Multiple webcam issues

[schmidtbag]

I've been working on this project that ultimately results in recording from
2x 1080p webcams side-by-side on the same video, but gstreamer seems to be
struggling with this despite using known-working commands and hardware that
works fine with other applications.  I have two problems in particular, and
I'm gussing they're both related.

The webcams in question are the Logitech C615, and my OS is Arch Linux 64
bit.

1.  When initializing gstreamer, it takes roughly 15 seconds to start doing
anything as long as these cameras are involved.  So far it doesn't seem to
matter what I change in the command, the delay is always the same.  For
example, this:
gst-launch-1.0 v4l2src device=/dev/video1 !
video/x-raw,framerate=30/1,width=1280,height=720 ! xvimagesink

The performance is fine, CPU usage always remains very low, but the delay is
quite long.  If I tell it to record to a file, it's actually recording the
entire time, but it's only showing the first frame during the those 15
seconds.  When using a different webcam, there is no delay.

2.  If I try doing a side-by-side recording, things get much worse.  I tried
doing a very basic side-by-side recording.  If I set both input sources to
both cameras, only one camera turns on and gstreamer eventually just
self-closes.  If I do one webcam and one "videotestsrc", it will run but the
framerate is abysmal.  Example:
gst-launch-1.0 v4l2src device=/dev/video1 !
vido/x-raw,framerate=30,width=640,height=480 ! m.sink_0   videotestsrc !
video/x-raw,width=640,height=480 ! m.sink_1   videomixer name=m
sink_0::xpos=0 sink_1::xpos=640 ! video/x-raw,width=1280,height=480 !
xvimagesink

Note the CPU usage is still very low.  When using a different webcam, the
frame rate is very smooth.


Any suggestions?  ffmpeg doesn't seem to have these problems, but I intend
to run these cameras on a platform where only gstreamer can run smoothly
(when it actually works).



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