[FFmpeg-devel] Sponsoring feature for H.264 decoding with scaling factor (1/2, 1/4...) (if possible)

[Michael Niedermayer]

On Sun, Jun 19, 2016 at 09:59:22AM +0200, Eric Beuque wrote:
> 2016-06-18 13:46 GMT+02:00 Michael Niedermayer <michael at niedermayer.cc>:
> 
> > On Fri, Jun 17, 2016 at 07:26:59PM +0200, Eric Beuque wrote:
> > > 2016-06-17 19:16 GMT+02:00 Michael Niedermayer <michael at niedermayer.cc>:
> > >
> > > > On Fri, Jun 17, 2016 at 05:39:23PM +0200, Eric Beuque wrote:
> > > > > Hi,
> > > > >
> > > > > i'm posting here for a feature that is missing in ffmpeg (or may be i
> > > > > missed something), which consist of decoding H.264 frame with a
> > scaling
> > > > > factor of 1/2, 1/4, 1/8...
> > > > >
> > > > > I found the parameter lowres, which works well with MJPEG stream but
> > it's
> > > > > not working with the H.264 decoder.
> > > > >
> > > > > I don't know if it's something possible to implement in the decoder,
> > but
> > > > if
> > > > > yes, my compagny agreed to sponsor the feature (depending on the
> > cost of
> > > > > course), if a developer qualified is interested to do it.
> > > > >
> > > > > Is someone know if it is possible, and if it can exist someone
> > interested
> > > > > to develop this feature?
> > > >
> > > > is it acceptable if the encoder enforces some restrictions on the used
> > > > features ?
> > > >
> > > > most general h264 likely cannot efficiently be decoded in lower
> > > > resolution with acceptable quality
> > > > Restricting the used intra modes may make it possible to do it though
> > > > i dont know what the quality would be but better than without
> > > > restrictions
> > > >
> > > > [...]
> > > >
> > >
> > >
> > > Actually, the main goal is for motion detection algorithm, so we need
> > small
> > > resolution and best quality is not required. We just need good estimation
> > > for the moving pixel.
> > >
> > > For now, we decode in H.264, and scale it close 320x240, the then we
> > > perform motion detection on it. The goal is to speed up the process and
> > > reduce the memory usage since decoding a 2048x1536 picture lead around a
> > 5
> > > MB for the decoded image in memory.
> > >
> > > So i think it could be OK.
> >
> > i dont think that withut restrictions it would be good enough for
> > motion detection
> >
> 
> I don't really understand what will be changed in the decoder with or
> without restriction...

some of the features of h.264 will not decode "right" without the
full resolution


> But do you think it is possible to get (with or
> without the restriction) an acceptable result with saving memory and/or CPU
> time?

memory usage should be reduced, cpu probably less so, the whole
bitstream still needs to be decoded.

also if you are in control of the encoder to restrict what it does
it may be simpler to use mpeg4-asp, H.263 or mpeg2 which have lowres
support in FFmpeg without these problems

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The misfortune of the wise is better than the prosperity of the fool.
-- Epicurus
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