Gash - Plane Problems

I started implementing the function for NextGenGraphics that converts a bunch of raw data into pixel values. This of course has to take into account the bits per pixel of the graphics adapter and whether it shuffles the planes around like EGA. I thought it would be easy until I remembered the mask plane. If included in CGA, there would be three bits per pixel, which would be very messy if it was arranged linearly. Sadly, there is no format documentation on masked CGA data or CGAGRAPH, so I just have to figure out where CGA keeps the mask information before I can move on.