[FFmpeg-devel] AAC-Main (round 2)

[Michael Niedermayer]

On Fri, Nov 07, 2008 at 07:20:45PM -0500, Alex Converse wrote:
> Hi,
> 
> Attached are a series of patches that implements AAC Main in FFmpeg. These
> are very similar to the first round. The only major change was avoiding
> unnecessary type punning.
> 
> While AAC-Main is rarely used, faad2 supports it, flash claims to support it
> (I haven't tested this), and we claim to support it but do not.
> 
> Notes:
> 1) Frequency domain prediction is described only in ISO/IEC 13818-7 not in
> 14496-3.

> 2) The prediction operation uses 16-bit floats, using 32-bit floats does not
> give adequate accuracy so emulation routines to round to 16-bit are
> included.

> 3) As only 16-bit floats are required it could be possible to store the
> prediction state with half the memory but I'm not sure how to approach that
> situation without resorting to IEEE type punning.
> 
> Regards,
> 
> Alex Converse

[...]
> +static void reset_predictor_group(PredictorState * ps, int group_num) {
> +    int i;
> +    if (group_num)
> +        for (i = group_num-1; i < MAX_PREDICTORS; i+=30)
> +            reset_predict_state(&ps[i]);
> +}

i think it would be clearer if the if() was moved out of the function


[...]
> @@ -786,6 +837,95 @@ static int decode_spectrum_and_dequant(AACContext * ac, float coef[1024], GetBit
>      return 0;
>  }
>  
> +static av_always_inline float flt16_round(float pf) {
> +    int exp;
> +    pf = frexp(pf, &exp);
> +    pf = ldexp(roundf(ldexp(pf, 8)), exp-8);
> +    return pf;
> +}
> +
> +static av_always_inline float flt16_even(float pf) {
> +    int exp;
> +    pf = frexpf(pf, &exp);
> +    pf = ldexp(rintf(ldexp(pf, 8)), exp-8);
> +    return pf;
> +}
> +
> +static av_always_inline float flt16_trunc(float pf) {
> +    int exp;
> +    pf = frexpf(pf, &exp);
> +    pf = ldexp(truncf(ldexp(pf, 8)), exp-8);
> +    return pf;
> +}
> +

are these faster or slower than the code suggested in the spec?


> +static void predict(AACContext * ac, PredictorState * ps, float* coef, int output_enable) {

> +    const float a = 0.953125;

61.0/64 (could be in a comment too if the literal float is prefered ...)


> +    const float alpha = 0.90625;

29.0/32


> +
> +    float e0, e1;
> +    float pv;
> +
> +    float k1, k2;
> +
> +    if (ps->var0 <= 1)
> +        k1 = 0;
> +    else
> +        k1 = ps->cor0*flt16_even(a/ps->var0);
> +
> +    if (ps->var1 <= 1)
> +        k2 = 0;
> +    else
> +        k2 = ps->cor1*flt16_even(a/ps->var1);
> +
> +    pv = k1*ps->r0 + k2*ps->r1;
> +    pv = flt16_round(pv);
> +    if (output_enable)
> +        *coef += pv/-1024;
> +
> +    e0 = *coef*-1024;
> +    e1 = e0-k1*ps->r0;
> +

> +    ps->cor1 = alpha*ps->cor1 + ps->r1*e1;
> +    ps->var1 = alpha*ps->var1 + 0.5 * (ps->r1*ps->r1 + e1*e1);
> +    ps->cor0 = alpha*ps->cor0 + ps->r0*e0;
> +    ps->var0 = alpha*ps->var0 + 0.5 * (ps->r0*ps->r0 + e0*e0);
> +
> +    ps->r1 = a*(ps->r0-k1*e0);
> +    ps->r0 = a*e0;
> +

> +    ps->r0   = flt16_trunc(ps->r0);
> +    ps->r1   = flt16_trunc(ps->r1);
> +    ps->cor0 = flt16_trunc(ps->cor0);
> +    ps->cor1 = flt16_trunc(ps->cor1);
> +    ps->var0 = flt16_trunc(ps->var0);
> +    ps->var1 = flt16_trunc(ps->var1);

the flt16_trunc() calls could be done when the values are calculated

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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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