The IP Over NBMA (ION) Archive

[Van Jacobson <van@ee.lbl.gov>]

> An additional tip:  when using vic, sliding the Quality bar to
> the right (from default 10 towards highest quality 1) improves
> the resolution of the image.

No, please don't do this -- it will actually make things worse.
Vic uses an adaptive quantizer that will do as good a job as
possible sending slides if you leave it at the default setting.
Vic tracks the motion in every 8x8 pixel image block on the
screen.  When a block is changing rapidly (e.g., someone is
changing slides), vic sends as coarsely quantized image (using
the q that you set on the menu panel) that helps to rapidly
paint the large, solid areas.  As soon as a block's motion
stops, vic repaints the block at twice the quality (half the q
you selected) and as soon as most of the total image motion
stops, all the image is repainted at the highest quality
possible with h.261 (q=1).  So if you push the quantizer down,
vic has to expend much more of its bit budget sending a high
resolution version of worthless data (the image of a slide being
removed or placed on the projector) which only serves make the
vic updates seriously lag the speaker & delays getting the high
res version of the slide painted.  [I have looked, in detail, at
what happens when sending sides with vic.  Using the default
q=10 + clicking on the 'sending slides' button on the menu panel
(which makes vic expend more of its bandwidth buget on the hi
res updates) gets the highest quality slides out sooner than any
other scheme or q setting.  (If you'd like to look at this
yourself, rtp_record a presentation then build the "h261_play"
tool in the vic distribution; it will let you view the video
both continuously & single step, and optionally highlight only
the blocks that change so you can see exactly what vic was
doing.)]

If you find pushing q down to 3 or 4 really improved things, you
might have an old copy of vic (the adaptive quantizer didn't go
in until around the beta-1 release).  Or there might be a
problem in your video capture -- ground loops can make noise
artifacts that crawl across the screen & fool vic into thinking
that there's motion when there isn't.  Or if the camera is
handheld rather than on a tripod, it's probably impossible to
hold it still enough for vic to switch to high quality mode.  Or
if your camera operator is into pan & scan (a *really* bad idea
for mbone sessions) and leaves their hand on the tripod control
arm, or if your camera is set to autofocus (which almost always
'hunts' on slide images & degrades the image quality), a lot of
extraneous motion can get introduced.  But any of these problems
will drastically cut down on the video frame rate & quality, no
matter what q you use.  They should be fixed beforehand so
you can simply leave vic alone & let it dynamically choose the
right quality to send at.

 - Van