Tuesday, April 2, 2013

Software A/V connectors for gmerlin

As mentioned earlier, I programmed generic connectors for A/V frames and compressed packets. They are much more sophisticated than the old API (based on callback functions), because they also do implicit format conversion and buffer management. The result is a simplified plugin API (consisting of fewer functions) and simplified applications. The stuff is implemented in gavl (include gavl/connectors.h), so it can be used in gmerlin as well as in gmerlin_avdecoder without introducing new library dependencies. There are 3 types of modules:
  • Sources work in pull mode and do format conversion. They are used by input- and recording plugins
  • Sinks work in push mode and are used by output and encoding plugins
  • Connectors connect multiple sinks to a source
Example for the API usage
Assuming you want to read audio samples from a media file and send it to a sink. When you get an audio source (e.g. from gemerlin_avdecoder with bgav_get_audio_source()), your application can look like:

gavl_audio_source_t * src;
gavl_audio_sink_t * sink;
gavl_audio_frame_t * f;
gavl_source_status_t st;

/* Get source */
src = bgav_get_audio_source(dec, 0);

/* Tell the source to deliver the format needed by the sink */
gavl_audio_source_set_dst(src, 0, gavl_audio_sink_get_format(sink));

/* Processing loop */
  /*  Get a frame of internally allocated memory from the sink 
   *  (e.g. shared or mmamp()ed memory). Return value can be NULL.
  f = gavl_audio_sink_get_frame(sink);

  /* Read a frame from the source, if f == NULL we'll get a frame 
   * allocated and owned by the source itself
  st = gavl_audio_source_read_frame(src, &f);

  if(st != GAVL_SOURCE_OK)

  if(gavl_audio_sink_put_frame(sink, f) != GAVL_SINK_OK)

If you want to use the gavl_audio_connector_t, things get even a bit simpler:

gavl_audio_source_t * src;
gavl_audio_sink_t * sink;
gavl_audio_connector_t * conn;

/* Get source */
src = bgav_get_audio_source(dec, 0);

/* Create connector */
conn = gavl_audio_connector_create(src);

/* Connect sink (you can connect multiple sinks) */
gavl_audio_connector_connect(conn, sink);

/* Initialize */

/* Processing loop */

The gmerlin plugin API was changed to use only the sources and sinks for passing around frames. Text subtitles are transported in gavl packets, overlay subtitles are transported in video frames.

In addition to the lower level gavl converters, the sources support some more format conversions. For audio frames, we do buffering such that the number of samples per frame you read from the source can be different from what the source natively delivers. For video, we support a simple framerate conversion, which works by repeating of dropping frames.

The video processing API is completely analogous to the audio API described above. For compressed packets, things are slightly different because we don't do format conversion on compressed packets.

A number of gmerlin modules (e.g. the player and transcoder) are already converted to the new API. In many cases, lots of redundant code could be kicked out, so the resulting code is much simpler and easier to understand.

No comments: