Capturing jpegs from an h264 stream

[Alex Hewson]

On 04/06/2013 14:32, Wes Miller wrote:
> Ummm....  I haven't (can't) try this as I have no Raspberry Pi, but does this
> actually work?
>
>      raspivid -n -t 1000000 -vf -b 2000000 -fps 25 -o - |  \
>      gst-launch-1.0 fdsrc fd=0 ! decodebin ! videorate !
>      video/x-raw,framerate=1/1 ! jpegenc ! multifilesink
> location=img_%03d.jpeg
>
> First, you're mssing a \ on line 2 (or it got lost in the posting).
Yes, that was simply eaten by the posting.  The command is syntactically 
correct and does run.  But no jpegs come out of the end.


> This is a Linux pipeline married to a gst pipeline.  Does gst_launch accept
> piped input?  Are you actually producing piped input?
Yes and yes - I've tested this by directing the output of raspivid into 
a file.  It's a valid video.  The gstreamer 'fdsrc fd=0' will accept 
data on stdin and I've checked this works by outputting it again 
straight to a filesink.


>   Should the Linux pipe
> char (|) perhaps be && to launch both these commands at the same time and
> (hope, hope, hope) get gst lunch to read from stdin (fd = 0) synchronously?
Both are running at once - normal for the Linux pipes implementation.  
And the above filesync verifies it.  Data certainly is entering gstreamer.


> And when it does read, the multifilesink is going to write one file per
> jpeg/frame.  Is 1/1 a valid framerate?  What happens to the other 24 frames
> per second?  Might you need a wrapper for the raspvid line that reruns the
> command once per second so you only get 1 frame per second?
Aha, this brings us to the reason for using gstreamer - it isn't just 
the jpegs I want.  If it was I agree that calling the binary for 
generating stills (raspistill) would do the trick.  But once I have the 
jpeg capture working I want to add a 'tee' to my gstreamer pipeline and 
record a video file as well, and while raspivid has the camera device 
open nothing else can touch it.



-- 
Alex Hewson
m: +44 7895 265219 | e: mock at mocko.org.uk | Skype: alex.hewson