How to cut a rectangular area from ui image

Please help me, I’m trying to cut a rectangle from the picture that I get from the camera device. I know how to cut this area, but I do not know how to get rect of this area with respect to my picture. How to get this rect in pixels so that I can use the GetPixels() function and crop this pixel block?90994-cropimage.jpg

Trying to achieve the same result. Any luck since?