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

[Alex Converse]

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:
>> On Sun, Jan 18, 2009 at 5:48 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>> > On Sat, Jan 17, 2009 at 08:18:04PM -0500, Alex Converse wrote:
>> >> On Sat, Jan 17, 2009 at 7:52 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>> >> > On Sat, Jan 17, 2009 at 01:29:13PM -0500, Alex Converse wrote:
>> >> >> Attached is my RDFT patch with all the feedback applied. Also attached
>> >> >> is a patch that makes use of it in QDM2 decoding. I thought I had
>> >> >> unwrapped the math perfectly but it looks like things changed for the
>> >> >> better.
>> >> >>
>> >> >> Also new is a flag for FFT sign convention. This is because the
>> >> >> inverse transform is not the same as the forward transform with the
>> >> >> opposite sign convention, scaled.
>> >> >>
>> >> >> The reference files are decoded with the binary coder from the qt63 dlls.
>> >> >>
>> >> >> $ tests/tiny_psnr ../qdm2/nadasurf_popular_dl.wav old.wav 2 0 44
>> >> >> stddev: 1305.89 PSNR: 34.00 bytes: 39632896/ 39632896
>> >> >> $ tests/tiny_psnr ../qdm2/nadasurf_popular_dl.wav new.wav 2 0 44
>> >> >> stddev: 1145.39 PSNR: 35.14 bytes: 39632896/ 39632896
>> >> >> $ tests/tiny_psnr ../qdm2/switzler084d_dl.wav old.wav 2 0 44
>> >> >> stddev:  377.46 PSNR: 44.78 bytes: 20660224/ 20660224
>> >> >> $ tests/tiny_psnr ../qdm2/switzler084d_dl.wav new.wav 2 0 44
>> >> >> stddev:  305.00 PSNR: 46.63 bytes: 20660224/ 20660224
>> >> > [...]
>> >> >> +/**
>> >> >> + * 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
>> >> >> + * @param sign_convention The sign of j of the forward FFT.
>> >> >> + */
>> >> >> +int ff_rdft_init(RDFTContext *s, int nbits, int inverse, int sign_convention)
>> >> >> +{
>> >> >> +    int n = 1 << nbits;
>> >> >> +    int i;
>> >> >> +    const double theta = sign_convention*(1-2*inverse)*2*M_PI/n;
>> >> >> +
>> >> >> +    s->nbits           = nbits;
>> >> >> +    s->inverse         = inverse;
>> >> >> +    s->sign_convention = sign_convention;
>> >> >> +
>> >> >> +    if (ff_fft_init(&s->fft, nbits-1, (sign_convention==1) ^ inverse) < 0)
>> >> >> +        goto fail;
>> >> >> +
>> >> >
>> >> >> +    s->tsin = av_malloc((n>>2) * sizeof(FFTSample));
>> >> >
>> >> > i do prefer that no tables are duplicated
>> >> >
>> >>
>> >> Where can I pull these from?
>> >
>> > I dont know, its very possible there are no such global tables yet,
>> > if so, just add new global/static tables ...
>> >
>> > [...]
>>
>> Like this?
>
> yes
>
>
>>
>> Also the addition to the Makefile in rdft-qdm.diff seemed unnecessary
>> (as it was already covered by a dependency in configure) so I removed
>> it and am resubmitting that patch as well.
>
> [...]
>
>> @@ -747,6 +749,23 @@ void ff_imdct_half_sse(MDCTContext *s, FFTSample *output, const FFTSample *input
>>  void ff_mdct_calc(MDCTContext *s, FFTSample *out, const FFTSample *input);
>>  void ff_mdct_end(MDCTContext *s);
>>
>> +/* Real Discrete Fourier Transform */
>> +
>> +typedef struct {
>
> not doxygen compatible

fixed

>
> [...]
>
>> +/**
>> + * 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.

>
>
>> + * @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).

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 }

> also the doxy should be in the header IMHO
>

fixed

>
>> + */
>> +int ff_rdft_init(RDFTContext *s, int nbits, int inverse, int sign_convention)
>
> should be av_cold

fixed

--Alex