[vlc-devel] [RFC] VLC RTSP trickplay support

Rémi Denis-Courmont rem at videolan.org
Sat Feb 21 13:59:03 CET 2009

Le samedi 21 février 2009 14:37:03 jean-paul.saman at planet.nl, vous avez 
écrit :
> Currently the Qt4 interface shows a 'reverse' playback button when the
> demux supports the DEMUX_CAN_CONTROL_PACE feature. At the moment only our
> live555 demux module does use this. It basically means that the button is
> shown for every RTSP server even if it doesn't support 'Scale'
> functionality.
> According to RFC 2326 the 'Scale' header is optional and thus potentially
> not supported by every RTSP server out there. The only way to determine if
> this feature is supported by the RTSP server is to send him a PLAY request
> with a Scale header present. If the RTSP server does not support this, then
> the response will not include the Scale header.

No. The presence of the optional Scale header only means the server 
understands the Scale parameter. It does not mean that it will support any 
specific Scale value:

   The server should try to approximate the viewing rate, but may
   restrict the range of scale values that it supports. The response
   MUST contain the actual scale value chosen by the server.

Indeed, the VLC broadcast RTSP will return "Scale: 1." if it finds a Scale in 
a PLAY request. That's regardless of the client-requested Scale value 
however. Obviously it cannot support scaling in broadcast mode. Still, the 
server understands the Scale parameter.

> At the moment VLC has no clue if the RTSP server supports "Scale" or not.
> If it could then it would be possible to distinguish between RTSP servers
> that do and those that do not support 'Scale'.

That's a protocol problem. We have the exact same problem with seeking.

We cannot fix that with RTSP/1.0.

Rémi Denis-Courmont

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