we need to preview videos with overlayed so-called "safe areas" according to this EBU document: SAFE AREAS FOR 16:9 TELEVISION PRODUCTION.
Those consist of three different "zones", each with the same percentage size-to-the-edge of the video frame for any resolution:
- Action Safe Area (GREEN box): 3.5%
- Graphics Safe Area (RED box): 5%
- 4:3 Safe Area (GREY box): 16.25%
It should be easily achieveable using the drawbox command, but how can be dynamically adapted to any video resolution?