Hi, I'm working on an application that uses libav for playback. I'd
like to support as many kinds of streams as possible, so I'm looking
to make my code as robust as I can. I've been reviewing Dranger's
tutorial (http://www.dranger.com/ffmpeg/tutorial05.html) and the
source for FFPlay
(http://svn.mplayerhq.hu/ffmpeg/trunk/ffplay.c?view=markup)
and I notice they both use an ugly hack where they capture the
current packet's PTS value in a user-defined get_buffer function.
According to Dranger:
However, the frame we get from avcodec_decode_video() gives
us an AVFrame, which doesn't contain a useful PTS value.
(Warning: AVFrame does contain a pts variable, but this
will not always contain what we want when we get a frame.)
I would really like to know why. It seems to me that having a useful
pts in AVFrame is one of the most important things libavcodec could do
for me. This hack of storing the current packet PTS in a global
variable and grabbing a copy of it as a side-effect of a get_buffer
function seems extremely bug-prone. Case-in-point, I was looking
through the documentation of AVCodecContext and see that there is also
a reget_buffer function that reuses the buffers of an existing AVFrame,
if possible. Does this mean that FFPlay already has a bug in it?
If a codec decided to use reget_buffer instead of get_buffer it would
bypass the PTS-copy, and end up with a new frame with an old PTS value.
I'm really hoping that this is an old bug that doesn't need a
workaround anymore...
Any insight is appreciated.
Other questions I'd love to know the answer to, if anyone knows:
* Can av_decode_video ever allocate two frames during one call?
(yes, it can only return one, but seems like in complex streams
it might internally allocate two at once, to be returned later)
* If the AVFrame->pts really isn't usable, could I just use the pts of
the first packet which does *not* result in a completed frame as
the pts of the next completed frame?
Thanks in advance,
-Mike
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