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

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)
         })
     }