Plot histogram of Sobel operator magnitude and angle in OpenCV - opencv

Question

I want to plot histogram in OpenCV C++. The task is that x-axis should be angle and y-axis should be magnitude of histogram. I calculate magnitude and angle by using Sobel operator. Now how can I plot histogram by using magnitude and angle?
Thanks in advance. The simple code of problem is
// Read image
Mat img = imread("abs.jpg");
img.convertTo(img, CV_32F, 1 / 255.0);
/*GaussianBlur(img, img, Size(3, 3), 0, 0, BORDER_CONSTANT);*/
// Calculate gradients gx, gy
Mat gx, gy;
Sobel(img, gx, CV_32F, 1, 0, 1);
Sobel(img, gy, CV_32F, 0, 1, 1);
// C++ Calculate gradient magnitude and direction (in degrees)
Mat mag, angle;
cartToPolar(gx, gy, mag, angle, 1);
imshow("magnitude of image is", mag);
imshow("angle of image is", angle);

Answer 1

Ok, So the first part of it is to calculate the histogram of each of them. Since both are separated already (in their own Mat) we do not have to split them or anything, and we can use them directly in the calcHist function of OpenCV.
By the documentation we have:
void calcHist(const Mat* images, int nimages, const int* channels, InputArray mask, OutputArray hist, int dims, const int* histSize, const float** ranges, bool uniform=true, bool accumulate=false )
So you would have to do:
cv::Mat histMag, histAng;
// number of bins of the histogram, adjust to your liking
int histSize = 10;
// degrees goes from 0-360 if radians then change acordingly
float rangeAng[] = { 0, 360} ;
const float* histRangeAng = { rangeAng };
double minval, maxval;
// get the range for the magnitude
cv::minMaxLoc(mag, &minval, &maxval);
float rangeMag[] = { static_cast<float>(minval), static_cast<float>(maxval)} ;
const float* histRangeMag = { rangeMag };
cv::calcHist(&mag, 1, 0, cv::NoArray(), histMag, 1, &histSize, &histRangeMag, true, false);
cv::calcHist(&angle, 1, 0, cv::NoArray(), histAng, 1, &histSize, &histRangeAng, true, false);
Now you have to plot the two histograms found in histMag and histAng.
In the turtorial I posted in the comments you have lines in the plot, for the angle it would be something like this:
// Draw the histograms for B, G and R
int hist_w = 512; int hist_h = 400;
int bin_w = cvRound( (double) hist_w/histSize );
cv::Mat histImage( hist_h, hist_w, CV_8UC3, Scalar( 0,0,0) );
/// Normalize the result to [ 0, histImage.rows ]
cv::normalize(histAng, histAng, 0, histImage.rows, cv::NORM_MINMAX, -1, Mat() );
// Draw the lines
for( int i = 1; i < histSize; i++ )
{
cv::line( histImage, cv::Point( bin_w*(i-1), hist_h - cvRound(histAng.at<float>(i-1)) ) ,
cv::Point( bin_w*(i), hist_h - cvRound(histAng.at<float>(i)) ),
cv::Scalar( 255, 0, 0), 2, 8, 0 );
}
With this you can do the same for the magnitude, or maybe turn it into a function which draws histograms if they are supplied.
In the documentation they have another option, to draw rectangles as the bins, adapting it to our case, we get something like:
// Draw the histograms for B, G and R
int hist_w = 512; int hist_h = 400;
int bin_w = std::round( static_cast<double>(hist_w)/static_cast<double>(histSize) );
cv::Mat histImage( hist_h, hist_w, CV_8UC3, Scalar( 0,0,0) );
/// Normalize the result to [ 0, histImage.rows ]
cv::normalize(histAng, histAng, 0, histImage.rows, cv::NORM_MINMAX, -1, Mat() );
for( int i = 1; i < histSize; i++ )
{
cv::rectangle(histImage, cv::Point(bin_w*(i-1), hist_h - static_cast<int>(std::round(histAng.at<float>(i-1)))), cv::Point(bin_w*(i), hist_h),);
}
Again, this can be done for the magnitude as well in the same way. This are super simple plots, if you need more complex or beautiful plots, you may need to call an external library and pass the data inside the calculated histograms. Also, this code has not been tested, so it may have a typo or error, but if something fails, just write a comment and we can find a solution.
I hope this helps you, and sorry for the late answer.