Along with the other approaches mentioned, another interesting one is to intercept the graphics library function calls before they enter the library code.

This strategy has been successfully used for a variety of game automation, including WoW auction house arbitrage and a multiplayer Starcraft II AI / bot.

This method is advantageous because it doesn't have the overhead or uncertainty of going from pixels back to information but it also does not require patching or inspecting the game's address space which is fragile and often triggers anti-cheating or anti-botting systems.

You can read a good write-up about this method here:

http://graphics.stanford.edu/~mdfisher/D3D9Interceptor.html