[MPlayer-users] Mencoder fails: Cannot initialize video driver

[Rich Felker]

On Thu, Jan 24, 2008 at 01:44:36AM +0000, Da Rock wrote:
> 
> Sorry, what exactly does the keyint do? I thought it was simply the
> fps output based on the context it was put in. What is the bitrate
> overhead you're talking about too (while we're on this subject)?

Please don't top-post.

Keyint has nothing to do with fps. It's the interval between
successive I (intra) frames. In all modern video codecs, most frames
are encoded as a difference from a past (or sometimes future) frame.
Only a few frames are encoded as stand-alone images that can be
decoded on their own. These are the I frames and they are MUCH larger
than other frames because they cannot take advantage of similarity to
adjacent frames.

However, the vast, vast majority of players can only seek to I frames.
(Seeking to a non-I frame is basically just seeking to an I frame
followed by decoding a large number of subsequent frames.) Lowering
the keyint therefore improves efficient fine-grained seeking, but at
the expense of compression efficiency. If keyint is 120, that means 1
I-frame every 120 frames. So if your video is 30 fps, that would be
one I frame (point you can seek to) every 4 seconds.

Rich