Experimental Physics and
Industrial Control System

Emmanuel Mayssat <[email protected]> · Wed, 10 Oct 2007 17:55:19 -0700

On Wed, 2007-10-10 at 19:22 -0400, D. Peter Siddons wrote:
> Doesn't the Axis server do some (lossy?) compression?
> Pete.
> 

Yes...
I was actually dumbfounded when I read the bandwidth requirement for the
Gig-e camera. It is actually in the name! You need a gigabyte network. 
Those camera don't use compression at all, hence the high bandwidth
requirement.

With the axis, you can use motion JPEG and MPEG-4.
Motion JPEG is a succession of JPEG images (lossy compression
algorithm). MPEG-4 not only compress the images individually, but also
between successive images (stream of images). So with MPEG-4 you have
even more losses.

Now, the question becomes should I care about the loss of information in
the video stream. My answer: it depends on the amount of loss. JPEG
(motion jpeg) losses are such that they cannot be detected by the human
eye. JPEG's fundamental idea is to reduce things that appear invisible
to the human eye and by doing so, tremendously increase the compression
ratio. Those losses would be detected by a computer though, a computer
can carefully look at the RGB value of each individual pixel.

Now, should I care about loss? Probably not for electron beam centering
or surveillance. 

For beam centering:
1/ my image is mostly black (image is mostly uniform, JPEG compression
ratio is huge.)
2/ what I care about is not the image itself but statistics (loss/gain
of intensity on a few pixel doesn't matter much and is averaged out) 
3/ image is mostly static (e- beam stay at the same place), so I could
use mpeg4 for even more compression
4/ frame rate is configurable (do you really need 30Hz or is 10Hz
enough?)

All in all, if I refer to Mark's calculation, my bandwidth requirement
with same frame rate and same image size can be 100 times smaller.
At the end of the day, it seems that you select your hardware based on
your application...

--
Emmanuel

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System