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

Ross Finlayson finlayson at live555.com
Tue Jan 16 20:33:53 CET 2007

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.

>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 

>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.

Ross Finlayson
Live Networks, Inc.

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