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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
Download patch file Download diff file Expand all files Collapse all files

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)
{

9
libcelt/entdec.h
View file

@ -110,6 +110,15 @@ ec_uint32 ec_dec_bits(ec_dec *_this,unsigned _ftb);
This must be at least one, and no more than 2**32-1.
Return: The decoded bits.*/
ec_uint32 ec_dec_uint(ec_dec *_this,ec_uint32 _ft);
/*Decodes a symbol given its CDF.
No call to ec_dec_update() is necessary after this call.
_cdf: The CDF, such that symbol s falls in the range [_cdf[s],_cdf[s+1]).
The first value must be 0, the last value must be (1<<_ftb), and the
values must be monotonicly non-decreasing.
_ftb: The number of bits of precision in the cumulative distribution.
Return: The decoded symbol s, which must have been encoded with
ec_encode_bin(enc,_cdf[s],_cdf[s+1],_ftb).*/
int ec_dec_cdf(ec_dec *_this,const int *_cdf,unsigned _ftb);
/* Decode a bit that has a _prob/65536 probability of being a one */
int ec_dec_bit_prob(ec_dec *_this,unsigned _prob);

22
libcelt/rangedec.c
View file

@ -203,6 +203,28 @@ int ec_dec_bit_prob(ec_dec *_this,unsigned _prob){
return val;
}
int ec_dec_cdf(ec_dec *_this,const int *_cdf,unsigned _ftb){
ec_uint32 r;
ec_uint32 d;
ec_uint32 s;
ec_uint32 t;
int val;
r=_this->rng>>_ftb;
d=_this->dif;
_cdf++;
val=0;
t=0;
s=IMUL32(r,(1<<_ftb)-_cdf[0]);
while(d<=s){
t=s;
s=IMUL32(r,(1<<_ftb)-_cdf[++val]);
}
_this->dif=d-s;
_this->rng=(val?t:_this->rng)-s;
ec_dec_normalize(_this);
return val;
}
ec_uint32 ec_dec_tell(ec_dec *_this,int _b){
ec_uint32 r;
int l;