> and makes no sense for the alpha channel (wave-based sharpening filters create opaque halos). Alpha channels are generally problematic, since if alpha is sma

> and makes no sense for the alpha channel (wave-based sharpening filters create opaque halos). Alpha channels are generally problematic, since if alpha is small, big changes in the rgb channels result in small changes in the actual color. You can't linearly interpolate in this color space, just like you can't in a polor coordinate space like HSL. Pre-multiplied alpha should fix most of these problems (though you still need to be careful with fancy filters, no not end up with values outside the valid range)