[gst-devel] Muxing subtitles to matroska [Was: Re: dvdsrc status update, sundry questions]

Michal Benes michal.benes at itonis.tv
Wed Dec 13 09:43:59 CET 2006

Mark Nauwelaerts píše v St 13. 12. 2006 v 00:03 +0100:
> Wim Taymans wrote:
> > On Fri, 2006-12-08 at 17:42 -0700, Jason Gerard DeRose wrote:
> >> Hello everyone,
> > Hi,
> >>
> >> 3) Is 'videomixer' suitable for overlaying the decoded subtitles? Once I
> >> have dvdsubdec working again, is 'videomixer' what should be used?
> > 
> > For DVD I would create a subtitle mixer that takes video on one pad and
> > the encoded subpicture on another pad. The element is purely chain
> > based, for each video picture received it blends (while unpacking) the
> > current subtitle on top of it (with matching timestamps, of course).
> > When you connect the demuxers subtitle pad directly to the subtitle
> > decoder/mixer and the raw video path of the video decoder to the mixer
> > video path, you will get the subpicture before the video frames it
> > applies to. You can insert an order-preserving queue (like decodebin2
> > does) to make sure none of the video/subtitle streams run ahead of
> > eachother (multiqueue).
> Small question on this.
> It seems that this (new) mixer element (with the techniques as given above) has
> much to do with "programming around" the limitations of collectpads (in dealing
> with sparse streams like subtitles) that is used in e.g. videomixer (?)
> Could it be helpful / somehow possible to (re-)use the techniques given above in
> a sort of upgraded collectpads that could then also be of use to muxers that
> might be interested/capable in muxing subtitles, e.g. a matroskamux ?

Hi Mark,

	I have a simple hack that does this. However it is a hack, therefore I
never tried to commit it upstream. The technique is according to

	Since collectpads is not yet handling update NEWSEGMENTS properly, I
handle them in matroskamux. I catch the update NEWSEGMENT events at
sinks and transform them to empty buffers (which I call "filler
buffers"). Collectpads class is happy with these buffers and calls
gst_matroska_mux_collected method (in which I discard filler buffers

This is the relevant part of gst_matroska_mux_handle_sink_event

      gint64 start, stop, time;
      gdouble rate;
      GstFormat format;
      gboolean update;

      gst_event_parse_new_segment (event, &update, &rate, &format,
          &start, &stop, &time);

      /* Send a filler buffer to the pad
         if an update new-segment event has been received. */
      if (update && (format == GST_FORMAT_TIME)) {
        GstBuffer *buffer = gst_buffer_new_and_alloc (0);

        GST_BUFFER_TIMESTAMP (buffer) = time;

        gst_pad_chain (pad, buffer);


	Unfortunately, I am not able to post the complete patch, because it is
mixed with my other hack and it would be complicated to separate these


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