How can I skip files in an imaqdx sequence

Question

I would like to start an imaq image sequence but skip approximately 100 images before beginning storage of the images into a buffer.  In addition, I may want to stop aquiring and begin again without ending the sequence.  The reason I need to do this is for precise timing of image capture.  The camera that the sequence collects images from sends an external trigger to a device which then begins a timed operation.  If I had unlimited buffer space, I would not need to skip images, but I can't switch over to a 64-bit system at this time.
It seems to me that the best option would be to only save the images that I need.  The imaq version of sequence had a "skip" input that allowed specified images to be skipped, thus saving buffer space.  The imaq dx version does not have this.  Is there a work-around?  Perhaps there is an imaqdx sequence sub-VI that has a "skip" input like the imaq version of sequence or something similar?

Answer 1

Using a Producer-Consumer pattern, you should have no trouble at all keeping up with streaming 30 fps to disk.  The basic idea of this pattern is that you have one process, the Producer, running in a loop that that makes "data" (in this case images) at some rate.  As they are made, they are put on a Queue, typically of some finite length (for efficiency), for export.  There is another process, the Consumer, also running in a loop and trying to empty the Queue.  The beauty of a Queue is that the Consumer will simply wait patiently for something to process, then process everything until the Queue is empty, when it waits again.
If you are not familiar with the Producer-Consumer pattern, there is an example, called Queue Basics, in LabVIEW (Help, Find Examples).  The Producer is the Top Left Loop, the Consumer is the Top Right Loop.
You should be able to set the camera up to produced images (in Grab mode) at 30 frames/sec.  Allocate "extra" buffers (experiment -- you may only need a few) to hold the images.
Here is the Producer loop:
Allocate a Queue to hold Image buffers from the camera.  I'd suggest that 10 elements should be enough (gives you 0.33 seconds for processing).
Set up the camera (Grab, 30 FPS, allocate buffers).
Start the Camera.
Bring into your Producer Loop the Image Queue and the Camera I/O Reference.  Within the loop, ask the Camera for the Next Buffer, and put it on the Queue.  Wire a Stop control to the Stop button to stop taking images.
On exiting the loop, stop the Camera.  Check if the Queue is empty (either using the Queue Status, or simply wait a second), then Release the Queue. 
[In this simple example, I'm assuming you are only taking a single video, hence when you push the Stop button, above, everything gets ready to shut down.  It is easy to modify this code to allow you to start and stop, saving multiple videos].
Here is the Consumer loop:
1   Open a file to hold the video.  Use the appropriate palette for the type of file you are writing (for example, I know there are VIs to handle .avi files).
2.  Bring the File reference into your Producer loop.  Also, bring a branch of your Queue into the loop.
3.  Dequeue an image.  It's not a bad idea to put a time-out (say, 100 msec) on the Dequeue, and to follow the Dequeue with a Case statement, where the True (Timed Out?) case does nothing, and the False case handles your image.
4.  Write the image to your already-opened file.
5.  When dealing with parallel (e.g. Producer-Consumer) loops, it's always a "trick" to get them to both stop at the same time.  You've already coded a Stop of the Producer, which stops the Camera, and then Releases the Queue.  Well, if the Queue is released while the Consumer is waiting on a Dequeue, an error will be generated (possibly 1122, I'm not sure).  You can wire the Error line to the Stop condition, since when the Queue has vanished, there's nothing to Consume, so you can safely exit the Consumer.  [It is Good Practice to find out the number of the "Waiting on non-existant Queue" error and to use this as the Exit condition, since you might have other errors that you'd want to handle differently, such as "No more Disk Space"].
Bottom line -- unless there is something very peculiar about your setup, you should have absolutely no trouble recording 30 fps video with LabVIEW.
BS