Premultiplied in GIMP

This is really just a note to self (because I keep forgetting how to do it): when you want to do pre-multiplied stuff in GIMP, first take the image with regular alpha into a layer, then create layer mask (specify "copy of layer's alpha channel"). Then set background color to black and remove alpha channel from the layer (but keep the mask). This way the layer gets blended on a black background (effectively pre-multiplying it) and the layer mask keeps a copy of the alpha channel intact. This can then be saved as PNG or whatever and it'll look right when blended as pre-multiplied.