CCEffectBloom - Fix threshold math

When applying the luminance threshold, don't check the luminance of the blurred result pixel and instead
check the luminance of the inputs to the blur operation. If the luminance of an input falls below the
threshold just use black as the input instead. Because the threshold happens before the blur, the
boundaries of the thresholded areas are still nicely blurred. Doing the threshold after the blur results
in hard boundaries that don't achieve the effect we want.

Note that the addition of the luminance calculation and the step() function does have a negative
performance impact. We're going with this implementation for now but this is an argument for
supporting multiple shaders / implementations per-effect. With multiple implementations, we could
select one that does not pay the performance penalty for the threshold check if it's not needed.

Author: Thayer J Andrews

cocos2d/CCEffectBloom.m

@@ -169,17 +169,27 @@ -(void)buildFragmentFunctions
 
     [shaderString appendString:@"if(u_enableGlowMap == 0.0) {\n"];
 
+    [shaderString appendString:@"const vec3 luminanceWeighting = vec3(0.2125, 0.7154, 0.0721);\n"];
+    [shaderString appendString:@"vec4 srcPixel; float luminanceCheck;\n"];
+
     // Inner texture loop
-    [shaderString appendFormat:@"src += texture2D(cc_PreviousPassTexture, v_blurCoordinates[0]) * %f;\n", standardGaussianWeights[0]];
+    [shaderString appendFormat:@"srcPixel = texture2D(cc_PreviousPassTexture, v_blurCoordinates[0]);\n"];
+    [shaderString appendString:@"luminanceCheck = step(u_luminanceThreshold, dot(srcPixel.rgb, luminanceWeighting));\n"];
+    [shaderString appendFormat:@"src += luminanceCheck * srcPixel * %f;\n", standardGaussianWeights[0]];
 
     for (NSUInteger currentBlurCoordinateIndex = 0; currentBlurCoordinateIndex < numberOfOptimizedOffsets; currentBlurCoordinateIndex++)
     {
         GLfloat firstWeight = standardGaussianWeights[currentBlurCoordinateIndex * 2 + 1];
         GLfloat secondWeight = standardGaussianWeights[currentBlurCoordinateIndex * 2 + 2];
         GLfloat optimizedWeight = firstWeight + secondWeight;
-        
-        [shaderString appendFormat:@"src += texture2D(cc_PreviousPassTexture, v_blurCoordinates[%lu]) * %f;\n", (unsigned long)((currentBlurCoordinateIndex * 2) + 1), optimizedWeight];
-        [shaderString appendFormat:@"src += texture2D(cc_PreviousPassTexture, v_blurCoordinates[%lu]) * %f;\n", (unsigned long)((currentBlurCoordinateIndex * 2) + 2), optimizedWeight];
+
+        [shaderString appendFormat:@"srcPixel = texture2D(cc_PreviousPassTexture, v_blurCoordinates[%lu]);\n", (unsigned long)((currentBlurCoordinateIndex * 2) + 1)];
+        [shaderString appendString:@"luminanceCheck = step(u_luminanceThreshold, dot(srcPixel.rgb, luminanceWeighting));\n"];
+        [shaderString appendFormat:@"src += luminanceCheck * srcPixel * %f;\n", optimizedWeight];
+
+        [shaderString appendFormat:@"srcPixel = texture2D(cc_PreviousPassTexture, v_blurCoordinates[%lu]);\n", (unsigned long)((currentBlurCoordinateIndex * 2) + 2)];
+        [shaderString appendString:@"luminanceCheck = step(u_luminanceThreshold, dot(srcPixel.rgb, luminanceWeighting));\n"];
+        [shaderString appendFormat:@"src += luminanceCheck * srcPixel * %f;\n", optimizedWeight];        
     }
 
     // If the number of required samples exceeds the amount we can pass in via varyings, we have to do dependent texture reads in the fragment shader
@@ -194,16 +204,17 @@ -(void)buildFragmentFunctions
 
             GLfloat optimizedWeight = firstWeight + secondWeight;
             GLfloat optimizedOffset = (firstWeight * (currentOverlowTextureRead * 2 + 1) + secondWeight * (currentOverlowTextureRead * 2 + 2)) / optimizedWeight;
-            
-            [shaderString appendFormat:@"src += texture2D(cc_PreviousPassTexture, v_blurCoordinates[0] + singleStepOffset * %f) * %f;\n", optimizedOffset, optimizedWeight];
-            [shaderString appendFormat:@"src += texture2D(cc_PreviousPassTexture, v_blurCoordinates[0] - singleStepOffset * %f) * %f;\n", optimizedOffset, optimizedWeight];
+
+            [shaderString appendFormat:@"srcPixel = texture2D(cc_PreviousPassTexture, v_blurCoordinates[0] + singleStepOffset * %f);\n", optimizedOffset];
+            [shaderString appendString:@"luminanceCheck = step(u_luminanceThreshold, dot(srcPixel.rgb, luminanceWeighting));\n"];
+            [shaderString appendFormat:@"src += luminanceCheck * srcPixel * %f;\n", optimizedWeight];
+
+            [shaderString appendFormat:@"srcPixel = texture2D(cc_PreviousPassTexture, v_blurCoordinates[0] - singleStepOffset * %f);\n", optimizedOffset];
+            [shaderString appendString:@"luminanceCheck = step(u_luminanceThreshold, dot(srcPixel.rgb, luminanceWeighting));\n"];
+            [shaderString appendFormat:@"src += luminanceCheck * srcPixel * %f;\n", optimizedWeight];
         }
     }
 
-    [shaderString appendString:@"const vec3 luminanceWeighting = vec3(0.2125, 0.7154, 0.0721);\n\
-     float luminance = dot(src.rgb, luminanceWeighting);\n\
-     if(luminance < u_luminanceThreshold)\n discard;\n"];
-    
     [shaderString appendString:@"} else {\n"];
     [shaderString appendString:@"\
         dst = texture2D(cc_MainTexture, cc_FragTexCoord1);\
@@ -328,7 +339,7 @@ -(void)buildRenderPasses
 
         pass.shaderUniforms[CCShaderUniformPreviousPassTexture] = previousPassTexture;
         pass.shaderUniforms[weakSelf.uniformTranslationTable[@"u_enableGlowMap"]] = [NSNumber numberWithFloat:0.0f];
-        pass.shaderUniforms[weakSelf.uniformTranslationTable[@"u_luminanceThreshold"]] = [NSNumber numberWithFloat:_luminanceThreshold];
+        pass.shaderUniforms[weakSelf.uniformTranslationTable[@"u_luminanceThreshold"]] = [NSNumber numberWithFloat:0.0f];
         pass.shaderUniforms[weakSelf.uniformTranslationTable[@"u_intensity"]] = [NSNumber numberWithFloat:_transformedIntensity];
 
         GLKVector2 dur = GLKVector2Make(0.0, 1.0 / (previousPassTexture.pixelHeight / previousPassTexture.contentScale));
@@ -344,7 +355,7 @@ -(void)buildRenderPasses
 
         pass.shaderUniforms[CCShaderUniformPreviousPassTexture] = previousPassTexture;
         pass.shaderUniforms[weakSelf.uniformTranslationTable[@"u_enableGlowMap"]] = [NSNumber numberWithFloat:1.0f];
-        pass.shaderUniforms[weakSelf.uniformTranslationTable[@"u_luminanceThreshold"]] = [NSNumber numberWithFloat:_luminanceThreshold];
+        pass.shaderUniforms[weakSelf.uniformTranslationTable[@"u_luminanceThreshold"]] = [NSNumber numberWithFloat:0.0f];
         pass.shaderUniforms[weakSelf.uniformTranslationTable[@"u_intensity"]] = [NSNumber numberWithFloat:_transformedIntensity];
 
     } copy]];