[FFmpeg-devel] [PATCH 1/4] zmbvenc: don't sum the entropy when blocks are equal

[matthew.w.fearnley at gmail.com]

From: Matthew Fearnley <matthew.w.fearnley at gmail.com>

If *xored is 0, then histogram[0]==bw*bh and histogram[1..255]==0.

Because histogram[0] is skipped over for the entropy calculation, the
return value is always 0 when *xored==0, so we don't need to waste time
calculating it.

This addition both clarifies the behaviour of the code and improves
the speed when the block matches.
---
 libavcodec/zmbvenc.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/libavcodec/zmbvenc.c b/libavcodec/zmbvenc.c
index 4d9147657d..2f041dae32 100644
--- a/libavcodec/zmbvenc.c
+++ b/libavcodec/zmbvenc.c
@@ -71,6 +71,7 @@ static inline int block_cmp(ZmbvEncContext *c, uint8_t *src, int stride,
     int i, j;
     uint8_t histogram[256] = {0};
 
+    /* build frequency histogram of byte values for src[] ^ src2[] */
     *xored = 0;
     for(j = 0; j < bh; j++){
         for(i = 0; i < bw; i++){
@@ -82,6 +83,10 @@ static inline int block_cmp(ZmbvEncContext *c, uint8_t *src, int stride,
         src2 += stride2;
     }
 
+    /* early out if src and src2 are equal */
+    if (!*xored) return 0;
+
+    /* sum the entropy of the non-zero values */
     for(i = 1; i < 256; i++)
         sum += c->score_tab[histogram[i]];
 
-- 
2.17.1