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

[Benjamin Larsson]

Hello.

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);

You could add a scale argument and the scaling code in the codec(s)
could be removed.


>      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?

Insignificant.

> For the above case the maximum deviation was
> 0.00000011920928955078125.
> 
> Also, off topic, should that sqrt(2.0) be switched for M_SQRT2?

Switch it.

> 
> Rob


The IMC, Cook and Nellymoser parts can be applied.

MvH
Benjamin Larsson