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Add a generic CDF decoding routine.

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This decodes a value encoded with ec_encode_bin() without using any
 divisions.
It is only meant for small alphabets.
If a symbol can take on a large number of possible values, a binary
 search would be better.

This patch also converts spread_decision to use it, since it is
 faster and introduces less rounding error to encode a single
 decision for the entire value than to encode it a bit at a time.
This commit is contained in:
Timothy B. Terriberry 2010-12-17 10:49:00 -08:00 • committed by Jean-Marc Valin
parent 3fed34ae00
commit a0b664df3d
3 changed files with 36 additions and 13 deletions
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18
libcelt/celt.c
View file

@ -54,6 +54,7 @@
#include "plc.h"
static const int trim_cdf[12] = {0, 2, 4, 9, 19, 41, 87, 109, 119, 124, 126, 128};
static const int spread_cdf[5] = {0, 7, 9, 30, 32};
#define COMBFILTER_MAXPERIOD 1024
#define COMBFILTER_MINPERIOD 16
@ -957,9 +958,8 @@ int celt_encode_with_ec_float(CELTEncoder * restrict st, const celt_sig * pcm, i
st->spread_decision = spreading_decision(st->mode, X, &st->tonal_average, st->spread_decision, effEnd, C, M);
}
/* Probs: NONE: 21.875%, LIGHT: 6.25%, NORMAL: 65.625%, AGGRESSIVE: 6.25% */
ec_enc_bit_prob(enc, st->spread_decision>>1, 47104);
ec_enc_bit_prob(enc, st->spread_decision&1,
(st->spread_decision>>1) ? 5699 : 14564);
ec_encode_bin(enc, spread_cdf[st->spread_decision],
spread_cdf[st->spread_decision+1], 5);
ALLOC(offsets, st->mode->nbEBands, int);
@ -1823,8 +1823,7 @@ int celt_decode_with_ec_float(CELTDecoder * restrict st, const unsigned char *da
ALLOC(tf_res, st->mode->nbEBands, int);
tf_decode(st->start, st->end, C, isTransient, tf_res, LM, dec);
spread_decision = ec_dec_bit_prob(dec, 47104)<<1;
spread_decision |= ec_dec_bit_prob(dec, (spread_decision>>1) ? 5699 : 14564);
spread_decision = ec_dec_cdf(dec, spread_cdf, 5);
ALLOC(pulses, st->mode->nbEBands, int);
ALLOC(offsets, st->mode->nbEBands, int);
@ -1844,14 +1843,7 @@ int celt_decode_with_ec_float(CELTDecoder * restrict st, const unsigned char *da
}
ALLOC(fine_quant, st->mode->nbEBands, int);
{
int fl;
alloc_trim = 0;
fl = ec_decode_bin(dec, 7);
while (trim_cdf[alloc_trim+1] <= fl)
alloc_trim++;
ec_dec_update(dec, trim_cdf[alloc_trim], trim_cdf[alloc_trim+1], 128);
}
alloc_trim = ec_dec_cdf(dec, trim_cdf, 7);
if (C==2)
{