[Libav-user] Black frames

orestesb orestesb at comcast.net
Thu Feb 11 00:15:22 CET 2016


Somebody know how can i add black frames with some text to obtain a output video.
Thank you

Sent from my T-Mobile 4G LTE Device

-------- Original message --------
From: Dirr Bernhard <Bernhard.Dirr at newtec.de> 
Date: 2/10/2016  4:29 PM  (GMT-05:00) 
To: "This list is about using libavcodec, libavformat, libavutil, libavdevice and libavfilter." <libav-user at ffmpeg.org> 
Subject: Re: [Libav-user] Keeping all frames in mp4 with h264 codec 

Hello Arthur,
fine to hear about your success.
I am not "The Master of libav", I just used it, and got some help from community. You surely searched the documentation on ffmpeg.org. How often you have to call avcodec_encode_video2 after providing last frame depends on codec. When it uses heavy compression by coding only the differences between successive frames, you have to call the function more often, and its ok, when you get got_packet as false for the first few frames. It is the codecs responsibility, when it decides to output some data. At end of coding process you have to ensure to get all retained output from codec.

Your question 1) You have to query codec at end of frames, until it has no more data to output.
Your question 2) libav does not know, how many packets the codec has pending. So you have to "loop", until you get no more data.
Your questions 3 & 4) You should always call avcodec_encode_video2 with NULL after encoding real frames, independend of used codec, until you get no more data.

Here I would like to thank the ffmpeg/libav community for there engagement. They provided the solution to the requirements in my project:
- lossless codec: FFV1
- direct output to FTP without the option to reposiotion to some tables at the beginning of container: NUT Container

regards and good luck to your project

Von: Libav-user [libav-user-bounces at ffmpeg.org] im Auftrag von Arthur Muller [muller at vki.com]
Betreff: Re: [Libav-user] Keeping all frames in mp4 with h264 codec

Hi Bernhard,

>after real frames are handled did you call avcodec_encode_video2 with NULL as frame as many times as got_packet returns true?
>Depending on codec you have to "flush" encoder several times, until all frames have gone to output.

Success!! Thanks!

But I do need one clarification: it just so happens that I have 36 frames, half return got_packet=0 and half return got_packet=1. So I called avcodec_encode_video2 18 times with a NULL frame. So here are my questions:

1) You mention I call this function with a NULL frame as many times as got_packet returns true (=1). Shouldn’t it be the other way around? That is, I should only flush as many times as got_packet returns false (=0)?

2) Is there a way to know exactly how many times I should call this function with a NULL frame? I can certainly count the number of got_packets={0 or 1}; but I’m wondering whether the ffmpeg api wouldn’t have that information itself in a way that would be more self-contained.

3) Is it safe to call this function with a NULL packet several times – even more than necessary to flush everything?

4) Finally, is it safe to call this function several times for other codecs – even if nothing needs to be flushed? In other words, would ffmpeg treat this call as a no-op if it’s not necessary?

Thanks again!

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