The VideoLAN Forums

Posted: **21 May 2015 12:44**

Hello, i'm using libvlc 2.0.8 on Ubuntu and I'm trying to read a file with 2 video tracks. The goal is to recompose the frame from track 1 with the frame from track 2.

Right now I'm able to select which track to read with libvlc_video_set_track and save frames in memory by setting my own callbacks with libvlc_video_set_callbacks

My question is how can I read both tracks at the same time ?

Do I have to set different callbacks for each track or are they handled by one callback ? I tried both cases without success.

1. Different callbacks for each track (pseudo code):

Code: Select all

libvlc_video_set_track(0)
libvlc_video_set_callbacks(lock_cb0, display_cb_0, unlock_cb0)

libvlc_video_set_track(1)
libvlc_video_set_callbacks(lock_cb1, display_cb_1, unlock_cb1)

It seems like only the last set of callbacks are taken into account.

2. One set of callbacks for both tracks (pseudo code):

Code: Select all

display_cb(void *opaque, void *picture)
{
	print libvlc_video_get_track() // always returns the same track id.
	myfifo.push(picture)
}

Although libvlc_video_get_track always returns the same track id, the frames stored in myfifo are actually from track 1 and 2. So how can I know exactly where the decoded frame came from ?

Any help is appreciated, thanks !

Posted: **21 May 2015 13:11**

1. The whole point of selecting a track is to play only that track. Don't do that.
2. You have to use the format (setup) callback if you need to match buffers from the same output. Outputs and buffers are not directly mapped to tracks.

Posted: **21 May 2015 21:06**

Thanks for the help !

So If I understand correctly I should define a callback like this ?

Code: Select all

unsigned my_setup_cb(void **opaque, char *chroma, unsigned *width, unsigned *height, unsigned *pitches, unsigned *lines) 
{
	int track_id = libvlc_video_format_cb_get_track_id(opaque); // Returns the track ID of the current decoded frame ?
	if( track_id == 0 )
		do_something();
	else
		do_something_else();
	...
}

libvlc_video_format_cb_get_track_id is not defined in my version of libvlc but I found that info here : http://vlc-devel.videolan.narkive.com/J ... p-callback
So I may have to update my libvlc.

Then call libvlc_video_set_format_callbacks instead of libvlc_video_set_format

Is that correct ?
By the way, I don't really know what should opaque be ?

opaque : pointer to the private pointer passed to libvlc_video_set_callbacks() [IN/OUT]

Posted: **21 May 2015 22:03**

Again, if you want to map lock, unlock and display callbacks to a given video output instance, then you need the setup callback. There exists no mappings between track ID and output instance, for various reasons.

The first parameter to setup is a pointer to a void pointer. The initial void pointer value is that which you passed to libvlc_video_set_callbacks(), but you can change it so that each output has its own private data.

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