How to specify the cropping area of an image with matlab?

I have got the grayscale map after edge detection using the canny algorithm, and I need to crop a part of the image. But I don't know the coordinates of the top left and bottom right corners of the cropping area (rectangle). How do I determine the coordinate values of these two points?

Answer 1

[Image Analyst]

See the attached drawrectangle demo.

Demos for other shapes are also attached.