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

Michael Niedermayer michaelni
Tue Jan 20 19:50:37 CET 2009

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?

[...]

--

Breaking DRM is a little like attempting to break through a door even
though the window is wide open and the only thing in the house is a bunch
of things you dont want and which you would get tomorrow for free anyway
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