Capturing multiple webcams (uvcvideo) with OpenCV

Question

I am trying to simultaneously stream the images from 3 Logitech Webcam Pro 900 devices using OpenCV 2.1 on Ubuntu 11.10. The uvcvideo driver gets loaded for these.

Capturing two devices works fine, however with three I run into the out of space error for the third:

libv4l2: error turning on stream: No space left on device

I seem to be running into this issue: http://renoirsrants.blogspot.com.au/2011/07/multiple-webcams-on-zoneminder.html and I have attempted to do the quirks=128 (or pretty much any other power-of-two value) trick but to no avail. I also tried on another machine with two USB 2.0 hubs and connecting two cameras to one and the third camera to the second, which resulted into the same problem. I am initializing roughly as follows (using N cameras so the result is actually put into an STL vector):

cv::VideoCapture cap0(0); //(0,1,2..)

and attempting to capture all the cameras in a loop as

cap0.retrieve(frame0);

This works fine for N=2 cameras. When I set N=3 the third window opens but no image appears and the console is spammed full of V4L2 errors. Similarly, when I set N=2, and attempt to open the third camera in say Cheese (simple webcam capture application), this doesn't work either.

Now comes the big but: After trying guvcview by starting three instances of that, I was able to view three cameras at once (with no problems in terms of frame rate or related), so it does not seem to be a hardware issue. I figure there is some property that I should set, but I'm not sure what that is. I have looked into MJPEG (which these cameras seem to support), but haven't succeeded into setting this property, or detect in which mode (yuyv?) they are running if I start them from OpenCV.

Thoughts?

Answer 1

I had this exact problem, using three logitech quickcam pro 9000 cameras (using ubuntu). I could read from two, but not three. In my case, I wasn't using opencv, but was accessing the cameras through V4L2 directly, using memory-mapped IO. Simply put, there was not enough USB bandwidth to allocate three buffers.

I was reading in the uncompressed frames, however. As soon as I switched the format to MJPEG, the data was small enough, and I could read from the three cameras. I used libjpeg to decode the MJPEG stream.

I haven't looked into how to change the image format using OpenCV, but I do know that it needs to be MJPEG to fit all that data.

Before I switched to MJPEG, I spent a lot of time trying to access each camera one at a time, streaming a single frame before switching to the next. Not recommended!