Capturing multiple webcams (uvcvideo) with OpenCV on Linux

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.

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Answer 1

OpenCV can be built to use either v4l or libv4l, and only the v4l version supports compressed formats, while the libv4l version supports just one uncompressed format for OpenCV 2.4.11. (See autosetup_capture_mode_v4l2() for v4l and the code following line 692 for libv4l.) OpenCV 3.0.0 does not improve much over 2.4.11 here; it still supports only uncompressed formats for libv4l.

Since your error mentions libv4l2, you seem to have the libv4l version and OpenCV captured uncompressed in your case. To build a v4l version of OpenCV, your cmake command should contain

-D WITH_LIBV4L=OFF

(WITH_LIBV4L was enabled by default for me.)

A note on bandwidth and USB. USB 2.0 (which virtually all webcams use) has a bandwidth of 480 Mbit/s. 640x480 at 30 fps and 24 bits/pixel uncompressed is about 221 Mbit/s, so one can use up USB 2.0 bandwidth quickly with uncompressed webcams. One gets 480 Mbit/s for each USB host controller, see this answer on how to list them. (USB hubs do not add host controllers, and several USB ports on a motherboard are typically connected to the same host controller. All devices and hubs connected to a host controller share the bandwidth.)

For webcams that reserve more USB bandwidth than they need, e.g., those with footnote [13] on the UVC driver page, the FIX_BANDWIDTH quirk can help. But the quirk works only for uncompressed formats (unless you do the kernel hack in Acorn's answer here). In my case (Ubuntu 14.04, many Microsoft LifeCam Cinemas at 320x240), the quirk worked when I used the libv4l version of OpenCV (four LifeCams on an ASMedia USB host controller worked well) but for the v4l version -- which I confirmed to use MJPEG -- I got a VIDIOC_STREAMON: No space left on device error as soon as I tried to capture from a second LifeCam! (For the same machine, Intel and VIA host controllers did better and each worked with two LifeCams for v4l; the LifeCam reserves 48% of USB 2.0 bandwidth.)