Improve text redering and do all color operation in gamma space (#2071)

crates/epaint/CHANGELOG.md

@@ -4,6 +4,7 @@ All notable changes to the epaint crate will be documented in this file.
 
 ## Unreleased
 * ⚠️ BREAKING: Fix text being too small ([#2069](https://github.com/emilk/egui/pull/2069)).
+* ⚠️ BREAKING: epaint now expects integrations to do all color blending in gamma space ([#2071](https://github.com/emilk/egui/pull/2071)).
 
 
 ## 0.19.0 - 2022-08-20

crates/epaint/src/color.rs

@@ -326,7 +326,7 @@ impl Rgba {
     /// Transparent white
     #[inline(always)]
     pub fn from_white_alpha(a: f32) -> Self {
-        crate::epaint_assert!(0.0 <= a && a <= 1.0);
+        crate::epaint_assert!(0.0 <= a && a <= 1.0, "a: {}", a);
         Self([a, a, a, a])
     }
 

crates/epaint/src/image.rs

@@ -195,17 +195,19 @@ impl FontImage {
 
     /// Returns the textures as `sRGBA` premultiplied pixels, row by row, top to bottom.
     ///
-    /// `gamma` should normally be set to 1.0.
-    /// If you are having problems with text looking skinny and pixelated, try
-    /// setting a lower gamma, e.g. `0.5`.
-    pub fn srgba_pixels(&'_ self, gamma: f32) -> impl ExactSizeIterator<Item = Color32> + '_ {
+    /// `gamma` should normally be set to `None`.
+    ///
+    /// If you are having problems with text looking skinny and pixelated, try using a low gamma, e.g. `0.4`.
+    pub fn srgba_pixels(
+        &'_ self,
+        gamma: Option<f32>,
+    ) -> impl ExactSizeIterator<Item = Color32> + '_ {
+        let gamma = gamma.unwrap_or(0.55); // TODO(emilk): this default coverage gamma is a magic constant, chosen by eye. I don't even know why we need it.
         self.pixels.iter().map(move |coverage| {
-            // This is arbitrarily chosen to make text look as good as possible.
-            // In particular, it looks good with gamma=1 and the default eframe backend,
-            // which uses linear blending.
-            // See https://github.com/emilk/egui/issues/1410
-            let a = fast_round(coverage.powf(gamma / 2.2) * 255.0);
-            Color32::from_rgba_premultiplied(a, a, a, a) // this makes no sense, but works
+            let alpha = coverage.powf(gamma);
+            // We want to multiply with `vec4(alpha)` in the fragment shader:
+            let a = fast_round(alpha * 255.0);
+            Color32::from_rgba_premultiplied(a, a, a, a)
         })
     }
 

crates/epaint/src/shadow.rs

@@ -14,7 +14,7 @@ pub struct Shadow {
 }
 
 impl Shadow {
-    /// Tooltips, menus, …
+    /// Tooltips, menus, …, for dark mode.
     pub fn small_dark() -> Self {
         Self {
             extrusion: 16.0,
@@ -22,15 +22,15 @@ impl Shadow {
         }
     }
 
-    /// Tooltips, menus, …
+    /// Tooltips, menus, …, for light mode.
     pub fn small_light() -> Self {
         Self {
             extrusion: 16.0,
-            color: Color32::from_black_alpha(32),
+            color: Color32::from_black_alpha(20),
         }
     }
 
-    /// Subtle and nice on dark backgrounds
+    /// Used for widnows in dark mode.
     pub fn big_dark() -> Self {
         Self {
             extrusion: 32.0,
@@ -38,11 +38,11 @@ impl Shadow {
         }
     }
 
-    /// Subtle and nice on white backgrounds
+    /// Used for widnows in light mode.
     pub fn big_light() -> Self {
         Self {
             extrusion: 32.0,
-            color: Color32::from_black_alpha(40),
+            color: Color32::from_black_alpha(16),
         }
     }