[FFmpeg-devel] [RFC] Frame rate emulation

Ramiro Ribeiro Polla ramiro
Mon Jun 11 16:01:50 CEST 2007

Michael Niedermayer wrote:

>On Wed, Jun 06, 2007 at 03:31:03PM -0300, Ramiro Ribeiro Polla wrote:
>>x11grab provides its own frame rate emulation inside its read_frame 
>>function. Many other grab formats will need it (GDI, VFW...), so it 
>>would be best to make a format level frame rate emulation in FFmpeg, and 
>>not each demuxer (also I believe they don't belong in the demuxers 
>>FFmpeg already provides a frame rate emulation (added for DV), but it 
>>works depending on a codec's demand, and not at a format level, like 
>>avcodec.h says:
>>    /**
>>     * Frame rate emulation. If not zero, the lower layer (i.e. format 
>>     * has to read frames at native frame rate.
>>     * - encoding: Set by user.
>>     * - decoding: unused
>>     */
>>It's in Fabrice's TODO "suppress rate_emu from AVCodecContext".
>>I don't know DV's needs for rate emulation, but couldn't it be passed to 
>>ffmpeg.c's AVOutputStream instead of AVCodecContext? It would depend on 
>>the format then, and not the codec.
>>Also, I plan to add that to AVInputStream, right before av_read_frame, 
>>where it will be useful for grabbing demuxers.
>>Can there be 2 places where rate emulation occurs (reading from the 
>>format, and writing to the codec)?
>>Is it ok for DV for the rate emulation to occur at writing to the output 
>>format, so it's checked in AVOutputStream?
>first, AVInputStream / AVOutputStream are private structs of ffmpeg.c
>libav* is used by more than just ffmpeg.c thus moving essential 
>functionality into ffmpeg.c is not ok

But ffmpeg.c is currently responsible for the actual frame rate emulation.
Anyways, it's your decision, so I won't bother with output frame rate 
emulation anymore.
I'll just leave that in Fabrice's TODO =)

>second, its the input formats / demuxers job to read the frames properly
>that includes reading them at the proper time for realtime stuff, sane
>capture APIs should provide the needed functionality to capture at a specific
>time. simply waiting in ffmpeg.c and then calling the demuxer is a incredibly
>inaccurate way to do it, keep in mind ffmpeg does encoding, writeing and
>others too, using a seperate thread would probably be needed ...

To avoid duplicating the same code in all grabbing formats that require 
it (only X11 for the moment, but VFW and GDI will also need it), is it 
ok to move the input frame rate emulation from x11grab.c to 
libavformat/utils.c under the name ff_rate_emu (or something similar)? 
Or should it be av_rate_emu?

Ramiro Polla

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