Nice little trick. And pretty useful, too, I imagine. I suppose the reason JPEG files don't allow for transparency information is that the DCT would mess it up

Nice little trick. And pretty useful, too, I imagine. I suppose the reason JPEG files don't allow for transparency information is that the DCT would mess it up? (I'm imagining little waves of semi-opaqueness appearing just outside the boundary of an opaque portion of an image. And little waves of semi-transparency appearing just inside. Ick.) Of course, there's no law that says JPEG formats have to use the DCT for absolutely everything. Heck, [ordinary JPEG + PNG mask] probably wouldn't be a bad file format. Now if we could just get people to start supporting it ....