I did not internalize the fact that your confusion over the meaning of alpha arose from trying to process alpha channels in PNG files. There is a mismatch between PNGs definition of alpha and Plan 9's. In Plan 9 images, the r, g, and b channels are premultiplied by alpha. In PNG, they are not, so when you display a PNG image with alpha channel you must do (r, g, b) *= alpha yourself before handing it off to libdraw.
Russ
