Experimental Physics and
Industrial Control System

"D. Peter Siddons" <[email protected]> · Thu, 11 Oct 2007 04:17:56 -0400

Hi Emmanuel,

I'm always nervous of throwing away information! You're probably
right in that it doesn't matter for your applications, but in a more
quantitative situation (e.g. computed tomography), where you might want
the frame rate, I would avoid compression if it was not lossless. And as
Mark says, so what if I need another Ethernet card!

Pete.
Emmanuel Mayssat wrote:

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

--
D. Peter Siddons
Detector Development Group Leader
National Synchrotron Light Source
Brookhaven National Laboratory
Upton, NY 11973
email: [email protected]
Phone: (631) 344-2738

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System