RE: video lag

["Mark Rivers" <rivers@cars.uchicago.edu>]

Hi John,

areaDetector plugins like NDStdArray (which converts the images to EPICS
waveforms) can run in 2 modes: blocking and non-blocking.  

In non-blocking mode the plugin runs in its own thread and gets its
images from a queue where the driver has put them during the callback
when each new image arrives.  The size of that queue is defined in the
startup script.  If the queue is large and the plugin cannot keep up,
then there can be lag in getting the images sent to EPICS.  EPICS does
not queue arrays for Channel Access, so there should not be a source of
lag there, nor on the client end.  Decreasing the queue size can
decrease the maximumm lag, but at the risk of dropping frames if the
plugin cannot keep up.

In blocking mode the plugin code runs in the driver thread, and there is
no queue.  This will eliminate that source of lag.  But the downside is
that if the plugin code takes a long time to execute then the driver may
not done processing the plugin by the time the next frame arrives, so it
may miss frames.

There may also be some image queueing down in the CMU Firewire driver, I
am not sure about that.  You could read their documentation to see if it
is discussed.

Mark


-----Original Message-----
From: tech-talk-bounces@aps.anl.gov
[mailto:tech-talk-bounces@aps.anl.gov] On Behalf Of John Dobbins
Sent: Friday, July 16, 2010 2:13 PM
To: EPICS Tech-Talk
Subject: video lag

All,

I am using areaDetector (1-6) on Windows with the CMU Firewire driver, a
Flea2 camera. The IOC host is a Pentium 4, 1.8 GHz, 1 GB of RAM.

I just did a simple test of the total lag by sticking my hand in front
of the camera and noting when the image changes on a remotely connected
display, specifically ImageJ reading image over network via Channel
Access. I was surprised to learn the total lag is about two seconds.
Camera frame rate is set to 3.75 Hz. I am not sure how much delay is
attributable to the network.

Can anyone provide approximate lag for their set-ups?

Does areaDetector buffer frames in a way that would produce lag? For
this test I had an NDStdArray connected directly to the camera driver.

Thanks,

John Dobbins

Lab for Elementary Particle Physics
Cornell University