How to configure shaders with Retropie. This will give it more of a retro look on HD TVs.
Note that the directory structure is from Retropie 2.3, but easy to apply to later versions.

Details on how to easily try out shaders is here:
http://www.dailymotion.com/video/x2hskeu_retropie-choosing-shaders-for-retropie-retroarch_videogames