This is more of a vision question than a general LabVIEW question so you may want to post your question to the Machine Vision forums for more traffic.
The only software trigger I know of would be to have a state where you monitor a shared variable and once that variable hits the desired value move on to the next state where the image is acquired. Kind of described here: https://forums.ni.com/t5/Machine-Vision/software-trigger-vision-builder/td-p/1159263
As far as a hardware trigger, I know under the Communications palette in Vision Builder you are able to generate hardware pulses if you are using an NI frame grabber or smart camera, but I'm not sure how it would work for a USB camera.
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Aaron F.
National Instruments
