Crop multiple portion from one image

Question

I have a image from which I need create multiple smaller images by drawing rectangles on the parent image, using opencv I know how to draw a rectangle and cut it from the image using Mat and Rect, but the main problem I am facing is that the image is entered through user input and the user can choose to select any portion he/she wants to crop out, so everything needs to be dynamically operated. The attached image will clarify things a bit I am new to opencv with android so any help is appreciated. also if one can provide solution without using opencv is also welcome.