I am trying to implement a graphics library using Linux for STB (Set Top Box) based on a existing graphics library (which is already implemented on a different platform). But I wonder to find that they set the display mode (NTSC/PAL) and many other video related stuff when initializing the graphics context. As far as I know that for a STB there are separate 'Graphics Plane ' and 'Video Plane ' for displaying graphics and video respectively. Then why does video matter when implementing graphics library ?
Please correct me if I am wrong. Do I have to initialize video when implementing graphics library ? If yes then why ?

Thanking you all.