[Ffmpeg-devel] [RFC] mdct window generation

[Michael Niedermayer]

Hi

On Sat, Jul 08, 2006 at 04:38:16PM +0200, Benjamin Larsson wrote:
> With the upcoming ffmpeg native aac and ac3 codecs something like this
> is needed to prevent code duplication. Currently each codec generate
> it's own mdct window, with more audio codecs being added it would be
> nice to have something like this patch. Note that the code in this patch
> isn't correct and that the code to use this in wma, vorbis and cook is
> missing.

[...]
> +/**
> + * Generate a Kaiser Window.
> + */
> +static void k_window_init(int alpha, float *window, int n, int iterations)
> +{
> +    int j, k;
> +    float a, x;
> +    a = alpha * M_PI / n;
> +    a = a*a;
> +    for(k=0; k<n; k++) {
> +        x = k * (n - k) * a;
> +        window[k] = 1.0;
> +        for(j=iterations; j>0; j--) {
> +            window[k] = (window[k] * x / (j*j)) + 1.0;
> +        }
> +    }
> +}
> +
> +/**
> + * Generate a Kaiser-Bessel Derived Window.
> + * @param alpha  determines window shape
> + * @param window array to fill with window values
> + * @param n      length of the window
> + * @param iter   number of iterations to use in BesselI0
> + */
> +static void kbd_window_init(int alpha, float *window, int n, int iterations)
> +{
> +    int k, n2;
> +    float *kwindow;
> +
> +    n2 = n >> 1;
> +    kwindow = &window[n2];
> +    k_window_init(alpha, kwindow, n2, iterations);
> +    window[0] = kwindow[0];
> +    for(k=1; k<n2; k++) {
> +        window[k] = window[k-1] + kwindow[k];
> +    }
> +    for(k=0; k<n2; k++) {
> +        window[k] = sqrt(window[k] / (window[n2-1]+1));
> +        window[n-1-k] = window[k];
> +    }
> +}
> +
> +/**
> + * Generate a MDCT window.
> + * @param window_type   one of the diffrent 
> + * @param window        array to fill with window values
> + * @param size          length of the window
> + * @param kbd_alpha     optional argument, used for KBD windows
> + */
> +
> +void ff_generate_mdct_window(enum MDCT_window window_type, float* window, int size, int kbd_alpha) {
> +    int i;
> +    float alpha;
> +
> +    switch(window_type) {
> +        case SINE:
> +            alpha = M_PI / (2.0 * (float)size);
> +            for(i=0 ; i<size ; i++)
> +                window[i] = sin((i + 512.0/(float)size) * alpha);
> +            break;
> +        case KBD:
> +            kbd_window_init(kbd_alpha, window, size, 50);
> +            break;
> +        case VORBIS:
> +            for(i=0 ; i<size ; i++)
> +                window[i]=sin(0.5*M_PI*(sin(((float)i+0.5)/(float)size*M_PI))*(sin(((float)i+0.5)/(float)size*M_PI)));
> +            break;
> +    }
> +}

is there _anything_ shared between the 3 cases? if no then IMO they should
be 3 seperate functions

furthermore, the code to generate a window MUST be smaller then the
window of course, otherwise its just wasting space

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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