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avcodec/aacenc_is: replace pow(x, 0.75) by x/sqrtf(sqrtf(x))

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This is quite an accurate approximation; testing shows ~ 2ulp error in
the floating point result. Tested with FATE.

Alternatively, if one wants "full accuracy", one can use powf, or sqrt
instead of sqrtf. With powf, one gets 1 ulp error (theoretically should be 0, as
0.75 is exactly representable) on GNU libm, with sqrt, 0 ulp error.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

Abstracted into pos_pow34 utility function

Signed-off-by: Claudio Freire <klaussfreire@gmail.com>
This commit is contained in:
Claudio Freire 2016-01-14 00:38:22 -03:00
parent a0a47a09b0
commit 62dfe1d40d
2 changed files with 6 additions and 1 deletions
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2
libavcodec/aacenc_is.c
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@ -54,7 +54,7 @@ struct AACISError ff_aac_is_encoding_err(AACEncContext *s, ChannelElement *cpe,
FFPsyBand *band0 = &s->psy.ch[s->cur_channel+0].psy_bands[(w+w2)*16+g];
FFPsyBand *band1 = &s->psy.ch[s->cur_channel+1].psy_bands[(w+w2)*16+g];
int is_band_type, is_sf_idx = FFMAX(1, sce0->sf_idx[w*16+g]-4);
float e01_34 = phase*pow(ener1/ener0, 3.0/4.0);
float e01_34 = phase*pos_pow34(ener1/ener0);
float maxval, dist_spec_err = 0.0f;
float minthr = FFMIN(band0->threshold, band1->threshold);
for (i = 0; i < sce0->ics.swb_sizes[g]; i++)

5
libavcodec/aacenc_utils.h
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@ -45,6 +45,11 @@ static inline void abs_pow34_v(float *out, const float *in, const int size)
}
}
static inline float pos_pow34(float a)
{
return sqrtf(a * sqrtf(a));
}
/**
* Quantize one coefficient.
* @return absolute value of the quantized coefficient