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I bought a new usb webcamera,plug it .

ls -l /dev/video*
crw-rw----+ 1 root video 81, 0 Nov 21 10:57 /dev/video0
crw-rw----+ 1 root video 81, 1 Nov 21 10:57 /dev/video1

Why only one usb camera shows two Video4Linux devices?

showkey
  • 342

1 Answers1

3

Very shortly
Your application should use /dev/video0, for usual stuff, as /dev/video1 gives you meta-data image, to be used by algorithms, to improve image quality.

A bit more deeply

Look at the Device caps section, as they are different for both devices:

$ v4l2-ctl -D -d /dev/video0
...
Device Caps   : 0x04200001
    Video Capture
    Streaming
    Extended Pix Format

$ v4l2-ctl -D -d /dev/video1
...


Device Caps   : 0x04A00000
    Streaming
    Extended Pix Format

The device caps is documented as follows:

  * @device_caps:  capabilities accessed via this particular device (node)

The bits, that are different for these are:

 #define V4L2_CAP_VIDEO_CAPTURE     0x00000001 # bit 1
 #define V4L2_CAP_META_CAPTURE      0x00800000 # bit 23

The applications need to check this in order to know what kind of buffer it will receive, as this should be used by the streaming ioctls: VIDIOC_REQBUFS, VIDIOC_QBUF, VIDIOC_DQBUF

Read more about this discussion from https://bugzilla.kernel.org/show_bug.cgi?id=199575

Andri Busch
  • 298