Opencv – Capturing multiple webcams (uvcvideo) with OpenCV on Linux

opencvv4l2webcam

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?

Best Answer

I had this problem too and have a solution that lets me capture 2 cameras at 640x480 with mjpeg compression. I am using a Creative "Live Cam Sync HD VF0770" which incorrectly reports its bandwidth requirements. The quirks=128 fix works for 320x240 uncompressed video. But for compressed (mjpg) format the quirks=128 does not work (it does nothing for compressed formats).

To fix this I modified the uvc driver as follows:

download the kernel sources

mkdir -p ~/Software/kernel-git
cd ~/Software/kernel-git
git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
git checkout v3.2
# NOTE: `uname -r`  shows me my current kernel is 3.2.0-60-generic
# For a different kernel use a different tag

copy uvc dir:

mkdir -p ~/Software/uvcvideo_driver
cd ~/Software/uvcvideo_driver
#cp -a ~/Software/kernel-git/linux/drivers/media/usb/uvc .
cp ~/Software/kernel-git/linux/drivers/media/video/uvc .

modify Makefile

cd ~/Software/uvcvideo_driver/uvc
vi Makefile

        obj-m += aauvcvideo.o
        aauvcvideo-objs  := uvc_driver.o uvc_queue.o uvc_v4l2.o uvc_video.o uvc_ctrl.o \
              uvc_status.o uvc_isight.o
        all:
          make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

        clean:
          make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

Force bandwith to 0x400 when compressed.

cd ~/Software/uvcvideo_driver/uvc
vw uvc_video.c
Find the uvc_fixup_video_ctrl() function.  At the end of the function add:
      if (format->flags & UVC_FMT_FLAG_COMPRESSED) {
        ctrl->dwMaxPayloadTransferSize = 0x400;
      }

build the aauvcvideo module:

make

remove old module and insert new one:

sudo rmmod uvcvideo
sudo insmod ./aauvcvideo.ko quirks=128

run gucview twice with compression in 2 different windows to test

guvcview --device=/dev/video1 --format=mjpg --size=640x480
guvcview --device=/dev/video2 --format=mjpg --size=640x480

Good luck! -Acorn

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