[MPlayer-dev-eng] writing a 'screenshot' function

[Paul TT]

i've seen before on this list questions about this, and:
- i dont' want to use 'postition' method to get a screenshot,
- neither output automatically in this way to png or jpg,
- neither use (as i do now) vo x11 and import from imagemagik to do it

i've seen a keybinding for 's'->MP_CMD_SCREENSHOT using VOCTRL_SCREENSHOT in video_out, so i'd like to try to implement it.
my real question is: 
is there a common interface for all vo drivers to get current image attributes or it depends by the driver in use???
just to know if i must implement it into vo_<drivername>.c or elsewhere in a common place :-)
(if not i'll insert into vo_xv.c)