[FFmpeg-devel] [PATCH] avfilter/dctdnoiz: rewrite [f/i]dct

[Ronald S. Bultje]

Hi,

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[][]. You can use the forward transforms
provided in libvpx (for vp9) or x265 (hevc), they're quite precise, and
already optimized.

Ronald