[FFmpeg-devel] [PATCH] avfilter/dctdnoiz: rewrite [f/i]dct
u at pkh.me
Mon Aug 4 17:59:22 CEST 2014
On Mon, Aug 04, 2014 at 11:44:43AM -0400, Ronald S. Bultje wrote:
> On Mon, Aug 4, 2014 at 11:42 AM, Clément Bœsch <u at pkh.me> wrote:
> > On Mon, Aug 04, 2014 at 11:17:29AM -0400, Ronald S. Bultje wrote:
> > > Hi,
> > >
> > > On Sun, Aug 3, 2014 at 4:27 PM, Clément Bœsch <u at pkh.me> wrote:
> > >
> > > > This removes the avcodec dependency and make the code almost twice as
> > > > fast. More to come.
> > > >
> > > > The DCT factorization is based on "Fast and numerically stable
> > > > algorithms for discrete cosine transforms" from Gerlind Plonkaa &
> > > > Manfred Tasche (DOI: 10.1016/j.laa.2004.07.015).
> > >
> > >
> > > I have no comments on the patch itself, but can you explain why we're
> > > re-implementing a custom f/idct rather than using the one provided in
> > > lavcodec? It seems to me that going from fixedpoint/simd'ed to float/c
> > > would be slower, not faster, so there must be more to this patch than
> > what
> > > I'm getting from it...
> > >
> > OK so as said in private, I didn't find an accurate (not wrongly "JPEG"
> > like I originally said) 16x16 DCT in libavcodec.
> > You suggested to use the HEVC or VP9 DCT. That's indeed one solution, but
> > we currently have only IDCT for those (AFAIK), and I needed a float
> > implementation.
> You mean forward. idct is inverse, fdct is forward, such that
> idct(fdct(data)) =~ data.
Yeah sure I meant we have only the IDCT and I also needed the FDCT. The
"float implementation" was another point.
> You can use the forward transforms
> provided in libvpx (for vp9) or x265 (hevc), they're quite precise, and
> already optimized.
Yeah so basically I would have to maintain that port instead of my
implementation, which doesn't look ideal either (the point of using an
existing code in FFmpeg is that its maintenance would have been shared).
Also, in order to use them, it would make sense to port the whole filter
to fixed point. And unfortunately between the IDCT-average part and the
DCT 3x3 for color [de]correlation, I wasn't really confident to do that
regarding the accuracy (which you definitely wants for a non real-time
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