Re: Frame rate performance

["D. Peter Siddons" <siddons@bnl.gov>]

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: siddons@bnl.gov
Phone: (631) 344-2738