Pods/GPUImage/framework/Source/GPUImageShiTomasiFeatureDetectionFilter.m
#import "GPUImageShiTomasiFeatureDetectionFilter.h"

@implementation GPUImageShiTomasiFeatureDetectionFilter

#if TARGET_IPHONE_SIMULATOR || TARGET_OS_IPHONE
NSString *const kGPUImageShiTomasiCornerDetectionFragmentShaderString = SHADER_STRING
( 
 varying highp vec2 textureCoordinate;
 
 uniform sampler2D inputImageTexture;
 uniform lowp float sensitivity;
 
 void main()
 {
     mediump vec3 derivativeElements = texture2D(inputImageTexture, textureCoordinate).rgb;
     
     mediump float derivativeDifference = derivativeElements.x - derivativeElements.y;
     mediump float zElement = (derivativeElements.z * 2.0) - 1.0;
     
     // R = Ix^2 + Iy^2 - sqrt( (Ix^2 - Iy^2)^2 + 4 * Ixy * Ixy)
     mediump float cornerness = derivativeElements.x + derivativeElements.y - sqrt(derivativeDifference * derivativeDifference + 4.0 * zElement * zElement);

     gl_FragColor = vec4(vec3(cornerness * sensitivity), 1.0);
 }
);
#else
NSString *const kGPUImageShiTomasiCornerDetectionFragmentShaderString = SHADER_STRING
(
 varying vec2 textureCoordinate;
 
 uniform sampler2D inputImageTexture;
 uniform float sensitivity;
 
 void main()
 {
     vec3 derivativeElements = texture2D(inputImageTexture, textureCoordinate).rgb;
     
     float derivativeDifference = derivativeElements.x - derivativeElements.y;
     float zElement = (derivativeElements.z * 2.0) - 1.0;
     
     // R = Ix^2 + Iy^2 - sqrt( (Ix^2 - Iy^2)^2 + 4 * Ixy * Ixy)
     float cornerness = derivativeElements.x + derivativeElements.y - sqrt(derivativeDifference * derivativeDifference + 4.0 * zElement * zElement);
     
     gl_FragColor = vec4(vec3(cornerness * sensitivity), 1.0);
 }
);
#endif

#pragma mark -
#pragma mark Initialization and teardown

- (id)init;
{
    if (!(self = [self initWithCornerDetectionFragmentShader:kGPUImageShiTomasiCornerDetectionFragmentShaderString]))
    {
        return nil;
    }
    
    self.sensitivity = 1.5;
    
    return self;
}


@end
