shithub: dav1d

Info  •  Files  •  Log  •  Branches

Internalize the EC_PROB_SHIFT precision reduction

All of the msac decode functions use 15-bit CDFs with the exception of
msac_decode_bool_prob() which takes a (15 - EC_PROB_SHIFT)-bit
probability.
This patch internalizes the reduction in precision from the EC_SMALL_MUL
experiment (hiding the define) and gives msac calls a consistent API.

--- a/src/decode.c

+++ b/src/decode.c

@@ -2165,7 +2165,7 @@

             const Av1Block *const b = &f->frame_thread.b[t->by * f->b4_stride + t->bx];

             is_split = b->bl != bl;

         } else {

-            is_split = msac_decode_bool(&t->ts->msac, gather_top_partition_prob(pc, bl) >> EC_PROB_SHIFT);

+            is_split = msac_decode_bool(&t->ts->msac, gather_top_partition_prob(pc, bl));

             if (DEBUG_BLOCK_INFO)

                 printf("poc=%d,y=%d,x=%d,bl=%d,ctx=%d,bp=%d: r=%d\n",

                        f->frame_hdr->frame_offset, t->by, t->bx, bl, ctx,

@@ -2193,7 +2193,7 @@

             const Av1Block *const b = &f->frame_thread.b[t->by * f->b4_stride + t->bx];

             is_split = b->bl != bl;

         } else {

-            is_split = msac_decode_bool(&t->ts->msac, gather_left_partition_prob(pc, bl) >> EC_PROB_SHIFT);

+            is_split = msac_decode_bool(&t->ts->msac, gather_left_partition_prob(pc, bl));

             if (f->cur.p.layout == DAV1D_PIXEL_LAYOUT_I422 && !is_split)

                 return 1;

             if (DEBUG_BLOCK_INFO)

--- a/src/msac.c

+++ b/src/msac.c

@@ -34,6 +34,7 @@

 #include "src/msac.h"

+#define EC_PROB_SHIFT 6

 #define EC_MIN_PROB 4  // must be <= (1<<EC_PROB_SHIFT)/16

 #define EC_WIN_SIZE (sizeof(ec_win) << 3)

@@ -115,7 +116,7 @@

     uint16_t r = s->rng;

     unsigned ret;

     assert((dif >> (EC_WIN_SIZE - 16)) < r);

-    v = ((r >> 8) * f >> (7 - EC_PROB_SHIFT)) + EC_MIN_PROB;

+    v = ((r >> 8) * (f >> EC_PROB_SHIFT) >> (7 - EC_PROB_SHIFT)) + EC_MIN_PROB;

     vw   = v << (EC_WIN_SIZE - 16);

     ret  = dif >= vw;

     dif -= ret*vw;

@@ -179,7 +180,7 @@

 }

 unsigned msac_decode_bool_adapt(MsacContext *const c, uint16_t *const cdf) {

-    const unsigned bit = msac_decode_bool(c, *cdf >> EC_PROB_SHIFT);

+    const unsigned bit = msac_decode_bool(c, *cdf);

     if(c->allow_update_cdf){

         // update_cdf() specialized for boolean CDFs

--- a/src/msac.h

+++ b/src/msac.h

@@ -43,8 +43,6 @@

     int allow_update_cdf;

 } MsacContext;

-#define EC_PROB_SHIFT 6

-

 void msac_init(MsacContext *c, const uint8_t *data, size_t sz, int disable_cdf_update_flag);

 unsigned msac_decode_symbol(MsacContext *s, const uint16_t *cdf,

                             const unsigned n_symbols);