Opencv vs matlab disk structuring element

Question

I want to create a disk structuring element in OpenCv. I need my SE to look like

sel = strel('disk',5); 

I want to do this using

 cvstructuringElementEx(cols,rows,anchor_x,anchor_y,shape,*values); 

What do I need to do for this, and what values ​​of anchor_x and anchor_y give the same center point SE with MATLAB?

Answer 1

According to the docs you can try:

 cv::Mat sel = cv::getStructuringElement(MORPH_ELLIPSE, cv::Size(9,9)); 

This gave me the following structuring element:

 0 0 0 0 1 0 0 0 0 0 1 1 1 1 1 1 1 0 0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 0 0 1 1 1 1 1 1 1 0 0 0 0 0 1 0 0 0 0 

In MATLAB I got:

 >> getnhood(strel('disk',5)) ans = 0 0 1 1 1 1 1 0 0 0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 0 0 0 1 1 1 1 1 0 0 

So it's not quite the same, but close enough :)

Answer 2

I need the exact structuring element, as in matlab, so I wrote this (not very elegant) function for my need. It works for shapes with an odd number of rows / columns between 3 and 21 (you can manually add other values ​​(check # zeros in Matlab).

The function is called as follows:

 int Radius = 1; // following call equivalent to Matlab sel = getnhood(strel('disk',Radius)) cv::Mat sel = strelDisk(Radius); 

And the actual function

 cv::Mat strelDisk(int Radius){ // THIS RETURNS STREL('DISK',RADIUS) LIKE IN MATLAB FOR RADIUS = ODD NUMBER BETWEEN 3-->21 cv::Mat sel((2*Radius-1),(2*Radius-1),CV_8U,cv::Scalar(255)); int borderWidth; switch (Radius){ case 1: borderWidth = 0; break; case 3: borderWidth = 0; break; case 5: borderWidth = 2; break; case 7: borderWidth = 2; break; case 9: borderWidth = 4; break; case 11: borderWidth = 6; break; case 13: borderWidth = 6; break; case 15: borderWidth = 8; break; case 17: borderWidth = 8; break; case 19: borderWidth = 10; break; case 21: borderWidth = 10; break; } for (int i=0; i<borderWidth; i++){ for (int j=0; j<borderWidth; j++){ if (i+j<8){ sel.at<uchar>(i,j)=0; sel.at<uchar>(i,sel.cols-1-j)=0; sel.at<uchar>(sel.rows-1-i,j)=0; sel.at<uchar>(sel.rows-1-i,sel.cols-1-j)=0; } } } return sel;