Earlier this week, a Beyond3D user claimed to have discovered that Halo 3 doesn't run natively in high definition. Using his HDTV's 1:1 mapping function, he discovered that Halo 3's vertical resolution was only 640; that's 80 pixels less than the lowest HD resolution, which is 720p (1280x720).

The latest Bungie Weekly Update has Luke Smith confessing that in fact, Halo 3 doesn't run natively at 720p like all other Xbox 360 games.

Halo 3 uses not one, but two frame buffers – both of which render at 1152x640 pixels. The reason we chose this slightly unorthodox resolution and this very complex use of two buffers is simple enough to see – lighting. We wanted to preserve as much dynamic range as possible – so we use one for the high dynamic range and one for the low dynamic range values. Both are combined to create the finished on screen image.

News-Source: http://news.teamxbox.com