[vlc-devel] VLC and ff / rewind, again ...
j.zorko at att.net
j.zorko at att.net
Sun May 4 03:11:52 CEST 2003
Hello, all ...
I am tasked to make VLC, while acting as a media server (not decoding / playing the media), able
to respond to fast-forward and rewind commands, and do the right thing. It was easy enough
adding this functionality on a purely temporal basis ... rewind seeks back n seconds, fast-forward
seeks forward n seconds, and this keeps happening until told to stop, and I merely changed the
remote control interface to call input_Seek( p_input, i_new_pos, INPUT_SEEK_SECONDS) in between
the calls to select(). This way the input thread gets the chance to read some data, the FIFO gets
filled, the output module gets to send this data out to the network, and the end result is that any
client VLCs tuning in to the server will hear something representative of the fast-forward or rewind
Now, this is all cool, but for video it really stinks -- seeking to some arbitrary time in the media
file tends to a.) produce some really jerky or bad video artifacts in any client VLCs tuned in to the
server VLC, and sometimes the client VLCs even crash (decoder fed data it wasn't expecting,
perhaps). So I need to extend this server-based rewind / ff mechanism to rewind not based on
time, but (for video at least) based on a start code (I frames / GOPs perhaps).
VLC is not built for this, evidently -- the decoder is not instantiated in a VLC acting only as a
server, the decoder lives on the other side of the FIFO wall, etc. So i'm implementing a new way to
seek on the input stream, by calling input_Seek( p_input, i_new_pos, INPUT_SEEK_STARTCODE).
I've got the plumbing all in there so far, using the VLC ways of doing things, and i've added a
Seek_Special() method to modules/access/file.c that gets called when I want it to. This function
will basically to a lseek() backwards or forwards, read some data, find the start code it's looking for
(or the nth occurence of the start code it's looking for), do one final lseek() to that point, and
return. Everything is done and working except the last (lseek()) part (the Seek_Special() function
basically seeks forward or backward 1000 bytes at a time and returns, it doesn't do the read and
look for the start code yet, but this is simple enough to add).
OK ... you know, this feels wrong, though. This is basically duplicating a bit of the functionality of
the decoder stuff that lives on the other side of the FIFO, but only enough to decode the media
down to a specific start code. Since neither VLC nor VLS does RTSP, for instance, this seems like
the only way I can do it, after studying the VLC source for a few days. Can anyone who is
intimately familiar with VLC tell me if there is a cleaner way to do this?
John, VLC hacker (who wants to do it right)
Falling You - exploring the beauty
of voice and sound
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