[gst-devel] Image conversion through a pipeline

[Tim-Philipp Müller]

On Thu, 2009-04-30 at 16:10 +0200, Olivier Aubert wrote:

Hi,

> Two years ago, I posted some code
> ( http://article.gmane.org/gmane.comp.video.gstreamer.devel/17603 ) that
> allowed me to convert a frame (from the playbin 'frame' property) into
> PNG, through a fakesrc/fakesink-based pipeline. A condition variable is
> used to "transform" the asynchronous gstreamer behaviour into a
> synchronous "snapshot" call.
> 
> It has worked perfectly for 2 years, and stopped working in recent
> gstreamer releases (at least 0.10.22). It appears that the "handoff"
> signal from fakesink is generated only on its first occurrence (i.e. the
> first time a snapshot is taken), but never after.
> I have tried to use True or False as return values for the handoff
> signal handler, to no avail.
> 
> I know that the preferred way is now to use appsrc/appsink, but the
> fakesrc/sink version is still more generic for the moment (i.e. the
> Debian packages have only recently began to ship appsrc/appsink), so I
> would like to understand what changed in gstreamer to break my code.
> 
> Does anyone have a clue ?

I'm not sure what exactly has changed, but I'm surprised your code
worked all this time before, since what it does is really quite, erm,
unusual, esp. the bit where you insert buffers in the middle of the
pipeline by pushing the snapshot buffer onto the queue pad from the
application thread.

Anyway, so basically when you set the conversion pipeline to PLAYING,
fakesrc will start a streaming thread and start pushing zero-sized
buffers with no caps on them downstream. Videoscale should/will reject
these [no caps, 0 size] and the fakesrc streaming thread will stop with
a not-negotiated flow error; it will also push an eos event downstream
when it does that. This in turn will make basesink drop any buffers you
push into the pipeline after it has stopped streaming:

gstbasesink.c:2931:gst_base_sink_chain_unlocked:<sink> we are EOS,
dropping object, return UNEXPECTED

If for whatever reason you don't want to use appsrc, then you need to
use fakesrc and fakesrc's "handoff" callback rather than just push
buffers into the pipeline in the middle from other threads. Or write
your own source deriving from GstPushSrc.


> Subsidiary question: how come I cannot do a "get_state" on the
> conversion pipeline ? ( fakesrc name=src ! queue name=queue !
> videoscale ! ffmpegcolorspace ! video/x-raw-rgb,width=160 ! pngenc !
> fakesink name=sink signal-handoffs=true ). 

Why "can't you"? I mean, what happens? What did you expect to happen -
etc.?

Cheers
 -Tim