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libavcodec/zmbvenc: block scoring improvements/bug fixes

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- Improve block choices by counting 0-bytes in the entropy score
- Make histogram use uint16_t type, to allow byte counts from 16*16
(current block size) up to 255*255 (maximum allowed 8bpp block size)
- Make sure score table is big enough for a full block's worth of bytes
- Calculate *xored without using code in inner loop
This commit is contained in:
Matthew Fearnley 2019-02-09 13:10:20 +00:00 committed by Tomas Härdin
parent ff03418348
commit 2d80b56ce0
1 changed files with 16 additions and 6 deletions
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22
libavcodec/zmbvenc.c
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@ -55,7 +55,7 @@ typedef struct ZmbvEncContext {
int keyint, curfrm;
z_stream zstream;
int score_tab[256];
int score_tab[ZMBV_BLOCK * ZMBV_BLOCK + 1];
} ZmbvEncContext;
@ -69,20 +69,26 @@ static inline int block_cmp(ZmbvEncContext *c, uint8_t *src, int stride,
{
int sum = 0;
int i, j;
uint8_t histogram[256] = {0};
uint16_t histogram[256] = {0};
*xored = 0;
/* Build frequency histogram of byte values for src[] ^ src2[] */
for(j = 0; j < bh; j++){
for(i = 0; i < bw; i++){
int t = src[i] ^ src2[i];
histogram[t]++;
*xored |= t;
}
src += stride;
src2 += stride2;
}
for(i = 1; i < 256; i++)
/* If not all the xored values were 0, then the blocks are different */
*xored = (histogram[0] < bw * bh);
/* Exit early if blocks are equal */
if (!*xored) return 0;
/* Sum the entropy of all values */
for(i = 0; i < 256; i++)
sum += c->score_tab[histogram[i]];
return sum;
@ -278,7 +284,11 @@ static av_cold int encode_init(AVCodecContext *avctx)
int i;
int lvl = 9;
for(i=1; i<256; i++)
/* Entropy-based score tables for comparing blocks.
* Suitable for blocks up to (ZMBV_BLOCK * ZMBV_BLOCK) bytes.
* Scores are nonnegative, lower is better.
*/
for(i = 1; i <= ZMBV_BLOCK * ZMBV_BLOCK; i++)
c->score_tab[i] = -i * log2(i / (double)(ZMBV_BLOCK * ZMBV_BLOCK)) * 256;
c->avctx = avctx;