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

[Aaron Colwell]

What are you using to convert the MPEG stream into YUV4MPEG2?

Aaron

On Fri, Feb 18, 2005 at 09:58:44AM -0800, Aaron Colwell wrote:
> I'll take a look at this. Even with my changes I've still seen cases where the
> encoder makes a huge change in Q for the keyframe. I plan on tracking that
> down as well. Having clips that exhibit this problem always help. Thanks for
> including a link to your content. I'll keep you posted on my findings.
> 
> Aaron
> 
> On Fri, Feb 18, 2005 at 06:49:21PM +0100, Ivan Popov wrote:
> > 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
> > 
> > 
> _______________________________________________
> Theora-dev mailing list
> Theora-dev at xiph.org
> http://lists.xiph.org/mailman/listinfo/theora-dev
>