What i've been told, by the graphics programmers on dod/marmoset(and various other people) is this: Take a Single, uncompressed 24 bit image Save it as single channel, and it will be 1/2 the size Save it as a 3 channel compressed image, and it will be 1/4 the size Save it as a 4 channel(IE spec+gloss alpha)compressed image…
dxt1 (RGB) compresses 6:1 dxt3 (RGBA) 4:1 (sharp alpha) dxt5 (RGBA) 4:1, (smooth alpha) 3dc (RG) 2:1 (for 8 bit data) 4:1 (for 16 bit ), compresses 32 bit better (2x16bit), ie higher quality normals there is no mainstream single channel compression. storage on disk is typically the same as in memory + a few bytes…