Detecting nonzero matrix elements in a rectangle

I have an image that I performed Sobel Edge Detection on. I am planning on drawing rectangles on top of the edge detected image and counting the number of nonzero matrix elements within those rectangles using the nnz command.

My question is how exactly do I use the nnz command to work only within those rectangles? I have the 4 corner coordinates of the rectangle already, and I will be performing this calculation on at at least 5 rectangles within the image.

Thank you for any help in advance.

Answer 1

[David Young]

Say your edge array is E and opposite corners of a rectangle are at integer coordinates (x0,y0) and (x1,y1), with the former nearer the top left of the image. Then you could use this

nEdgePixels = nnz(E(y0:y1, x0:x1));

Note that I'm using the common convention that X refers to the column subscript in the array and Y refers to the row subscript.

[Michael]

Exactly what I wanted. Thank you!