[vlc-devel] Re: VLC RTSP client support for Transport Stream 'trick play' operations

Glen Gray glen at lincor.com
Tue Jan 16 16:57:45 CET 2007

Derk-Jan Hartman wrote:
> VLC currently does FFW/RW by simply doing a seek. I need to put the 
> scale func. as a preferred method over the current functionality in the 
> core, which would be some simple additions. Glen Gray should already 
> have some code to start this of as a matter of fact.

My patches are rather out of date with the current branch and where very 
much specific to our own implementation requirements with regard to 
Kasenna servers.

We control vlc via the rc interface, and as such most of our interface 
changes have been done from that perspective. My patches over-rode the 
rc versions of the fastforward and rewind command to allow you to pass 
over a speed, 6, 12, 30 for example. If no values where entered, the 
INPUT_RATE_MAX|MIN values where set based on if fastforward|rewind 
command was issues.

I added a DEMUX_SET_RATE to the livedotcom.cpp Control function. The 
DEMUX_SET_RATE part of control would look for a passed in i_rate 
variable. If the i_rate variable was set to INPUT_RATE_MAX it would set 
a default of 30 (this is Kasenna specific). If it was INPUT_RATE_MIN, it 
would set a default of -30 (again, Kasenna specific).

I also had to add some code to vlc/src/input.c where it deals with the 
INPUT_CONTROL_SET_RATE_SLOWER|FASTER. It could call demux2_Control with 
a DEMUX_SET_RATE and check for a return value. If we didn't get a 
VLC_SUCCESS, it would carry on with the normal RATE control code.

In the liveMedia RTSP demuxer, using the media session, it would Pause 
the stream, set the scale value and offsets and resume playback to enter 
into Fastforward|Rewind mode.

Here's a sample of my code....

     i_rate = (int)va_arg( args, int );

     if( i_rate == INPUT_RATE_MAX )
       i_rate = 30;
     else if( i_rate == INPUT_RATE_MIN )
       i_rate = -30;
     else if( i_rate == INPUT_RATE_DEFAULT || i_rate == 0 )
       i_rate = 1;

     //First of all we need to Pause the stream
     if( !p_sys->rtsp->pauseMediaSession( *p_sys->ms ) )
         msg_Err( p_demux, "PAUSE failed %s",
                           p_sys->env->getResultMsg() );
         return VLC_EGENERIC;

     //Passing -1 for the start and end time will mean liveMedia won't
     //create a Range: section for the RTSP message. Server should pick
     //up from current position.
     if( ( !p_sys->rtsp->playMediaSession( *p_sys->ms,
                                           -1, -1, i_rate ) ) )
             msg_Err( p_demux, "PLAY with Scale %d failed %s",
                               i_rate, p_sys->env->getResultMsg() );
             return VLC_EGENERIC;

     /* Resync the stream */
     p_sys->i_start = (mtime_t)(f * (double)p_sys->i_length);
     p_sys->i_pcr_start = 0;
     p_sys->i_pcr       = 0;
     es_out_Control( p_demux->out, ES_OUT_RESET_PCR );
     return VLC_SUCCESS;

There was of course plenty of other pieces to the puzzle to get this to 
work, starting Dermot McGahon initially working with Derk-jan and Ross 
to get Kasenna playback and the foundations for the Scale support into 
liveMedia. All of which Ross has included in the liveMedia codebase as 
we've made submissions.

As stated, my patches are against a rather old version of VLC trunk. 
Dermot had made a push before Xmas to get as many of our patches 
accepted upstream and it's been left to me to get our trickplay support 
upstream too. Although, recently I've had far too much on project wise 
to even attempt this, I'm in a position now where at least I can spend 
some personal time on it in the evenings if work is too loaded (which it 
usually is)

We've been very keen to contribute this support back to VLC and liveMedia.
Glen Gray <glen at lincor.com>              Digital Depot, Thomas Street
Senior Software Engineer                            Dublin 8, Ireland
Lincor Solutions Ltd.                          Ph: +353 (0) 1 4893682

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