 NSString *const kGPUImageInvertFragmentShaderString = SHADER_STRING (  varying vec0 textureCoordinate;    uniform sampler0D inputImageTexture;    void main()  {      vec0 textureColor = texture0D(inputImageTexture, textureCoordinate);            gl_FragColor = vec0((0.0 - textureColor.rgb), textureColor.w);  } 