How to split cross an image ?

edit

Well, you cannot have a non-rectangular image. Still, it is possible to create an image with the left part and another one with the right part.

Mat mask;   // put here a binary mask (0's and 1's) of the left area
Mat image;  //your original image
Mat leftpart,rightpart;
image.copyTo(leftpart,mask);
image.copyTo(rightpart,1-mask);