in reply to Perl wrapper for v4l2-ctl

Hi stevieb, I played around with it awhile back and found a couple of problems which you maybe able to overcome. First with v4l2 I was only able to grab single pictures at a time, however, they could easily be assembled into a video. Second, I could not find an efficient way to display the video in reatime, and save it simultaneously. My advice would be to use mplayer or mencoder, which can be run in slave mode for scripting.

I have a raspberry pi too, and I love it. If I were attempting it, I would use either mplayer or mencoder run by perl in slave mode, which you can see a sample of in ztk-v4l-video-bloger/recorder. That link shows the use of slave mode, but the v4l part is outdated. It may help you though. Basically you get a window id of the Tk container, the canvas is easiest, and pass that window id to mplayer in slave mode.

I have 2 working mplayer snippets which you may find useful. First is just display the camera in realtime. 

mplayer -tv  device=/dev/video0:fps=29.97 tv://

[download]
The following nicely records video and audio 
mencoder \                                                            
+                                                                 
    tv:// \                                                           
+                                                                 
    -tv driver=v4l2:device=/dev/video0:fps=30:width=640:height=480:for
+ceaudio:alsa:adevice=hw.0,0 \                                    
    -ovc lavc \                                                       
+                                                                 
    -oac lavc \                                                       
+                                                                 
    -ffourcc divx \                                                   
+                                                                 
    -noskip  -mc 0 -ofps 30 \                                         
+                                                                 
    -o record-compressed-good-sync.avi

[download]

The problem is to get them to work simultaneously, so you can have a realtime Tk display and save to a file as you go. The mplayer and mencoder options show a -dumpstream and -dumpfile option, but I as yet have gotten them to work together. Some have suggested using a tee for output, but no luck there either. Maybe you will be the one to discover the coding trick needed.:-). So, I recommend using a c program to do the heavy video work, rather than Perl.

Here is an informative link describing the problem in full. play record same time.

I'm not really a human, but I play one on earth. ..... an animated JAPH

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Re^2: Perl wrapper for v4l2-ctl
by stevieb (Canon) on May 15, 2017 at 19:53 UTC

    Thanks to both of you for the replies.

    I didn't realize that streaming to two locations simultaneously (display and file) would be problematic.

    I ordered my camera from Amazon Canada this morning and it should be here in a couple of days. At that time I'll commence playing around with various options.

    On another note, my GPS receivers came in today, so I'll be picking that up after work, so hopefully I'll have a Perl module for the GPS side of things (wrapped C, of course). One will be used with the Pi for the same project (so I can embed the coordinates in the video directly), and the other for my Arduino Trinket Pro, as I'm making my own GPS device for when I'm out on my days-long mountain hikes (yes, I could buy one, but where's the fun in that ;)

[reply]
      Hi, you know, in terms of development time and cost, the easiest solution would be to have 2 raspberry pi's and 2 cameras. One to record, and one for realtime display. :-) What does a Pi and a camera cost $50? Plus you would have a backup to record with if your main recorder failed. Is $50 worth all the head-scratching required to get just 1 Pi to do it all? Like a few weeks of time for $50? :-)

      I also came across something for you to google around for. It is claimed that Gstreamer, which has a Perl module, has the ability to duplex the video feed from one camera. That is what we need, take 1 camera output and split it into 2 identical streams. It dosn't seem it should be that hard, but it eludes us for now. ffmpeg seems to be able to do it, see ffmpeg multiple outputs. I'm going to play with that.

      UPDATE0 These 2 links are the most promising leads. Use a kernel module 2 split the /dev/video stream, then use whatever to display 1 stream and record another. v4l2loopback module and splitting /dev/video to 2 streams .

      UPDATE1It works! Here is how to split /dev0 into /dev/video1 and /dev/video2 

      modprobe v4l2loopback devices=2   # as root
 # then                
ffmpeg -f v4l2 -i /dev/video0 -f v4l2 /dev/video1 -f v4l2 /dev/video2

      [download]
      Now you can watch /dev/video1 in realtime, and record /dev/video2. :-)
      I'm not really a human, but I play one on earth. ..... an animated JAPH
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