calculate gradient directions - c++

Question

I want calculate angles of gradients from depth map and group it for some directions (8 sectors)
But my function calculates only first 3 directions
cv::Mat calcAngles(cv::Mat dimg)//dimg is depth map
{
const int directions_num = 8;//number of directions
const int degree_grade = 360;
int range_coeff = 255 / (directions_num + 1);//just for visualize
cv::Mat x_edge, y_edge, full_edge, angles;
dimg.copyTo(x_edge);
dimg.copyTo(y_edge);
dimg.copyTo(full_edge);
//compute gradients
Sobel( dimg, x_edge, CV_8U, 1, 0, 5, 1, 19, 4 );
Sobel( dimg, y_edge, CV_8U, 0, 1, 5, 1, 19, 4 );
Sobel( dimg, full_edge, CV_8U, 1, 1, 5, 1, 19, 4 );
float freq[directions_num + 1];//for collect direction's frequency
memset(freq, 0, sizeof(freq));
angles = cv::Mat::zeros(dimg.rows, dimg.cols, CV_8U);//store directions here
for(int i = 0; i < angles.rows; i++)
{
for(int j = 0; j < angles.cols; j++)
{
angles.at<uchar>(i, j) = (((int)cv::fastAtan2(y_edge.at<uchar>(i, j), x_edge.at<uchar>(i, j))) / (degree_grade/directions_num) + 1
) * (dimg.at<uchar>(i, j) ? 1 : 0);//fastatan returns values from 0 to 360, if i not mistaken. I want group angles by directions_num sectors. I use first 'direction' (zero value) for zero values from depth map (zero value at my depth map suggest that it is bad pixel)
freq[angles.at<uchar>(i, j)] += 1;
}
}
for(int i = 0; i < directions_num + 1; i++)
{
printf("%2.2f\t", freq[i]);
}
printf("\n");
angles *= range_coeff;//for visualization
return angles;
}
Out from one of the frames:
47359.00 15018.00 8199.00 6224.00 0.00 0.00 0.00 0.00 0.00
(first value is "zero pixel", next is number of gradients in n-place but only 3 are not zero)
Visualization
Is there way out? Or these result is OK?
PS Sorry for my writing mistakes. English in not my native language.

Answer 1

You used CV_8U type for Sobel output. It is unsigned integer 8 bit. So it can store only positive values. That's why fastAtan2 returns less or equal than 90. Change type to CV_16S and use short type for accessing the elements:
cv::Sobel(dimg, x_edge, CV_16S, 1, 0, 5, 1, 19, 4);
cv::Sobel(dimg, y_edge, CV_16S, 0, 1, 5, 1, 19, 4);
cv::fastAtan2(y_edge.at<short>(i, j), x_edge.at<short>(i, j))