Do all blending in gamma space

This is an experiment to investigate solutions to
* https://github.com/emilk/egui/issues/1410

egui_glow/src/painter.rs

@@ -273,7 +273,7 @@ impl Painter {
         );
 
         if !cfg!(target_arch = "wasm32") {
-            self.gl.enable(glow::FRAMEBUFFER_SRGB);
+            self.gl.disable(glow::FRAMEBUFFER_SRGB);
             check_for_gl_error!(&self.gl, "FRAMEBUFFER_SRGB");
         }
 

egui_glow/src/shader/fragment.glsl

@@ -5,72 +5,32 @@
 uniform sampler2D u_sampler;
 
 #ifdef NEW_SHADER_INTERFACE
-    in vec4 v_rgba;
+    in vec4 v_rgba_gamma;  // 0-1 RGBA gamma
     in vec2 v_tc;
     out vec4 f_color;
     // a dirty hack applied to support webGL2
     #define gl_FragColor f_color
     #define texture2D texture
 #else
-    varying vec4 v_rgba;
+    varying vec4 v_rgba_gamma;  // 0-1 RGBA gamma
     varying vec2 v_tc;
 #endif
 
-#ifdef SRGB_SUPPORTED
-    void main() {
-        // The texture sampler is sRGB aware, and OpenGL already expects linear rgba output
-        // so no need for any sRGB conversions here:
-        gl_FragColor = v_rgba * texture2D(u_sampler, v_tc);
-    }
-#else
-    // 0-255 sRGB  from  0-1 linear
-    vec3 srgb_from_linear(vec3 rgb) {
-        bvec3 cutoff = lessThan(rgb, vec3(0.0031308));
-        vec3 lower = rgb * vec3(3294.6);
-        vec3 higher = vec3(269.025) * pow(rgb, vec3(1.0 / 2.4)) - vec3(14.025);
-        return mix(higher, lower, vec3(cutoff));
-    }
+// 0-255 sRGB  from  0-1 linear
+vec3 srgb_from_linear(vec3 rgb) {
+    bvec3 cutoff = lessThan(rgb, vec3(0.0031308));
+    vec3 lower = rgb * vec3(3294.6);
+    vec3 higher = vec3(269.025) * pow(rgb, vec3(1.0 / 2.4)) - vec3(14.025);
+    return mix(higher, lower, vec3(cutoff));
+}
 
-    vec4 srgba_from_linear(vec4 rgba) {
-        return vec4(srgb_from_linear(rgba.rgb), 255.0 * rgba.a);
-    }
+// 0-255 sRGBA  from  0-1 linear
+vec4 srgba_from_linear(vec4 rgba) {
+    return vec4(srgb_from_linear(rgba.rgb), 255.0 * rgba.a);
+}
 
-    // 0-1 linear  from  0-255 sRGB
-    vec3 linear_from_srgb(vec3 srgb) {
-        bvec3 cutoff = lessThan(srgb, vec3(10.31475));
-        vec3 lower = srgb / vec3(3294.6);
-        vec3 higher = pow((srgb + vec3(14.025)) / vec3(269.025), vec3(2.4));
-        return mix(higher, lower, vec3(cutoff));
-    }
-
-    vec4 linear_from_srgba(vec4 srgba) {
-        return vec4(linear_from_srgb(srgba.rgb), srgba.a / 255.0);
-    }
-
-    void main() {
-        // We must decode the colors, since WebGL1 doesn't come with sRGBA textures:
-        vec4 texture_rgba = linear_from_srgba(texture2D(u_sampler, v_tc) * 255.0);
-        /// Multiply vertex color with texture color (in linear space).
-        gl_FragColor = v_rgba * texture_rgba;
-
-        // WebGL1 doesn't support linear blending in the framebuffer,
-        // so we do a hack here where we change the premultiplied alpha
-        // to do the multiplication in gamma space instead:
-
-        // Unmultiply alpha:
-        if (gl_FragColor.a > 0.0) {
-            gl_FragColor.rgb /= gl_FragColor.a;
-        }
-
-        // Empiric tweak to make e.g. shadows look more like they should:
-        gl_FragColor.a *= sqrt(gl_FragColor.a);
-
-        // To gamma:
-        gl_FragColor = srgba_from_linear(gl_FragColor) / 255.0;
-
-        // Premultiply alpha, this time in gamma space:
-        if (gl_FragColor.a > 0.0) {
-            gl_FragColor.rgb *= gl_FragColor.a;
-        }
-    }
-#endif
+void main() {
+    vec4 tex_color_linear = texture2D(u_sampler, v_tc);
+    vec4 tex_color_gamma = srgba_from_linear(tex_color_linear) / 255.0;
+    gl_FragColor = v_rgba_gamma * tex_color_gamma;
+}

egui_glow/src/shader/vertex.glsl

@@ -14,30 +14,17 @@
 
 uniform vec2 u_screen_size;
 I vec2 a_pos;
-I vec4 a_srgba; // 0-255 sRGB
+I vec4 a_srgba; // 0-255 sRGBA gamma
 I vec2 a_tc;
-O vec4 v_rgba;
+O vec4 v_rgba_gamma; // 0-1 RGBA gamma
 O vec2 v_tc;
 
-// 0-1 linear  from  0-255 sRGB
-vec3 linear_from_srgb(vec3 srgb) {
-    bvec3 cutoff = lessThan(srgb, vec3(10.31475));
-    vec3 lower = srgb / vec3(3294.6);
-    vec3 higher = pow((srgb + vec3(14.025)) / vec3(269.025), vec3(2.4));
-    return mix(higher, lower, V(cutoff));
-}
-
-vec4 linear_from_srgba(vec4 srgba) {
-    return vec4(linear_from_srgb(srgba.rgb), srgba.a / 255.0);
-}
-
 void main() {
     gl_Position = vec4(
                       2.0 * a_pos.x / u_screen_size.x - 1.0,
                       1.0 - 2.0 * a_pos.y / u_screen_size.y,
                       0.0,
                       1.0);
-    // egui encodes vertex colors in gamma space, so we must decode the colors here:
-    v_rgba = linear_from_srgba(a_srgba);
+    v_rgba_gamma = a_srgba / 255.0;
     v_tc = a_tc;
 }

epaint/src/color.rs

@@ -311,22 +311,22 @@ impl Rgba {
     }
 
     pub fn from_luminance_alpha(l: f32, a: f32) -> Self {
-        crate::epaint_assert!(0.0 <= l && l <= 1.0);
-        crate::epaint_assert!(0.0 <= a && a <= 1.0);
+        crate::epaint_assert!(0.0 <= l && l <= 1.0, "l: {}", l);
+        crate::epaint_assert!(0.0 <= a && a <= 1.0, "a: {}", a);
         Self([l * a, l * a, l * a, a])
     }
 
     /// Transparent black
     #[inline(always)]
     pub fn from_black_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([0.0, 0.0, 0.0, a])
     }
 
     /// 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])
     }
 

epaint/src/image.rs

@@ -197,13 +197,10 @@ impl FontImage {
     /// 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> + '_ {
-        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
+        self.pixels.iter().map(move |&coverage| {
+            Color32::from(crate::Rgba::from_white_alpha(
+                coverage.clamp(0.0, 1.0).powf(gamma),
+            ))
         })
     }
 
@@ -252,10 +249,6 @@ impl From<FontImage> for ImageData {
     }
 }
 
-fn fast_round(r: f32) -> u8 {
-    (r + 0.5).floor() as _ // rust does a saturating cast since 1.45
-}
-
 // ----------------------------------------------------------------------------
 
 /// A change to an image.