Why not separate the format for RGB vs RGBA based on a header? Since images will never have both RGB and RGBA pixels, you would be able to get an extra few perc

Why not separate the format for RGB vs RGBA based on a header? Since images will never have both RGB and RGBA pixels, you would be able to get an extra few percent encoding by using different (and conflicting) tags for the two types of images. For example you could have QOI_COLOR_RGB which would use a 5 byte tag and QOI_DIFF24_RBG could store 7 bit differences for 2 colors and 6 bit differences for the other color. For RGBA, you could do something similar for the two tags that are RGB specific.