mut hsva = ui
+    let mut lcha = ui
         .ctx()
         .memory()
         .color_cache
         .get(srgba)
         .cloned()
-        .unwrap_or_else(|| Hsva::from(*srgba));
+        .unwrap_or_else(|| Lcha::from(*srgba));
 
-    let response = color_edit_button_hsva(ui, &mut hsva, alpha);
+    let response = color_edit_button_lcha(ui, &mut lcha, alpha);
 
-    *srgba = Color32::from(hsva);
+    *srgba = Color32::from(lcha);
 
-    ui.ctx().memory().color_cache.set(*srgba, hsva);
+    ui.ctx().memory().color_cache.set(*srgba, lcha);
 
     response
 }

egui_demo_lib/src/apps/demo/demo_window.rs

@@ -100,6 +100,7 @@ struct ColorWidgets {
     srgba_premul: [u8; 4],
     rgba_unmul: [f32; 4],
     rgba_premul: [f32; 4],
+    lcha: egui::color::Lcha,
 }
 
 impl Default for ColorWidgets {
@@ -110,6 +111,7 @@ impl Default for ColorWidgets {
             srgba_premul: [0, 187, 140, 127],
             rgba_unmul: [0.0, 1.0, 0.5, 0.5],
             rgba_premul: [0.0, 0.5, 0.25, 0.5],
+            lcha: (Rgba::from(egui::Color32::BLUE) * 0.5).into(),
         }
     }
 }
@@ -125,8 +127,14 @@ impl ColorWidgets {
             srgba_premul,
             rgba_unmul,
             rgba_premul,
+            lcha,
         } = self;
 
+        ui.horizontal(|ui| {
+            ui.color_edit_button_lcha(lcha);
+            ui.label(format!("LCHA: {} {} {} {}", lcha.l, lcha.c, lcha.h, lcha.a));
+        });
+
         ui.horizontal(|ui| {
             ui.color_edit_button_srgba_unmultiplied(srgba_unmul);
             ui.label(format!(

epaint/src/color.rs

@@ -228,7 +228,8 @@ impl Rgba {
 
     /// How perceptually intense (bright) is the color?
     pub fn intensity(&self) -> f32 {
-        0.3 * self.r() + 0.59 * self.g() + 0.11 * self.b()
+        // 0.3 * self.r() + 0.59 * self.g() + 0.11 * self.b()
+        Lcha::from_rgb([self.r(), self.g(), self.b()]).l
     }
 
     /// Returns an opaque version of self
@@ -382,6 +383,19 @@ impl Hsva {
         Self { h, s, v, a }
     }
 
+    pub fn from_rgb(rgb: [f32; 3]) -> Self {
+        let (h, s, v) = hsv_from_rgb(rgb);
+        Hsva { h, s, v, a: 1.0 }
+    }
+
+    pub fn from_srgb([r, g, b]: [u8; 3]) -> Self {
+        Self::from_rgb([
+            linear_from_gamma_byte(r),
+            linear_from_gamma_byte(g),
+            linear_from_gamma_byte(b),
+        ])
+    }
+
     /// From `sRGBA` with premultiplied alpha
     pub fn from_srgba_premultiplied(srgba: [u8; 4]) -> Self {
         Self::from_rgba_premultiplied([
@@ -434,19 +448,6 @@ impl Hsva {
         }
     }
 
-    pub fn from_rgb(rgb: [f32; 3]) -> Self {
-        let (h, s, v) = hsv_from_rgb(rgb);
-        Hsva { h, s, v, a: 1.0 }
-    }
-
-    pub fn from_srgb([r, g, b]: [u8; 3]) -> Self {
-        Self::from_rgb([
-            linear_from_gamma_byte(r),
-            linear_from_gamma_byte(g),
-            linear_from_gamma_byte(b),
-        ])
-    }
-
     // ------------------------------------------------------------------------
 
     pub fn to_opaque(self) -> Self {
@@ -585,3 +586,314 @@ fn test_hsv_roundtrip() {
         }
     }
 }
+
+// ----------------------------------------------------------------------------
+
+// /// A simple perceptual color space.
+// ///
+// /// https://bottosson.github.io/posts/oklab/
+// #[derive(Clone, Copy, Debug, Default, PartialEq)]
+// struct Oklab {
+//     /// Perceived lightness (0-1)
+//     pub l: f32,
+//     /// How green/red the color is ([-1, 1])
+//     pub a: f32,
+//     /// How blue/yellow the color is ([-1, 1])
+//     pub b: f32,
+// }
+
+// impl Oklab {
+//     pub fn from_linear_rgb(r: f32, g: f32, b: f32) -> Oklab {
+//         let (l, a, b) = lab_from_rgb([r, g, b]);
+//         Oklab { l, a, b }
+//     }
+
+//     pub fn to_linear_rgb(self) -> [f32; 3] {
+//         rgb_from_lab((self.l, self.a, self.b))
+//     }
+// }
+
+// /// Polar form of [`Oklab`], all coordinated in 0-1 range.
+// #[derive(Clone, Copy, Debug, Default, PartialEq)]
+// struct Oklch {
+//     /// Perceived lightness in [0, 1] range.
+//     pub l: f32,
+//     /// Chroma in [0, 1] range.
+//     pub c: f32,
+//     /// Hue in [0, 1] range.
+//     pub h: f32,
+// }
+
+// impl From<Oklab> for Oklch {
+//     fn from(i: Oklab) -> Oklch {
+//         use std::f32::consts::TAU;
+//         Oklch {
+//             l: i.l,
+//             c: i.a.hypot(i.b),
+//             h: (i.b.atan2(i.a) + TAU) % TAU / TAU,
+//         }
+//     }
+// }
+
+// impl From<Oklch> for Oklab {
+//     fn from(i: Oklch) -> Oklab {
+//         use std::f32::consts::TAU;
+//         let (sin_h, cos_h) = (i.h * TAU).sin_cos();
+//         Oklab {
+//             l: i.l,
+//             a: i.c * cos_h,
+//             b: i.c * sin_h,
+//         }
+//     }
+// }
+
+// impl From<Oklab> for Color32 {
+//     fn from(i: Oklab) -> Color32 {
+//         let [r, g, b] = i.to_linear_rgb();
+//         Rgba::from_rgb(r, g, b).into()
+//     }
+// }
+
+// impl From<Oklch> for Color32 {
+//     fn from(i: Oklch) -> Color32 {
+//         Oklab::from(i).into()
+//     }
+// }
+
+// #[test]
+// // #[ignore] // a bit expensive
+// fn test_oklab_roundtrip() {
+//     for r in 0..=255 {
+//         for g in 0..=255 {
+//             for b in 0..=255 {
+//                 let srgba = Color32::from_rgb(r, g, b);
+//                 let rgba = Rgba::from(srgba);
+//                 let oklab = Oklab::from_linear_rgb(rgba.r(), rgba.g(), rgba.b());
+//                 assert_eq!(srgba, Color32::from(oklab));
+//                 let oklch = Oklch::from(oklab);
+//                 assert_eq!(srgba, Color32::from(oklch),);
+//             }
+//         }
+//     }
+// }
+
+// ----------------------------------------------------------------------------
+
+/// oklab from linear rgb
+fn lab_from_rgb([r, g, b]: [f32; 3]) -> (f32, f32, f32) {
+    let x = 0.4121656120 * r + 0.5362752080 * g + 0.0514575653 * b;
+    let y = 0.2118591070 * r + 0.6807189584 * g + 0.1074065790 * b;
+    let z = 0.0883097947 * r + 0.2818474174 * g + 0.6302613616 * b;
+
+    let x = x.cbrt();
+    let y = y.cbrt();
+    let z = z.cbrt();
+
+    (
+        0.2104542553 * x + 0.7936177850 * y - 0.0040720468 * z,
+        1.9779984951 * x - 2.4285922050 * y + 0.4505937099 * z,
+        0.0259040371 * x + 0.7827717662 * y - 0.8086757660 * z,
+    )
+}
+
+/// linear rgb from oklab
+pub fn rgb_from_lab((l, a, b): (f32, f32, f32)) -> [f32; 3] {
+    let x = l + 0.3963377774 * a + 0.2158037573 * b;
+    let y = l - 0.1055613458 * a - 0.0638541728 * b;
+    let z = l - 0.0894841775 * a - 1.2914855480 * b;
+
+    let x = x.powi(3);
+    let y = y.powi(3);
+    let z = z.powi(3);
+
+    [
+        4.0767245293 * x - 3.3072168827 * y + 0.2307590544 * z,
+        -1.2681437731 * x + 2.6093323231 * y - 0.3411344290 * z,
+        -0.0041119885 * x - 0.7034763098 * y + 1.7068625689 * z,
+    ]
+}
+
+/// 0-1 normalized lch from oklab.
+fn lch_from_lab((l, a, b): (f32, f32, f32)) -> (f32, f32, f32) {
+    use std::f32::consts::TAU;
+    let c = a.hypot(b);
+    let h = (b.atan2(a) + TAU) % TAU / TAU;
+    (l, c, h)
+}
+
+/// Oklab from 0-1 normalized lch.
+fn lab_from_lch((l, c, h): (f32, f32, f32)) -> (f32, f32, f32) {
+    use std::f32::consts::TAU;
+    let (sin_h, cos_h) = (h * TAU).sin_cos();
+    let a = c * cos_h;
+    let b = c * sin_h;
+    (l, a, b)
+}
+
+/// 0-1 normalized lch from linear rgb
+fn lch_from_rgb(rgb: [f32; 3]) -> (f32, f32, f32) {
+    lch_from_lab(lab_from_rgb(rgb))
+}
+/// linear rgb from 0-1 normalized lch
+fn rgb_from_lch(lch: (f32, f32, f32)) -> [f32; 3] {
+    rgb_from_lab(lab_from_lch(lch))
+}
+
+/// Lightness, chroma, hue and alpha. All in the range [0, 1].
+/// No premultiplied alpha.
+/// Based on the the perceptual color space Oklab (https://bottosson.github.io/posts/oklab/).
+#[derive(Clone, Copy, Debug, Default, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+pub struct Lcha {
+    /// Perceived lightness in [0, 1] range.
+    pub l: f32,
+    /// Chroma in [0, 1] range.
+    pub c: f32,
+    /// Hue in [0, 1] range.
+    pub h: f32,
+    /// Alpha in [0, 1] range. A negative value signifies an additive color (and alpha is ignored).
+    pub a: f32,
+}
+
+impl Lcha {
+    pub fn new(l: f32, c: f32, h: f32, a: f32) -> Self {
+        Self { l, c, h, a }
+    }
+
+    /// From linear RGB.
+    pub fn from_rgb(rgb: [f32; 3]) -> Self {
+        let (l, c, h) = lch_from_rgb(rgb);
+        Lcha { l, c, h, a: 1.0 }
+    }
+
+    /// From `sRGBA` with premultiplied alpha
+    pub fn from_srgba_premultiplied(srgba: [u8; 4]) -> Self {
+        Self::from_rgba_premultiplied([
+            linear_from_gamma_byte(srgba[0]),
+            linear_from_gamma_byte(srgba[1]),
+            linear_from_gamma_byte(srgba[2]),
+            linear_from_alpha_byte(srgba[3]),
+        ])
+    }
+
+    /// From `sRGBA` without premultiplied alpha
+    pub fn from_srgba_unmultiplied(srgba: [u8; 4]) -> Self {
+        Self::from_rgba_unmultiplied([
+            linear_from_gamma_byte(srgba[0]),
+            linear_from_gamma_byte(srgba[1]),
+            linear_from_gamma_byte(srgba[2]),
+            linear_from_alpha_byte(srgba[3]),
+        ])
+    }
+
+    /// From linear RGBA with premultiplied alpha
+    pub fn from_rgba_premultiplied(rgba: [f32; 4]) -> Self {
+        #![allow(clippy::many_single_char_names)]
+        let [r, g, b, a] = rgba;
+        if a == 0.0 {
+            if r == 0.0 && b == 0.0 && a == 0.0 {
+                Lcha::default()
+            } else {
+                Lcha::from_additive_rgb([r, g, b])
+            }
+        } else {
+            let (l, c, h) = lch_from_rgb([r / a, g / a, b / a]);
+            Lcha { l, c, h, a }
+        }
+    }
+
+    /// From linear RGBA without premultiplied alpha
+    pub fn from_rgba_unmultiplied(rgba: [f32; 4]) -> Self {
+        #![allow(clippy::many_single_char_names)]
+        let [r, g, b, a] = rgba;
+        let (l, c, h) = lch_from_rgb([r, g, b]);
+        Lcha { l, c, h, a }
+    }
+
+    pub fn from_additive_rgb(rgb: [f32; 3]) -> Self {
+        let (l, c, h) = lch_from_rgb(rgb);
+        Lcha {
+            l,
+            c,
+            h,
+            a: -0.5, // anything negative is treated as additive
+        }
+    }
+    // ------------------------------------------------------------------------
+
+    pub fn to_rgb(&self) -> [f32; 3] {
+        rgb_from_lch((self.l, self.c, self.h))
+    }
+
+    pub fn to_rgba_premultiplied(&self) -> [f32; 4] {
+        let [r, g, b, a] = self.to_rgba_unmultiplied();
+        let additive = a < 0.0;
+        if additive {
+            [r, g, b, 0.0]
+        } else {
+            [a * r, a * g, a * b, a]
+        }
+    }
+
+    pub fn to_rgba_unmultiplied(&self) -> [f32; 4] {
+        let Lcha { l, c, h, a } = *self;
+        let [r, g, b] = rgb_from_lch((l, c, h));
+        [r, g, b, a]
+    }
+
+    pub fn to_srgba_premultiplied(&self) -> [u8; 4] {
+        let [r, g, b, a] = self.to_rgba_premultiplied();
+        [
+            gamma_byte_from_linear(r),
+            gamma_byte_from_linear(g),
+            gamma_byte_from_linear(b),
+            alpha_byte_from_linear(a),
+        ]
+    }
+
+    pub fn to_srgba_unmultiplied(&self) -> [u8; 4] {
+        let [r, g, b, a] = self.to_rgba_unmultiplied();
+        [
+            gamma_byte_from_linear(r),
+            gamma_byte_from_linear(g),
+            gamma_byte_from_linear(b),
+            alpha_byte_from_linear(a.abs()),
+        ]
+    }
+}
+
+impl From<Lcha> for Rgba {
+    fn from(hsva: Lcha) -> Rgba {
+        Rgba(hsva.to_rgba_premultiplied())
+    }
+}
+impl From<Rgba> for Lcha {
+    fn from(rgba: Rgba) -> Lcha {
+        Self::from_rgba_premultiplied(rgba.0)
+    }
+}
+
+impl From<Lcha> for Color32 {
+    fn from(hsva: Lcha) -> Color32 {
+        Color32::from(Rgba::from(hsva))
+    }
+}
+impl From<Color32> for Lcha {
+    fn from(srgba: Color32) -> Lcha {
+        Lcha::from(Rgba::from(srgba))
+    }
+}
+
+#[test]
+// #[ignore] // a bit expensive
+fn test_lcha_roundtrip() {
+    for r in 0..=255 {
+        for g in 0..=255 {
+            for b in 0..=255 {
+                let srgba = Color32::from_rgb(r, g, b);
+                let lcha = Lcha::from(srgba);
+                assert_eq!(srgba, Color32::from(lcha),);
+            }
+        }
+    }
+}