Question

While I can write a whole image to a file with

cvSaveImage("image.png", img);

how can I write only a given rectangle from the image I’m working on to a file?

Answer 1

There’s Mat constructor:

Mat(const Mat& m, const Rect& roi);

So just use it!

Or if you use c (not c++) interface than you have to set ROI (Region Of Interest):

http://nashruddin.com/OpenCV_Region_of_Interest_(ROI)/

Your code should look like this:

cvSetImageROI(img, rect);//rect is a ROI
cvSaveImage("image.png", img);

If you want to work again with full image than you have to reset ROI:

cvResetImageROI( IplImage* img )