Hello!

I’m trying to port the levels shader from Brad Larsons GPUImage2, but I get an error when trying to run the node:

```
11.09.2016 09.29.15,987 ShadersTest-RByiV8[10957]: VuoGlPool.cc:879 VuoGlShader_printShaderInfoLog() ERROR: 0:2: Invalid call of undeclared identifier 'LevelsControl'
```

It seems like the #define part of the fragment shader isn’t running/compiled from searching for the general error message, but I’m a bit stuck on how to figure this out, so any help would be greatly appreciated. The source for the shader:

```
/*
** Gamma correction
** Details: http://blog.mouaif.org/2009/01/22/photoshop-gamma-correction-shader/
*/
#define GammaCorrection(color, gamma) pow(color, 1.0 / gamma)
/*
** Levels control (input (+gamma), output)
** Details: http://blog.mouaif.org/2009/01/28/levels-control-shader/
*/
#define LevelsControlInputRange(color, minInput, maxInput) min(max(color - minInput, vec3(0.0)) / (maxInput - minInput), vec3(1.0))
#define LevelsControlInput(color, minInput, gamma, maxInput) GammaCorrection(LevelsControlInputRange(color, minInput, maxInput), gamma)
#define LevelsControlOutputRange(color, minOutput, maxOutput) mix(minOutput, maxOutput, color)
#define LevelsControl(color, minInput, gamma, maxInput, minOutput, maxOutput) LevelsControlOutputRange(LevelsControlInput(color, minInput, gamma, maxInput), minOutput, maxOutput)
varying vec2 textureCoordinate;
uniform sampler2D inputImageTexture;
uniform vec3 levelMinimum;
uniform vec3 levelMiddle;
uniform vec3 levelMaximum;
uniform vec3 minOutput;
uniform vec3 maxOutput;
void main()
{
vec4 textureColor = texture2D(inputImageTexture, textureCoordinate);
gl_FragColor = vec4(LevelsControl(textureColor.rgb, levelMinimum, levelMiddle, levelMaximum, minOutput, maxOutput), textureColor.a);
}
```