Hi Ken,
I've actually been working on an issue related to buffered acquisitions in IMAQdx over the last week. Let me see if I can address your questions.
1) Yes,
Buffer Number In is ignored when the VI is set to get the last or next buffer in the acquisition
2) According to the detailed help for the IMAQdx Get Image VI, the description for the setting for Buffer Number Mode states "
Buffer Number, which specifies an exact buffer number to wait for." The VI will sit and wait until you have reached the desired buffer number to continue. When you have only a size of 5 buffers configured, you are not restricted to getting buffers 0-4. Acquired images will continue to count up from 0 until you stop the ring acquisition (all the way up to the limit of an I32 or U32 number I think) and continually roll over the preallocated buffer whenever it fills up. The Developer Zone Tutorial on
Ring Acquisitions is a good primer for the subject, even though it deals with NI-IMAQ rather than IMAQdx.
So if you request buffer 5 in a 3-buffer acquisition, it will actually wait until buffer 5 is acquired in hardware (after 0-4):
[0][1][2] => [3][1][2] => [3][4][5] and then it will extract it from the buffer for processing.
Another thing to keep in mind is that in IMAQdx, approximately half of the allocated buffers in memory get locked by the driver for performance reasons so they can be overwritten for the acquisition process. Then the user can only request the last half of buffers to be acquired. In a 10-buffer acquisition, for example, suppose you're at [10][11][12][13][4][5][6][7][8][9]. The next buffer to acquire, 14, would be placed in the slot of 4. If I recall correctly, the driver will actually lock out 4, 5, 6, 7, and 8 to ensure that acquisitions go smoothly and would only let you request (extract) buffers 9-13 at that point. The acquisition attribute Overwrite Mode (settable in Measurement & Automation Explorer for the camera or programmatically using a property node under the Acquisition Attributes category of properties) determines what happens when you try to request a buffer that's no longer available to be extracted anymore (in this case, buffers 0-8). By default it gives you the oldest buffer available, but you can also set it to instead give the newest buffer available or fail and return an error.
