Determine x and y coordinate in node node-red-image-tools to crop image

Hello everyone
I have to crop image using node node-red-image-tools but i don't know how to determine x and y coordinate.
I already crop image before and input random number for x and y that show in figure below.
but i don't understand where x and y is coming from or how the formula to get x and y coordinate.
Anyone can explain? thanks for your help

Images are usually referenced from 0,0 being the top left of the image. So x is across the image, y is down the image. Then w is the width you want and h the height.