How to get a raw rgb frame

[SpiritOffice]

I would like to create a filter for a game. It should analyze the game frame per frame (or 1 time per second) and then show some advanced information for my viewers. As example: scan some progress bars and write percent digits on top of it.
The problem is: i can't understand, how to get the current RGB frame for my analyze. I have read through existing filters, but still do not get it.