Re: [gst-devel] Image conversion through a pipeline

[Olivier A. <oli...@li...>]

Hi Tim

Thanks for the answer and indications. I know that my solution was
hackish, but it indeed worked perfectly for the last 2 years. I will
have a quick go at the fakesrc 'handoff' variation, but I think I will
eventually use appsrc/appsink since it is the more appropriate
approach. 

For the get_state() issue, the call locks and never returns. This is
probably due to my weird buffer injection into the pipeline.

Cheers
Olivier

On Sat, 2009-05-02 at 17:28 +0100, Tim-Philipp Müller wrote:
> 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
> 
> 
> 
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