Add `Color32::lerp_to_gamma` (#4627)

Add `lerp_to_gamma` utility function to `Color32`

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

Cargo.lock

@@ -1165,6 +1165,7 @@ dependencies = [
  "cint",
  "color-hex",
  "document-features",
+ "emath",
  "serde",
 ]
 

crates/ecolor/Cargo.toml

@@ -30,6 +30,8 @@ default = []
 
 
 [dependencies]
+emath.workspace = true
+
 #! ### Optional dependencies
 
 ## [`bytemuck`](https://docs.rs/bytemuck) enables you to cast `ecolor` types to `&[u8]`.

crates/ecolor/src/color32.rs

@@ -1,4 +1,6 @@
-use crate::{gamma_u8_from_linear_f32, linear_f32_from_gamma_u8, linear_f32_from_linear_u8, Rgba};
+use crate::{
+    fast_round, gamma_u8_from_linear_f32, linear_f32_from_gamma_u8, linear_f32_from_linear_u8, Rgba,
+};
 
 /// This format is used for space-efficient color representation (32 bits).
 ///
@@ -235,4 +237,16 @@ impl Color32 {
             a as f32 / 255.0,
         ]
     }
+
+    /// Lerp this color towards `other` by `t` in gamma space.
+    pub fn lerp_to_gamma(&self, other: Self, t: f32) -> Self {
+        use emath::lerp;
+
+        Self::from_rgba_premultiplied(
+            fast_round(lerp((self[0] as f32)..=(other[0] as f32), t)),
+            fast_round(lerp((self[1] as f32)..=(other[1] as f32), t)),
+            fast_round(lerp((self[2] as f32)..=(other[2] as f32), t)),
+            fast_round(lerp((self[3] as f32)..=(other[3] as f32), t)),
+        )
+    }
 }

crates/egui_demo_lib/src/rendering_test.rs

@@ -342,7 +342,7 @@ impl Gradient {
             (0..=n)
                 .map(|i| {
                     let t = i as f32 / n as f32;
-                    lerp_color_gamma(left, right, t)
+                    left.lerp_to_gamma(right, t)
                 })
                 .collect(),
         )
@@ -634,12 +634,3 @@ fn mul_color_gamma(left: Color32, right: Color32) -> Color32 {
         (left.a() as f32 * right.a() as f32 / 255.0).round() as u8,
     )
 }
-
-fn lerp_color_gamma(left: Color32, right: Color32, t: f32) -> Color32 {
-    Color32::from_rgba_premultiplied(
-        lerp((left[0] as f32)..=(right[0] as f32), t).round() as u8,
-        lerp((left[1] as f32)..=(right[1] as f32), t).round() as u8,
-        lerp((left[2] as f32)..=(right[2] as f32), t).round() as u8,
-        lerp((left[3] as f32)..=(right[3] as f32), t).round() as u8,
-    )
-}