theora dev - Oct 2004 - Black frames need higher bitrates than moving ones?!

I'm capturing video from a webcam and encoding it using Theora through
Zen's
DirectShow filter.

I noticed that black frames (webcam lens shut) produce 45 KB/s (target bitrate 
is 400 kbps), while moving video is around 8-20 KB/s. How can that be? Is the 
encoder fooled by the background noise the webcam produces?
Maybe it has to do with the quality setting, though I don't know what
it's for
(there's already a bitrate setting) and what i.e. 30 means to it, but I
could
imagine that a quality calculation in the codec has problems with black 
frames...

Bye,
  Robert
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On Sun, Oct 03, 2004 at 03:56:30PM +0200, Robert Huitl wrote:
> I noticed that black frames (webcam lens shut) produce 45 KB/s (target
bitrate
> is 400 kbps), while moving video is around 8-20 KB/s. How can that be? Is
the
> encoder fooled by the background noise the webcam produces?
> Maybe it has to do with the quality setting, though I don't know what
it's for
> (there's already a bitrate setting) and what i.e. 30 means to it, but I
could
> imagine that a quality calculation in the codec has problems with black 
> frames...
It's quite possible this is the problem. Noise is expensive to encode, 
and without any actual signal to calibrate its efforts the encoder is 
trying very, very hard to represent the camera noise accurately. 
The Vorbis reference encoder behaves exactly this way when fed 
low-intensity analog noise vs. actual digital silence.

The quality setting tells the codec what 'quality level' you'd the
like
the output video to have; it then uses the minimum number of bits to 
achieve this quality, resulting in more efficient encoding since it 
doens't have to waste bits maintaining a constant bitrate when the scene 
isn't demanding. Generally, you should use this mode in preference to 
seting an actual bitrate.

 -r