framework/Source/GPUImageXYDerivativeFilter.m
#import "GPUImageXYDerivativeFilter.h"

// I'm using the Prewitt operator to obtain the derivative, then squaring the X and Y components and placing the product of the two in Z.
// In tests, Prewitt seemed to be tied with Sobel for the best, and it's just a little cheaper to compute.
// This is primarily intended to be used with corner detection filters.

@implementation GPUImageXYDerivativeFilter

#if TARGET_IPHONE_SIMULATOR || TARGET_OS_IPHONE
NSString *const kGPUImageGradientFragmentShaderString = SHADER_STRING
(
 precision highp float;
 
 varying vec2 textureCoordinate;
 varying vec2 leftTextureCoordinate;
 varying vec2 rightTextureCoordinate;
 
 varying vec2 topTextureCoordinate;
 varying vec2 topLeftTextureCoordinate;
 varying vec2 topRightTextureCoordinate;
 
 varying vec2 bottomTextureCoordinate;
 varying vec2 bottomLeftTextureCoordinate;
 varying vec2 bottomRightTextureCoordinate;
 
 uniform sampler2D inputImageTexture;
 
 uniform float edgeStrength;
 
 void main()
 {
     float topIntensity = texture2D(inputImageTexture, topTextureCoordinate).r;
     float topRightIntensity = texture2D(inputImageTexture, topRightTextureCoordinate).r;
     float topLeftIntensity = texture2D(inputImageTexture, topLeftTextureCoordinate).r;
     float bottomIntensity = texture2D(inputImageTexture, bottomTextureCoordinate).r;
     float bottomLeftIntensity = texture2D(inputImageTexture, bottomLeftTextureCoordinate).r;
     float bottomRightIntensity = texture2D(inputImageTexture, bottomRightTextureCoordinate).r;
     float leftIntensity = texture2D(inputImageTexture, leftTextureCoordinate).r;
     float rightIntensity = texture2D(inputImageTexture, rightTextureCoordinate).r;
     
     float verticalDerivative = -topLeftIntensity - topIntensity - topRightIntensity + bottomLeftIntensity + bottomIntensity + bottomRightIntensity;
     float horizontalDerivative = -bottomLeftIntensity - leftIntensity - topLeftIntensity + bottomRightIntensity + rightIntensity + topRightIntensity;
     verticalDerivative = verticalDerivative * edgeStrength;
     horizontalDerivative = horizontalDerivative * edgeStrength;
     
     // Scaling the X * Y operation so that negative numbers are not clipped in the 0..1 range. This will be expanded in the corner detection filter
     gl_FragColor = vec4(horizontalDerivative * horizontalDerivative, verticalDerivative * verticalDerivative, ((verticalDerivative * horizontalDerivative) + 1.0) / 2.0, 1.0);
 }
);
#else
NSString *const kGPUImageGradientFragmentShaderString = SHADER_STRING
(
 varying vec2 textureCoordinate;
 varying vec2 leftTextureCoordinate;
 varying vec2 rightTextureCoordinate;
 
 varying vec2 topTextureCoordinate;
 varying vec2 topLeftTextureCoordinate;
 varying vec2 topRightTextureCoordinate;
 
 varying vec2 bottomTextureCoordinate;
 varying vec2 bottomLeftTextureCoordinate;
 varying vec2 bottomRightTextureCoordinate;
 
 uniform sampler2D inputImageTexture;
 
 uniform float edgeStrength;

 void main()
 {
     float topIntensity = texture2D(inputImageTexture, topTextureCoordinate).r;
     float topRightIntensity = texture2D(inputImageTexture, topRightTextureCoordinate).r;
     float topLeftIntensity = texture2D(inputImageTexture, topLeftTextureCoordinate).r;
     float bottomIntensity = texture2D(inputImageTexture, bottomTextureCoordinate).r;
     float bottomLeftIntensity = texture2D(inputImageTexture, bottomLeftTextureCoordinate).r;
     float bottomRightIntensity = texture2D(inputImageTexture, bottomRightTextureCoordinate).r;
     float leftIntensity = texture2D(inputImageTexture, leftTextureCoordinate).r;
     float rightIntensity = texture2D(inputImageTexture, rightTextureCoordinate).r;
     
     float verticalDerivative = -topLeftIntensity - topIntensity - topRightIntensity + bottomLeftIntensity + bottomIntensity + bottomRightIntensity;
     float horizontalDerivative = -bottomLeftIntensity - leftIntensity - topLeftIntensity + bottomRightIntensity + rightIntensity + topRightIntensity;
     verticalDerivative = verticalDerivative * edgeStrength;
     horizontalDerivative = horizontalDerivative * edgeStrength;
     
     // Scaling the X * Y operation so that negative numbers are not clipped in the 0..1 range. This will be expanded in the corner detection filter
     gl_FragColor = vec4(horizontalDerivative * horizontalDerivative, verticalDerivative * verticalDerivative, ((verticalDerivative * horizontalDerivative) + 1.0) / 2.0, 1.0);
 }
);
#endif

#pragma mark -
#pragma mark Initialization and teardown

- (id)init;
{
    if (!(self = [self initWithFragmentShaderFromString:kGPUImageGradientFragmentShaderString]))
    {
		return nil;
    }
    
    self.edgeStrength = 1.0;
    
    return self;
}

@end
