VAAPI

VAAPI (Video Acceleration API) provides access to graphics hardware (GPU) acceleration for video processing.

Installation
Portage knows the global USE flag  for enabling support for VAAPI in other packages. Enabling this USE flag will pull in automatically:

The USE flags of libva are:

After setting this you want to update your system so the changes take effect:

Configuration
You may need to tweak the config files of the programs, you can make use of VAAPI.

Usage
vainfo shows if VAAPI is supported.

VLC
Vlc supports Vaapi natively. Activate the appropriate codecs through the Preferences Menu (VA API Acceleration).

MPV
Mpv also supports the vaapi acceleration through the commandline option --hwdec=vaapi

Mplayer
As of now, main branch Mplayer doesn't support vaapi. There is a development branch though that brings vaapi support. the branch is hwaccel-vaapi, available at gitorious. https://gitorious.org/vaapi/mplayer/source/1923fa10ed77bbf8408f2ce312d85a97dab1f0f3:

It doesn't seem there is a proper ebuild providing mplayer-vaapi, even on the overlays

You can use the following commands to get a vaapi enabled mplayer

You will get yourself an executable, that you can install with the not recommanded following procedure :

Instead, configure a player like smplayer to use your special executable (Preferences/Mplayer executable), then video driver (User defined/vaapi)