In Photoshop, on a transparent layer, I have an area with a black-to-white gradient. I want to turn it to a red-to-white gradient, so that pure black (0,0,0) becomes pure red (255,0,0). In the example below, I would want to turn the top bar into the bottom one.
I don't want to re-draw anything, since the gradient has a particular shape. I want to modify the existing image. How do I do this?