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

Derk-Jan Hartman hartman at videolan.org
Wed Jan 17 13:58:34 CET 2007

On 16-jan-2007, at 20:33, Ross Finlayson wrote:

> At 3:45 PM +0100 1/16/07, Derk-Jan Hartman wrote:
>> can we please not use the term "trickplay" ?? it's too confusing,  
>> spans too many topics etc.
> I'm surprised to hear this, because the term 'trick play' or 'trick  
> mode' seems to be in widespread use, to refer to the fast-forward  
> and reverse-play operations, at least.

Trickplay is a name for a set of features that originally stemmed  
from an mpeg2 trick to do FFW/RW. It's a term without technical  
meaning and doesn't specify what happens with timelines, bitrates etc  

That is my problem with it.


>> To decode: We need to support the optional paramaters  
>> start,end,scale of playMediaSession
>> start is only in use atm when you do seeking. (DEMUX_SET_POSITION)
>> Just so i can get a few things clear.
>> 1: if i set start=0, and the sdp specifies a starttime, is this  
>> still automatically taken into account?
> Mumble...  It turns out that the LIVE555 SDP parsing code currently  
> ignores the start time in the SDP "a=range:" attribute (it parses  
> only the end time).  However, I have yet to see a stream whose SDP  
> description contains anything other than 0 as the start time.  So,  
> for now at least, you should just assume that streams always start  
> at 0.
>> 2: scale: this corresponds to normal FFW, RW functionality  
>> correct? not to faster/slower (this would be speed?)
> Yes.  "scale" just affects the content that's generated by the  
> server, and the playing position within the file (e.g., as shown by  
> the GUI's cursor).  It doesn't change the rate at which timestamp  
> information appears in the data.
>> 3: is there a way to know wether or not Scale is a supported  
>> option by the server ?
> There's no way to know this until after the server responds to the  
> RTSP "PLAY" command; the actual scale used by the server will  
> appear in the "PLAY" response.  The LIVE555 code does parse this,  
> however. You can access it (after you've called "playMediaSession 
> ()") by calling "p_sys->ms->scale()" (this will be a 'float').
> So, you should call "playMediaSession()" by passing the scale value  
> requested by the user (default value: 1.0f).  Then, call "p_sys->ms- 
> >scale()" to get the actual scale value used by the server, and use  
> this to update the GUI.

Mmm, ok. We'll see.


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