[Libav-user] avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet) why most of the time frameFinished is zero ?

Joshua Kordani jkordani at lsa2.com
Tue May 27 17:50:58 CEST 2014

On 5/27/14 11:28 AM, wm4 wrote:
> On Tue, 27 May 2014 17:10:55 +0530
> sithruk sana <get2jils at gmail.com> wrote:
>> thanks for the reply.
>> I am doing the same by queueing the packet  as described in dranger
>> tutorial http://dranger.com/ffmpeg/ffmpegtutorial_all.html
> This tutorial is prehistoric, and following it will cause you more pain
> than not. Many things in it are just wrong or uses deprecated APIs. I
> recommend pretending that this tutorial doesn't exist.
> Normally, libavcodec decodes one frame for one packet. But you need
> to deal with "codec delay": at first you won't get any decoded frames,
> even if you put in some packets. Only later (when enough frames are
> queued internally), you will get 1 decoded frame for 1 packet. (You also
> need to feed it null-packets at the end of the video to get the last
> frames.)
If I have a buffer of encoded units, which I know contains a complete 
frame of encoded input, are you saying that I may have to pass multiple 
sets of these before I receive the decoded results of the first buffer?

Or are you saying that I can submit chunks of a frame's worth of encoded 
input, and (understandably) won't get a complete frame back until I pass 
in enough data for 1 frame?

Does the term "packet" in your reply correspond to something specific, 
as in, one "packet" of h264 encoded data corresponds to one NALU, or do 
you mean, one "packet" refers to enough encoded data, no matter the 
encoder, to make one frame?

> There are others reasons why it wouldn't output a frame. If the video
> frame is "damaged", you may need to set CODEC_FLAG2_SHOW_ALL or
> CODEC_FLAG_OUTPUT_CORRUPT (forgot which one) to make lavc output them.
> Also, it's possible to drop frames by setting skip_frame, but you
> probably don't use this.
this is useful, thank you

Joshua Kordani
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