[FFmpeg-devel] [PATCH] Generic sine window init function

[Michael Niedermayer]

On Sat, Jun 21, 2008 at 09:02:35PM +0100, Robert Swain wrote:
> Hello,
> 
> A few of the MDCT codecs (AAC, IMC, COOK, Nellymoser) use sine windows
> so it would make sense to share the sine window initialisation code
> between them as the . I have attached a patch for this. If there are
> other codecs that can use this that I've missed, let me know.
> 
> It is probably noteworthy that the tables for functions that normalise
> the window coefficients may differ from the direct calculation due to
> loss of precision during intermediate stages. For example, in imc.c:
> 
> +    ff_sine_window_init(q->mdct_sine_window, 2*COEFFS);
>      for(i = 0; i < COEFFS; i++)
> -        q->mdct_sine_window[i] = sin((i + 0.5) / 512.0 * M_PI) * sqrt(2.0);
> +        q->mdct_sine_window[i] *= sqrt(2.0);
> 
> mdct_sine_window is a float array so the sin() part of the calculation
> is limited to float precision in ff_sine_window_init and is then
> normalised with the sqrt(2.0) after. Is this a problem or is the
> difference insignificant? For the above case the maximum deviation was
> 0.00000011920928955078125.
> 

> Also, off topic, should that sqrt(2.0) be switched for M_SQRT2?

yes


[...]
> Index: libavcodec/imc.c
> ===================================================================
> --- libavcodec/imc.c	(revision 13854)
> +++ libavcodec/imc.c	(working copy)
> @@ -102,8 +102,9 @@
>          q->old_floor[i] = 1.0;
>  
>      /* Build mdct window, a simple sine window normalized with sqrt(2) */
> +    ff_sine_window_init(q->mdct_sine_window, COEFFS);
>      for(i = 0; i < COEFFS; i++)
> -        q->mdct_sine_window[i] = sin((i + 0.5) / 512.0 * M_PI) * sqrt(2.0);
> +        q->mdct_sine_window[i] *= sqrt(2.0);
>      for(i = 0; i < COEFFS/2; i++){
>          q->post_cos[i] = cos(i / 256.0 * M_PI);
>          q->post_sin[i] = sin(i / 256.0 * M_PI);
> Index: libavcodec/cook.c
> ===================================================================
> --- libavcodec/cook.c	(revision 13854)
> +++ libavcodec/cook.c	(working copy)
> @@ -239,9 +239,9 @@
>        return -1;
>  
>      /* Initialize the MLT window: simple sine window. */
> -    alpha = M_PI / (2.0 * (float)mlt_size);
> +    ff_sine_window_init(q->mlt_window, mlt_size);
>      for(j=0 ; j<mlt_size ; j++)
> -        q->mlt_window[j] = sin((j + 0.5) * alpha) * sqrt(2.0 / q->samples_per_channel);
> +        q->mlt_window[j] *= sqrt(2.0 / q->samples_per_channel);

The window should not be scaled by sqrt(2) there are plenty of other places
where the sqrt(2) could be applied. 
and it should not be in the context but static and shared
(of course that is not really your job to do so the patch is ok if you dont
want to work on that)

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Many things microsoft did are stupid, but not doing something just because
microsoft did it is even more stupid. If everything ms did were stupid they
would be bankrupt already.
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