I'm not sure what it's called, but, to get an idea, take a look at jefhatfield's avatar. It has the light source. I want to do the same thing in Photoshop with a contrast adjustment. I know there is a hard way to doing this: Selecting each row of pixels manually and adjusting the contrast accurately, sort of like using the gradient tool with the multiply option enabled. So how can I make a light shine, or whatever, by adjusting the contrast using one step, instead of going through all the pixels?