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replace brute force find_optimal_param() with a closed-form solution. 

overall flac encoding: 4-15% faster.
output is not identical to the previous algorithm due to occasional rounding
errors, but the differece is less than .0005% bitrate.

Originally committed as revision 10612 to svn://svn.ffmpeg.org/ffmpeg/trunk
This commit is contained in:
Loren Merritt 2007-09-29 05:41:27 +00:00
parent f6215b1b06
commit 14bd2a9f25
1 changed files with 10 additions and 11 deletions
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21
libavcodec/flacenc.c
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@ -447,20 +447,19 @@ static void copy_samples(FlacEncodeContext *s, int16_t *samples)
#define rice_encode_count(sum, n, k) (((n)*((k)+1))+((sum-(n>>1))>>(k)))
/**
* Solve for d/dk(rice_encode_count) = n-((sum-(n>>1))>>(k+1)) = 0
*/
static int find_optimal_param(uint32_t sum, int n)
{
int k, k_opt;
uint32_t nbits[MAX_RICE_PARAM+1];
int k;
uint32_t sum2;
k_opt = 0;
nbits[0] = UINT32_MAX;
for(k=0; k<=MAX_RICE_PARAM; k++) {
nbits[k] = rice_encode_count(sum, n, k);
if(nbits[k] < nbits[k_opt]) {
k_opt = k;
}
}
return k_opt;
if(sum <= n>>1)
return 0;
sum2 = sum-(n>>1);
k = av_log2(n<256 ? FASTDIV(sum2,n) : sum2/n);
return FFMIN(k, MAX_RICE_PARAM);
}
static uint32_t calc_optimal_rice_params(RiceContext *rc, int porder,