[FFmpeg-devel] [RFC] Frame rate emulation

[Michael Niedermayer]

Hi

On Mon, Jun 11, 2007 at 11:01:50AM -0300, Ramiro Ribeiro Polla wrote:
> Michael Niedermayer wrote:
> 
> >Hi
> >
> >On Wed, Jun 06, 2007 at 03:31:03PM -0300, Ramiro Ribeiro Polla wrote:
> >  
> >
> >>Hello,
> >>
> >>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 
> >>themselves).
> >>
> >>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 
> >>handler)
> >>     * 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?

it should be moved into a new file so its not compiled if no grabing
interface is

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I do not agree with what you have to say, but I'll defend to the death your
right to say it. -- Voltaire
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