Here you can see how I've defined my points so that all of the extra garbage in my shot is masked out. Note, that the points along the upper and lower edge have exact values of -240 and 240 in their second coordinates. I did that by keying those numbers into the coordinate boxes, after I set my points about where I wanted them. This is important, if you don't want to lose any of your image along one or more edges. It is much easier to set exact values numerically.