[FFmpeg-devel] [PATCH 1/3] avcodec/mpeg12: Reduce size of motion-vector VLC

[Andreas Rheinhardt]

It currently uses 9 bits per table, but there are no codes with
nine bits at all, while there are codes with eight, ten and eleven bits.
So reducing the table size to eight bits will not reduce the amount of
codes that can be parsed in the first step, but it allows to reduce the
size of the motion-vector VLC.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
---
 libavcodec/mpeg12.c    | 2 +-
 libavcodec/mpeg12vlc.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/libavcodec/mpeg12.c b/libavcodec/mpeg12.c
index ab6c19c615..e4f007aec5 100644
--- a/libavcodec/mpeg12.c
+++ b/libavcodec/mpeg12.c
@@ -149,7 +149,7 @@ av_cold void ff_mpeg12_init_vlcs(void)
                         ff_mpeg12_vlc_dc_chroma_code, 2, 2, 514);
         INIT_VLC_STATIC(&ff_mv_vlc, MV_VLC_BITS, 17,
                         &ff_mpeg12_mbMotionVectorTable[0][1], 2, 1,
-                        &ff_mpeg12_mbMotionVectorTable[0][0], 2, 1, 518);
+                        &ff_mpeg12_mbMotionVectorTable[0][0], 2, 1, 266);
         INIT_VLC_STATIC(&ff_mbincr_vlc, MBINCR_VLC_BITS, 36,
                         &ff_mpeg12_mbAddrIncrTable[0][1], 2, 1,
                         &ff_mpeg12_mbAddrIncrTable[0][0], 2, 1, 538);
diff --git a/libavcodec/mpeg12vlc.h b/libavcodec/mpeg12vlc.h
index c5abae96b6..70aca645cb 100644
--- a/libavcodec/mpeg12vlc.h
+++ b/libavcodec/mpeg12vlc.h
@@ -31,7 +31,7 @@
 #include "vlc.h"
 
 #define DC_VLC_BITS 9
-#define MV_VLC_BITS 9
+#define MV_VLC_BITS 8
 #define TEX_VLC_BITS 9
 
 #define MBINCR_VLC_BITS 9
-- 
2.25.1