Been at this for a while. I know it's simple but I can't figure out how to do it without this seriously expensive loop. What's the object I'm missing out on here?
I want to take an input image like the first attachment, and add "strips" of transparency to the top and bottom (represented in red on 2nd image). The image could also be smushed in from the top and bottom, I don't really care that much. The reason I want to do this is to layer it below this transparent iPhone image which has curvy corners so it needs GL transparency. So the image has to fit into the screen portion of the image (both the screen and the corners of the image are transparent).