Split frames (rolling image) seen in a grab acquisition but not in a snap acquisition

[parmstrong42]

I am trying to acquire digital images from a Indigo Systems Merlin Mid camera using a PCI-1422 frame grabber.  The camera description on NI is below:

http://sine.ni.com/apps/utf8/nipc.product?pid=2084

When I acquire using a "Snap" acquisition, everything works fine, no matter how many images I acquire.  When I try to move to a grab acquisition to get closer to the 60Hz frame rate, I get split frames from apparently different acquisitions on each image.  The split location moves vertically downward on each acquisition, leading to the appearance of a scrolling or rolling image.  It looks to me like someone needs to adjust the horizontal synch knob on an old TV!

This problem is present in both LabVIEW IMAQ and MAX.  In both cases Snap works fine, but Grab gives the rolling output.  I am using the .icd file provided by the manufacturer in MAX.  Any thoughts as to what the problem might be?

[petitOhu]

It seems that you display an image which is currently written. As if you see the image "k+1" before (above) the split, and the past image k after (below) the split.

In the snap mode, there is only one "write/acquisition" and you display the image after the end of the written.

Could you check that your rolling buffer is large enough to read an image which is not currently written ?

Could you use an producer consumer stucture to separate acquisition and diplaying  (the producer loop acquires only one image and copies it in the queue, and the consumer one displays the last image of the queue) ?

Hope this helps...

Best regards

[petitOhu]

You also may try to change the acquisition parameters. Try to decrease the framerate and the packet size. Does-it change anything ?

[BruceAmmons · Accepted solution]

I have found that this problem sometimes goes away if you reduce the size of the image being acquired.  In MAX, try reducing the number of image lines by one or two.

Bruce

[parmstrong42]

Bruce,

That seemed to be it. 

I thought that I had tried that before based on a post I had seen elsewhere on the forums.  However, when I was trying it then I was reducing the "height" value in MAX's Acquisition Window control.  No matter how much I changed the Height value, the problem never went away.  I was about to post that this was not the problem when I thought I should go back and try to reduce the "top" value.  Sure enough, reducing the "top" value by two worked.  Interestingly, reducing it by one line did not work.  I then went back to LabVIEW and IMAQ and the Grab acquisition works fine, even all the way up to the maximum frame rate of the camera.

So, apart from losing a couple of lines in the image, it is now working.

Thank you again!!