How do I use overlay_add command in mplayer?

Question

I managed to create a .pgm image file with GIMP, and noticed the potentially problematic #-comment it embeds into the file, but I still cannot get this command to display any noticeable result. I launch the mplayer with -slave mode, and input the command as follows

overlay_add file.pgm 0 10 10 16

expecting it to display a grayscale bitmap image on top of playing video at coordinates 10,10, but nothing happens.

Answer 1

the problem is with the vo driver, which was directx
the mplayer reported on load 'your card does not support overlay', which i ignored
after changing the output driver with a cli option -vo gl (to GL, in this case), the overlay_add worked at once
and #-comments inside the pgm file have no effect on it; although raw encoding order is a must