Normally if you install an image as the contents of a layer's mask layer, the alpha channel of the mask layer's image is what masks the other layer. I know how to do that.
I have a vague memory of reading that you can instead install a grayscale image with no alpha and use the brightness of the image as the mask. I've tried searching various ways, but can't find any documentation about that.
Am I imagining it?
It would be more memory efficient, and make your code cleaner, if you could draw into a mask image using a shade of gray.