Getting fingerprint orientation using gradient based method - c++

Question

I'm trying to get orientation image of a fingerprint using the method proposed in this paper.
I tried implementing the steps described in Section 3.1.1 of the paper, but I don't get the desired result.
Here are my OpenCV code:
Mat calculate_orientation(Mat img, Mat &coherence) {
Mat image = img.clone();
Mat orient_im = Mat::zeros(image.size(), image.type());
Mat grad_x, grad_y;
Sobel(image, grad_x, CV_32F, 1, 0, 3, 1, 0, BORDER_DEFAULT );
Sobel(image, grad_y, CV_32F, 0, 1, 3, 1, 0, BORDER_DEFAULT );
//Iterate per BLOCKSIZE and use BLOCKSIZE/2 as the center
for (int i=BLOCKSIZE/2 ; i<=image.rows-BLOCKSIZE/2 ; i+=BLOCKSIZE) {
for (int j=BLOCKSIZE/2 ; j<=image.cols-BLOCKSIZE/2 ; j+=BLOCKSIZE) {
//Iterate each pixel in the block
float vx = 0.0f, vy = 0.0f, angle;
//Coherence
float gx = 0.0f, gy = 0.0f, gxy = 0.0f;
for (int u=i-BLOCKSIZE/2 ; u<i+BLOCKSIZE/2 ; u++) {
for (int v=j-BLOCKSIZE/2 ; v<j+BLOCKSIZE/2 ; v++) {
gx = 2* grad_x.at<float>(u,v) * grad_y.at<float>(u,v);
gy = pow(grad_x.at<float>(u,v), 2) - pow(grad_y.at<float>(u,v), 2);
vx += gx;
vy += gy;
gxy += sqrt(pow(gx,2)+pow(gy,2));
}
}
if (vy == 0) {
angle = 90;
} else {
angle = 0.5 * atan(vx/vy) * 180.0f/CV_PI;
}
//The angle above is the angle perpendicular to ridge direction
orient_im.at<float>(i,j) = angle + 90;
//Coherence
float coh = sqrt(pow(vx,2)+pow(vy,2))/gxy;
coherence.at<float>(i,j) = coh;
}
}
return orient_im;
}
This is the input image.
And this is the result. The blue lines are orientation with coherence value of more than 0.5, and the red lines are orientation with coherence value of less than 0.5.
Only around half of the orientation seems right.
I know there are already a few questions about this, but I still haven't gotten the correct results, so pardon me for asking. Any help would be appreciated.