Fix alpha blending in WebGL2 backend (#650)

Add a render-to-texture step with an sRGBA8 texture

egui_web/src/shader/fragment_100es.glsl

@@ -30,7 +30,7 @@ vec4 linear_from_srgba(vec4 srgba) {
 }
 
 void main() {
-  // We must decode the colors, since WebGL doesn't come with sRGBA textures:
+  // 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).

egui_web/src/shader/fragment_300es.glsl

@@ -1,45 +0,0 @@
-precision mediump float;
-uniform sampler2D u_sampler;
-varying vec4 v_rgba;
-varying vec2 v_tc;
-
-// 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 sRGBA  from  0-1 linear
-vec4 srgba_from_linear(vec4 rgba) {
-  return vec4(srgb_from_linear(rgba.rgb), 255.0 * rgba.a);
-}
-
-void main() {
-  // The texture is set up with `SRGB8_ALPHA8`, so no need to decode here!
-  vec4 texture_rgba = texture2D(u_sampler, v_tc);
-
-  /// Multiply vertex color with texture color (in linear space).
-  gl_FragColor = v_rgba * texture_rgba;
-
-  // WebGL 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;
-  }
-}

egui_web/src/shader/main_fragment_300es.glsl

@@ -0,0 +1,13 @@
+precision mediump float;
+uniform sampler2D u_sampler;
+varying vec4 v_rgba;
+varying vec2 v_tc;
+
+void main() {
+  // The texture is set up with `SRGB8_ALPHA8`, so no need to decode here!
+  vec4 texture_rgba = texture2D(u_sampler, v_tc);
+
+  // Multiply vertex color with texture color (in linear space).
+  // Linear color is written and blended in Framebuffer and converted to sRGB later
+  gl_FragColor = v_rgba * texture_rgba;
+}

egui_web/src/shader/post_fragment_300es.glsl

@@ -0,0 +1,22 @@
+precision mediump float;
+uniform sampler2D u_sampler;
+varying vec2 v_tc;
+
+// 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 sRGBA  from  0-1 linear
+vec4 srgba_from_linear(vec4 rgba) {
+    return vec4(srgb_from_linear(rgba.rgb), 255.0 * rgba.a);
+}
+
+void main() {
+    gl_FragColor = texture2D(u_sampler, v_tc);
+
+    gl_FragColor = srgba_from_linear(gl_FragColor) / 255.;
+}

egui_web/src/shader/post_vertex_300es.glsl

@@ -0,0 +1,8 @@
+precision mediump float;
+attribute vec2 a_pos;
+varying vec2 v_tc;
+
+void main() {
+    gl_Position = vec4(a_pos * 2. - 1., 0.0, 1.0);
+    v_tc = a_pos;
+}