[gtk+] gsk: Add more modes to the blend shader

[Emmanuele Bassi <ebassi src gnome org>]

commit a840c7f00c1f152f7d9f1b73d3fd061187c8ea5d
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Sun Jul 3 23:02:56 2016 +0100

    gsk: Add more modes to the blend shader
    
    Use the compositing CSS spec at:
    
      https://www.w3.org/TR/compositing-1/#blending
    
    For the implementation.

 gsk/resources/glsl/blend.fs.glsl |   52 +++++++++++++++++++++++++++++++++----
 1 files changed, 46 insertions(+), 6 deletions(-)
---
diff --git a/gsk/resources/glsl/blend.fs.glsl b/gsk/resources/glsl/blend.fs.glsl
index 5d2f873..d632e58 100644
--- a/gsk/resources/glsl/blend.fs.glsl
+++ b/gsk/resources/glsl/blend.fs.glsl
@@ -1,22 +1,62 @@
-vec3 BlendMultiply(vec3 Csrc, vec3 Cdst) {
-  return Csrc * Cdst;
+vec3 BlendMultiply(vec3 Cb, vec3 Cs) {
+  return Cb * Cs;
+}
+
+vec3 BlendScreen(vec3 Cb, vec3 Cs) {
+  return Cb + Cs - (Cb * Cs);
+}
+
+vec3 BlendHardLight(vec3 Cb, vec3 Cs) {
+  vec3 m = BlendMultiply(Cb, 2.0 * Cs);
+  vec3 s = BlendScreen(Cb, 2.0 * Cs - 1.0);
+  vec3 edge = vec3(0.5, 0.5, 0.5);
+
+  /* Use mix() and step() to avoid a branch */
+  return mix(m, s, step(edge, Cs));
+}
+
+vec3 BlendOverlay(vec3 Cb, vec3 Cs) {
+  return BlendHardLight(Cs, Cb);
+}
+
+vec3 BlendDarken(vec3 Cb, vec3 Cs) {
+  return min(Cb, Cs);
+}
+
+vec3 BlendLighten(vec3 Cb, vec3 Cs) {
+  return max(Cb, Cs);
 }
 
 void main() {
-  vec4 src = Texture(map, vUv);
   vec4 mask = Texture(parentMap, vUv);
+  vec4 diffuse = Texture(map, vUv);
   vec3 res;
 
   if (blendMode == 0) {
-    res = src.xyz;
+    res = diffuse.xyz;
   }
   else if (blendMode == 1) {
-    res = BlendMultiply(src.xyz, mask.xyz);
+    res = BlendMultiply(mask.xyz, diffuse.xyz);
+  }
+  else if (blendMode == 2) {
+    res = BlendScreen(mask.xyz, diffuse.xyz);
+  }
+  else if (blendMode == 3) {
+    res = BlendOverlay(mask.xyz, diffuse.xyz);
+  }
+  else if (blendMode == 4) {
+    res = BlendDarken(mask.xyz, diffuse.xyz);
+  }
+  else if (blendMode == 5) {
+    res = BlendLighten(mask.xyz, diffuse.xyz);
+  }
+  else if (blendMode == 8) {
+    res = BlendHardLight(mask.xyz, diffuse.xyz);
   }
   else {
     // Use red for debugging missing blend modes
     res = vec3(1.0, 0.0, 0.0);
   }
 
-  setOutputColor(vec4(res, src.a * alpha));
+  setOutputColor(vec4(res, diffuse.a * alpha));
 }