GPU Accelerated Harris Corner Detection - image-processing

Question

I am trying to use GLSL to implement a Harris Corner Detection. But it does not work properly(I am guessing). First off it doesn't detect all corners and it detects alot of points that are not corners, another big problem is that it the threshold is very specific for each image. Maybe that is normal with a Harris detector?
All help is appreciated.
Shader Passes:
1st: Standard passthrough.
2nd: I turn the image into a grayscale image.
3rd: Sobel filtering the image and passing the x, y gradient intensitys and the product of the xy intensitys.
uniform sampler2D texture;
varying vec2 vUV;
void main() {
vec2 uv = vUV;
// Offset used to get access to neighbours
float w = 1.0/800.0;
float h = 1.0/600.0;
vec3 temp;
vec3 sum = vec3(0.0);
// Sobel - Edge Detection
// y gradient
vec3 texel0 = texture2D(texture, uv + vec2(-w, h)).xyz;
vec3 texel1 = texture2D(texture, uv + vec2(-w, 0)).xyz;
vec3 texel2 = texture2D(texture, uv + vec2(-w, -h)).xyz;
vec3 texel6 = texture2D(texture, uv + vec2(w, h)).xyz;
vec3 texel7 = texture2D(texture, uv + vec2(w, 0)).xyz;
vec3 texel8 = texture2D(texture, uv + vec2(w, -h)).xyz;
vec3 vertEdge = 1.0 * texel0 + (2.0*texel1) + 1.0 * texel2 -
(1.0 * texel6 + (2.0*texel7) + 1.0 * texel8);
// x gradient
vec3 texe0 = texture2D(texture, uv + vec2(-w,h)).xyz;
vec3 texe1 = texture2D(texture, uv + vec2(0, h)).xyz;
vec3 texe2 = texture2D(texture, uv + vec2(w, h)).xyz;
vec3 texe6 = texture2D(texture, uv + vec2(-w,-h)).xyz;
vec3 texe7 = texture2D(texture, uv + vec2(0,-h)).xyz;
vec3 texe8 = texture2D(texture, uv + vec2(w,-h)).xyz;
vec3 horizEdge = 1.0 * texe0 + (2.0*texe1) + 1.0 * texe2 -
(1.0 * texe6 + (2.0*texe7) + 1.0 * texe8);
// Gradient intensity values
float iy = (vertEdge.r + vertEdge.g + vertEdge.b) /3.0;
float ix = (horizEdge.r + horizEdge.g + horizEdge.b) /3.0;
// Absolute to get negative values
iy = abs(iy);
ix = abs(ix);
float gradProcduct = ix * iy;
gl_FragColor = vec4(ix,iy,gradProcduct, 0.0);
Not the best looking code - just want it to work for now
4th and 5th: Standard Gaussian Blur
6th: Calculating Harris Response.
If it is a corner i paint that pixel in magenta.
void main() {
vec2 uv = vUV;
float w = 1.0/800.0;
float h = 1.0/600.0;
float threshold = 0.05;
vec4 gradientInfo = texture2D(texture, uv);
/************** Harris Reponse **********************
R is calculated as R = det(M)- K(Trace(M)) which leads to
R = Ix^2*Ix^y - Ixy^2-K(ix^2+iy^2)^2
Ix = X-gradient intesity
Iy = Y-gradient intesity
Ixy = product of the X- and Y-gradient intensities
*********************************************************/
float R = pow(gradientInfo.r,2.0)*pow(gradientInfo.g,2.0)
- pow(gradientInfo.b,2.0)
- threshold * pow((pow(gradientInfo.r,2.0)+pow(gradientInfo.g,2.0)),2.0);
vec4 test;
//if(R > 0.000000000005)
if(R > 0.0000000000750){
// Extremley small values, ugly soloution for now to be able to use R in maxSupress
test = vec4(1.0, 0.0, 1.0, R*1000000000.0);
}
else
test = vec4(vec3(gradientInfo.xyz),0.0);
gl_FragColor = vec4( test);
}
Results
Result on a simple square
Result on a more complex figure with the same R and Threshold
And the result when the response check is mulitplied by a 1000. Doesn't really seem to work.
Below is the code for the maximum supression.
void main() {
vec2 uv = vUV;
float vOffset = 1.0/800.0;
float hOffset = 1.0/600.0;
vec4 neighbourPixels[9];
vec3 result;
int check = 0;
vec3 previous = texture2D(texture2, uv).xyz;
vec4 current = texture2D(texture, uv);
float temp = current.a;
vec4 neighbourArray[25];
if(current.a > 0.0){
for(int i = -2; i<3;i++){
for(int j = -2; j<3;j++){
if(temp < texture2D(texture, vUV.xy+vec2(i,j)*vec2(vOffset,hOffset)).a ){
//result = vec3(1.0,0.0,1.0);
check = 1;
break;
}
}
if(check==1){
break;
}
}
if(check==1){
result = vec3(1.0,0.0,0.0);
}
else{
result = vec3(0.0,1.0,1.0);
}
}
else{
result = previous.xyz;
}
gl_FragColor = vec4( result, 0.0);
}

Answer 1

You need to look for local maxima in the Harris response, rather than just thresholding. Dilate the response with a 3x3 or 5x5 box, then find pixels whether original response and dilated version are equal.