Merge pull request #991 from thayerandrews/develop

CCEffectContrast - Fix for negative contrast values

Author: slembcke

cocos2d/CCEffectContrast.m

@@ -54,7 +54,8 @@ -(void)buildFragmentFunctions
     CCEffectFunctionInput *input = [[CCEffectFunctionInput alloc] initWithType:@"vec4" name:@"inputValue" initialSnippet:CCEffectDefaultInitialInputSnippet snippet:CCEffectDefaultInputSnippet];
 
     NSString* effectBody = CC_GLSL(
-                                   return vec4(((inputValue.rgb - vec3(0.5)) * vec3(u_contrast) + vec3(0.5)), inputValue.a);
+                                   vec3 offset = vec3(0.5) * inputValue.a;
+                                   return vec4(((inputValue.rgb - offset) * vec3(u_contrast) + offset), inputValue.a);
                                    );
 
     CCEffectFunction* fragmentFunction = [[CCEffectFunction alloc] initWithName:@"contrastEffect" body:effectBody inputs:@[input] returnType:@"vec4"];