> BTW, on my Q840av, pixels are 4-byte long (32-bit) in 24-bit mode.  Is
> it the case with other 24-bit internal videos/NuBus cards?

For the most part it should be. The 4th byte is for 8 bits of alpha channel.
Even if it isn't exploited it's much faster for the CPU to move aligned longs
so most card designers would probably avoid using 3 bytes. If memory serves,
I also believe that the video board spec demands that 24 bit mode use 32
bits because of a Quickdraw constraint.

HOWEVER. There is one exception that I know of. The 8/24GC card supposedly
uses interleaved bitplanes rather than chunky pixels to do 24 bit mode in
the tiny amount of RAM that it has. Dealing with interleaved bit planes
would most likely require some serious coding. Though you could probably
rip off code from the Atari port's X server. They have video modes with
interleaved bit planes.