[FFmpeg-devel] About Unchanged Frames

Mike Melanson mike
Sun May 11 03:14:55 CEST 2008

Michael Niedermayer wrote:
> On Fri, May 09, 2008 at 11:42:01PM -0700, Mike Melanson wrote:
>> Hi,
>> The type of material I am trying to encode with ZMBV has a lot of 
>> unchanged frames. It seems that the best ZMBV can do with unchanged 
>> frames is 11-12 bytes. I seem to recall that it's possible to stuff an 
>> AVI file with an empty frame (i.e., a '00dc' chunk with a 0-length 
>> payload) that the demuxer will interpret as an empty frame (I have some 
>> H.264 AVI files that do this). How can an encoder communicate to FFmpeg 
>> that the AVI muxer should mux a 0-length frame? When I try to return 0 
>> from the encode_frame() function, FFmpeg construes that as a skipped 
>> frame and doesn't mux it.
> The avi muxer will add 0 frames as needed depending on dts.
> Anyway try skip_threshold,
> if it works (with mpeg4) then your code is buggy
> if it doesnt then mine is :)

This gave me a little to work with, although they seem unrelated (dts 
and skip threshold). I see there is an option called -skip_threshold and 
a few associated options and that mpegvideo_enc.c cares about them. So I 
can see how the parameters for comparing the current frame with the 
previous can be tweaked. But I still don't see how an encoder can 
communicate to the muxer that a frame is unchanged and that it's safe to 
mux an empty frame.

As a test, the file ( http://samples.mplayerhq.hu/fli-flc/a.fli ) has a 
reasonable number of unchanged frames.

ffmpeg -y -i a.fli a-mpeg4.avi
ffmpeg -skip_threshold 10 -y -i a.fli a-mpeg4.avi
ffmpeg -y -i a.fli -skip_threshold 10 a-mpeg4.avi

I chose threshold 10 at random. I was aiming for something non-zero, 
even though I have determined that a.fli has a number of frames 
unchanged from the previous. None of the resulting a.fli files contained 
  0-length frames. The smallest were either 36 or 40 bytes.

	-Mike Melanson

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