Hi Mariano Alvarez:
GrSetMode(GR_width_height_bpp_graphics,800,600,8 ); // for 8bpp GrSetMode(GR_width_height_bpp_graphics,800,600,24 ); // for 24bpp
But this is not the problem. The problem is a known bug in the 8bpp win32 driver, it operates with colors instead index colors to make the xor-operations. The problem is big, is a design error. But the 24 bpp driver works OK with your test.
Thanks for claryfying this. But - not that I want to insist on that topic - please note that the output of GrPlot() is in error even for the 24bpp. Bye.