Is it good to use raspberry pi camera module for serious computer vision project using video or is it okay to use a usb webcam?
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The Pi camera is 'run' by the GPU and can dump full frames into RAM at 15 frames a second .. this is 7.5MB/frame, 15fps = 112.5 Mega BYTES per second .. or you can have full HD resolution 30fps H264 encoded (by the GPU) along with some simultaneous still photos (Google MMAL) all at virtually zero CPU loading ..
On the other hand, the Pi USB is 'run' byte at a time by the CPU, and, at the cost of 100% CPU loading you might achieve a couple of hundred Mega BITS per second .. not that a web cam is going to deliver that anyway (even if it could, you then don't have any CPU cycles to do anything with it ..)
SO, unless CCTV resolutions (320x240 pixels) are what you want, it has to be the Pi camera.