# [FFmpeg-devel] [PATCH] RDFT for Bink and QDM2

Michael Niedermayer michaelni
Thu Jan 22 19:35:28 CET 2009

On Thu, Jan 22, 2009 at 01:08:50PM -0500, Alex Converse wrote:
> On Thu, Jan 22, 2009 at 12:58 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> > On Thu, Jan 22, 2009 at 11:56:24AM -0500, Alex Converse wrote:
> >> On Wed, Jan 21, 2009 at 6:14 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> >> > On Wed, Jan 21, 2009 at 06:03:36PM -0500, Alex Converse wrote:
> >> >> On Wed, Jan 21, 2009 at 5:53 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> >> >> > On Wed, Jan 21, 2009 at 04:45:09PM -0500, Alex Converse wrote:
> >> >> >> On Wed, Jan 21, 2009 at 4:25 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> >> >> >> > On Wed, Jan 21, 2009 at 01:12:30PM -0500, Alex Converse wrote:
> >> >> >> >> On Wed, Jan 21, 2009 at 8:36 AM, Michael Niedermayer <michaelni at gmx.at> wrote:
> >> >> >> >> >
> >> >> >> >> > On Tue, Jan 20, 2009 at 03:19:46PM -0500, Alex Converse wrote:
> >> >> >> >> > > On Tue, Jan 20, 2009 at 1:50 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> >> >> >> >> > > > On Tue, Jan 20, 2009 at 12:24:06PM -0500, Alex Converse wrote:
> >> >> >> >> > > >> On Tue, Jan 20, 2009 at 11:52 AM, Michael Niedermayer <michaelni at gmx.at> wrote:
> >> >> >> >> > > >> > On Mon, Jan 19, 2009 at 07:44:38PM -0500, Alex Converse wrote:
> >> >> >> >> > > > [...]
> >> >> >> >> > > >> >
> >> >> >> >> > > >> > [...]
> >> >> >> >> > > >> >
> >> >> >> >> > > >> >> +/**
> >> >> >> >> > > >> >> + * Sets up a real FFT.
> >> >> >> >> > > >> >> + * @param nbits           Log2 of the length of the input array
> >> >> >> >> > > >> >
> >> >> >> >> > > >> >> + * @param inverse         If TRUE, perform the inverse of the transform
> >> >> >> >> > > >> >
> >> >> >> >> > > >> > i suggest if 0 perform the forward transform, if 1 perform the inverse
> >> >> >> >> > > >>
> >> >> >> >> > > >> I believe these are equivalent.
> >> >> >> >> > > >
> >> >> >> >> > > > I searched for TRUE in the iso C standard and found no match.
> >> >> >> >> > > > But lets assume it where defined as 1, this doesnt document any
> >> >> >> >> > > > other value, is inverse=2 or 0 invalid? the forward transform?
> >> >> >> >> > > > 99% of the people will guess correct but why write something ambigous if
> >> >> >> >> > > > it can be written unambigous ...
> >> >> >> >> > > >
> >> >> >> >> > > >
> >> >> >> >> > > >>
> >> >> >> >> > > >> >
> >> >> >> >> > > >> >
> >> >> >> >> > > >> >> + * @param sign_convention The sign of j of the forward FFT.
> >> >> >> >> > > >> >
> >> >> >> >> > > >> > i do not understand this, j is a variable that has no clear relation to a FFT
> >> >> >> >> > > >> >
> >> >> >> >> > > >>
> >> >> >> >> > > >> j is the unit vector in the vertical direction on the complex plane. j
> >> >> >> >> > > >> = sqrt(-1).
> >> >> >> >> > > >
> >> >> >> >> > > > wasnt that i, i for imaginary?
> >> >> >> >> > > >
> >> >> >> >> > > >
> >> >> >> >> > > >>
> >> >> >> >> > > >> The forward DFT can be defined as:
> >> >> >> >> > > >> X_k = \sum_{n=0}^{N-1} x_n e^{-{2\pi j \over N} nk }
> >> >> >> >> > > >>
> >> >> >> >> > > >> It also can be defined as
> >> >> >> >> > > >> X_k = \sum_{n=0}^{N-1} x_n e^{{2\pi j \over N} nk }
> >> >> >> >> > > >
> >> >> >> >> > > > you can, but why would we need it?
> >> >> >> >> > > >
> >> >> >> >> > >
> >> >> >> >> > > Bink uses it. As per the comments, the inverse of this transform is
> >> >> >> >> > > not simply the transform with the opposite sign convention.
> >> >> >> >> >
> >> >> >> >> > maybe iam too tired but i see just one transform
> >> >> >> >> >
> >> >> >> >> > X[  k] = sum(n=0..N-1) x[n] e^(-2pi*i*n*k / N)
> >> >> >> >> >        | k= N-m
> >> >> >> >> > X[N-m] = sum(n=0..N-1) x[n] e^(-2pi*i*n*(N-m) / N)
> >> >> >> >> > X[N-m] = sum(n=0..N-1) x[n] e^(-2pi*i*n*N / N - 2pi*i*n*-m / N)
> >> >> >> >> > X[N-m] = sum(n=0..N-1) x[n] e^(-2pi*i*n + 2pi*i*n*m / N)
> >> >> >> >> > X[N-m] = sum(n=0..N-1) x[n] e^( 2pi*i*n*m / N)e^(-2pi*i*n)
> >> >> >> >> > X[N-m] = sum(n=0..N-1) x[n] e^( 2pi*i*n*m / N)
> >> >> >> >>
> >> >> >> >> The twiddle has to happen in the frequency domain so we have 4 cases:
> >> >> >> >
> >> >> >> > i have no clue what you talk about, but after a little testing i have
> >> >> >> > some doubt that your code works at all.
> >> >> >> > for example
> >> >> >> >
> >> >> >> >    ff_rdft_init(&one, 7, 1, 0);
> >> >> >> >
> >> >> >>
> >> >> >> sign_convention takes +/- 1. I'm sorry if that wasn't clear.
> >> >> >
> >> >> > followng shows empirically
> >> >> > that the transforms are identical short of trivial changes
> >> >> >
> >> >> >    ff_rdft_init(&one, bits, 1, -1);
> >> >> >    ff_rdft_init(&two, bits, 1,  1);
> >> >> >
> >> >> >    for(i=0; i<N; i++)
> >> >> >        src[i] = dst1[i] = (i + 2*i*i - 7*i*i*i + 123) % 1000;
> >> >> >
> >> >> >    for(i=0; i<N; i++){
> >> >> >        dst2[i] = dst1[i];
> >> >> >        if((i&1) && i!=1)
> >> >> >            dst2[i]*=-1;
> >> >> >    }
> >> >> >
> >> >> >    ff_rdft_calc(&one, dst1);
> >> >> >    ff_rdft_calc(&two, dst2);
> >> >> >
> >> >>
> >> >> Yes, it can be done as yet another pre/post processing loop. Or it can
> >> >> be simply included as part of the transform.
> >> >
> >> > or the decoder can be fixed so it stores the values with the correct signs
> >> >
> >>
> >> There is no such thing as "the correct sign." It's simply a matter of
> >> convention,
> >
> > Well, if you define teh RDFT as a real input DFT than there is no convention
> > that you can change unless you change the DFT as well
> >
>
> We have an option to choose the sign convention for the DFT it is
> simply the iDFT. You can think of the RDFT with opposite sign
> convention as the RiDFT.

RDFT, RiDFT, iRDFT, iRiDFT :)

[...]
> >
> >> and in this case, considering the code we already have,
> >> the problem is easier to solve with the positive sign convention.
> >
> > We should surely pick what is simpler, used by more important codecs
> > and allows more tables to be shared and is closer to established conventions
> > i dont know which of the 2 that neccessarily is ...
> >
>
> Assuming the forward and inverse RDFT are implemented, adding the
> opposite sign convention does not add any new tables.

well, at least sign_convention should be replaced by something less
error prone, maybe a enum combining it with inverse

[...]
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