[Theora-dev] Fix a bug that causes the encoder to spend way to many bits on keyframes

[Ivan Popov]

Hi Aaron,

On Thu, Feb 17, 2005 at 11:22:11AM -0800, Aaron Colwell wrote:
> Here is a fix that prevents the encoder from spending too many bits on
> keyframes. The main problem is that the old code computes the target keyframe
> bitrate incorrectly. This causes the code that computes the Q for the frame to

> have to use a much lower Q. This ends up causing a "popping" artifact where 
> everything is clear for a keyframe, but gets progressively blurry for each
> successive inter-frame.

I have been looking for the reason of that (or similar) effect.
No one on the list answered when I asked.

Now I tested the encoder with your change, and unfortunately it did not
improve the cases where I see artifacts - still your change looks right,
if I can believe the comments in encoder_toplevel.c.

I see terrible artifacts on "text screens" with e.g.

theora-encoder -a 0 -V 400 -o newref.400.ogg AmelieArchive.mpg
e.g. during the seconds 4 and 5

or very spectacular ones with

theora-encoder -a 0 -V 2000 -o newref.2000.ogg AmelieArchive.mpg
e.g. during the seconds 13-16...

(http://www.archive.org/download/AmelieArchive/AmelieArchive.mpg)

theora-encoder script runs theoraenc which (and the library)
is from today's svn.

Would you - or anybody - comment on my observations?

Best regards,
--
Ivan