ANS: Switch from PDFs to CDFs.

Make the RANS implementation operate on cumulative distribution
functions rather than individual probability distribution functions.
CDFs have shown themselves more flexible to work with.

Reduces decoding memory usage from scaling O(num_distributions *
symbol_resolution) to O(num_distributions).

No bitstream change. This is an purely implementation change.

Change-Id: I4e18d3a0a3d37a36a61487c3d778f9d088b0b374

diff --git a/test/vp10_ans_test.cc b/test/vp10_ans_test.cc
index 441583ad1b09df30c88340ab4131b0cef1377ccc..363161d7a428aefa9ee041e5cbc53dd289f75e88 100644 (file)
--- a/test/vp10_ans_test.cc
+++ b/test/vp10_ans_test.cc
@@ -147,7 +147,7 @@ bool check_vpxbool(const PvVec &pv_vec, uint8_t *buf) {
 }
 
 const rans_sym rans_sym_tab[] = {
-    {70, 186}, {70, 116}, {100, 16}, {16, 0},
+    {16, 0}, {100, 16}, {70, 116}, {70, 186},
 };
 const int kDistinctSyms = sizeof(rans_sym_tab) / sizeof(rans_sym_tab[0]);
 
@@ -170,13 +170,9 @@ std::vector<int> ans_encode_build_vals(const rans_sym *tab, int iters) {
 
 void rans_build_dec_tab(const struct rans_sym sym_tab[],
                         rans_dec_lut dec_tab) {
-  int val = 0;
-  int i;
-  for (i = ans_p8_precision - 1; i >= 0; --i) {
-    dec_tab[i].val = val;
-    dec_tab[i].prob = sym_tab[val].prob;
-    dec_tab[i].cum_prob = sym_tab[val].cum_prob;
-    if (i == sym_tab[val].cum_prob) ++val;
+  dec_tab[0] = 0;
+  for (int i = 1; dec_tab[i - 1] < ans_p8_precision; ++i) {
+    dec_tab[i] = dec_tab[i - 1] + sym_tab[i - 1].prob;
   }
 }
 

diff --git a/vp10/common/ans.h b/vp10/common/ans.h
index a1862f30d057273db5d2c3d87cab949db0676453..163a7a956afb4c9bdec0c803648d6500e2951e05 100644 (file)
--- a/vp10/common/ans.h
+++ b/vp10/common/ans.h
@@ -241,23 +241,16 @@ struct rans_dec_sym {
   AnsP8 cum_prob;  // not-inclusive
 };
 
-typedef struct rans_dec_sym rans_dec_lut[ans_p8_precision];
+// This is now just a boring cdf. It starts with an explict zero.
+// TODO(aconverse): Remove starting zero.
+typedef uint16_t rans_dec_lut[16];
 
-static INLINE void rans_build_dec_tab(const AnsP8 token_probs[],
-                                      rans_dec_lut dec_tab) {
-  int val = 0;
-  int cum_prob = 0;
-  int sym_end = token_probs[0];
+static INLINE void rans_build_cdf_from_pdf(const AnsP8 token_probs[],
+                                           rans_dec_lut cdf_tab) {
   int i;
-  for (i = 0; i < 256; ++i) {
-    if (i == sym_end) {
-      ++val;
-      cum_prob = sym_end;
-      sym_end += token_probs[val];
-    }
-    dec_tab[i].val = val;
-    dec_tab[i].prob = token_probs[val];
-    dec_tab[i].cum_prob = cum_prob;
+  cdf_tab[0] = 0;
+  for (i = 1; cdf_tab[i - 1] < ans_p8_precision; ++i) {
+    cdf_tab[i] = cdf_tab[i - 1] + token_probs[i - 1];
   }
 }
 
@@ -275,20 +268,32 @@ static INLINE void rans_write(struct AnsCoder *ans,
       (ans->state / p) * ans_p8_precision + ans->state % p + sym->cum_prob;
 }
 
+static INLINE void fetch_sym(struct rans_dec_sym *out, const rans_dec_lut cdf,
+                             AnsP8 rem) {
+  int i = 0;
+  // TODO(skal): if critical, could be a binary search.
+  // Or, better, an O(1) alias-table.
+  while (rem >= cdf[i]) {
+    ++i;
+  }
+  out->val = i - 1;
+  out->prob = cdf[i] - cdf[i - 1];
+  out->cum_prob = cdf[i - 1];
+}
+
 static INLINE int rans_read(struct AnsDecoder *ans,
                             const rans_dec_lut tab) {
   unsigned rem;
   unsigned quo;
-  int val;
+  struct rans_dec_sym sym;
   if (ans->state < l_base && ans->buf_offset > 0) {
     ans->state = ans->state * io_base + ans->buf[--ans->buf_offset];
   }
   quo = ans->state / ans_p8_precision;
   rem = ans->state % ans_p8_precision;
-  val = tab[rem].val;
-
-  ans->state = quo * tab[rem].prob + rem - tab[rem].cum_prob;
-  return val;
+  fetch_sym(&sym, tab, rem);
+  ans->state = quo * sym.prob + rem - sym.cum_prob;
+  return sym.val;
 }
 
 static INLINE int ans_read_init(struct AnsDecoder *const ans,

diff --git a/vp10/common/entropy.c b/vp10/common/entropy.c
index 8c7e27a9bc471ace02b2f4b271be494247621a31..df03224bfcfdf4d817a5c1c099ac3c852eefcd07 100644 (file)
--- a/vp10/common/entropy.c
+++ b/vp10/common/entropy.c
@@ -676,12 +676,12 @@ const vpx_prob vp10_pareto8_token_probs[COEFF_PROB_MODELS]
   {247, 1, 1, 1, 1, 1, 1, 1, 1, 1},
 };
 
-void vp10_build_pareto8_dec_tab(
+void vp10_build_pareto8_cdf_tab(
     const vpx_prob token_probs[COEFF_PROB_MODELS][ENTROPY_TOKENS - 2],
-    rans_dec_lut dec_tab[COEFF_PROB_MODELS]) {
+    rans_dec_lut cdf_tab[COEFF_PROB_MODELS]) {
   int p;
   for (p = 0; p < COEFF_PROB_MODELS; ++p) {
-    rans_build_dec_tab(token_probs[p], dec_tab[p]);
+    rans_build_cdf_from_pdf(token_probs[p], cdf_tab[p]);
   }
 }
 #endif  // CONFIG_ANS

diff --git a/vp10/common/entropy.h b/vp10/common/entropy.h
index 4fa330e33ff530f3104d5dd714c6add4647ee322..8ea01be4b639f2bf547b8a9a3b2477673a970878 100644 (file)
--- a/vp10/common/entropy.h
+++ b/vp10/common/entropy.h
@@ -170,9 +170,9 @@ extern const vpx_prob vp10_pareto8_full[COEFF_PROB_MODELS][MODEL_NODES];
 extern const vpx_prob
     vp10_pareto8_token_probs[COEFF_PROB_MODELS][ENTROPY_TOKENS - 2];
 
-void vp10_build_pareto8_dec_tab(
+void vp10_build_pareto8_cdf_tab(
     const vpx_prob token_probs[COEFF_PROB_MODELS][ENTROPY_TOKENS - 2],
-    rans_dec_lut dec_tab[COEFF_PROB_MODELS]);
+    rans_dec_lut cdf_tab[COEFF_PROB_MODELS]);
 #endif  // CONFIG_ANS
 
 typedef vpx_prob vp10_coeff_probs_model[REF_TYPES][COEF_BANDS]

diff --git a/vp10/common/onyxc_int.h b/vp10/common/onyxc_int.h
index 02d7e1761ce7415f1fa012a84ddc25cc5431b7ae..bcc69f3bd8fa64aeb02bfadc9ddbc920395fd2ea 100644 (file)
--- a/vp10/common/onyxc_int.h
+++ b/vp10/common/onyxc_int.h
@@ -324,6 +324,9 @@ typedef struct VP10Common {
   // - this is intentionally not placed in FRAME_CONTEXT since it's reset upon
   // each keyframe and not used afterwards
   vpx_prob kf_y_prob[INTRA_MODES][INTRA_MODES][INTRA_MODES - 1];
+#if CONFIG_ANS
+  rans_dec_lut token_tab[COEFF_PROB_MODELS];
+#endif  // CONFIG_ANS
 } VP10_COMMON;
 
 // TODO(hkuang): Don't need to lock the whole pool after implementing atomic

diff --git a/vp10/decoder/decodeframe.c b/vp10/decoder/decodeframe.c
index 2c2dc485b695b1e7be47de297bafe1930b31c13f..af6016a68d65d4d02d4065336be8d0bfd5ec9616 100644 (file)
--- a/vp10/decoder/decodeframe.c
+++ b/vp10/decoder/decodeframe.c
@@ -1858,7 +1858,7 @@ static void decode_block(VP10Decoder *const pbi, MACROBLOCKD *const xd,
           for (col = 0; col < max_blocks_wide; col += step)
             predict_and_reconstruct_intra_block(xd,
 #if CONFIG_ANS
-                                                pbi->token_tab, tok,
+                                                cm->token_tab, tok,
 #else
                                                 r,
 #endif
@@ -1959,7 +1959,7 @@ static void decode_block(VP10Decoder *const pbi, MACROBLOCKD *const xd,
             for (col = 0; col < max_blocks_wide; col += step)
               eobtotal += reconstruct_inter_block(xd,
 #if CONFIG_ANS
-                                                  pbi->token_tab, tok,
+                                                  cm->token_tab, tok,
 #else
                                                   r,
 #endif

diff --git a/vp10/decoder/decoder.c b/vp10/decoder/decoder.c
index 13a590a05dcf986bf5fd3bca9ac3d4495f2a3971..35c53df6d5902e8fa0f6d3d362902d9fd93b1d77 100644 (file)
--- a/vp10/decoder/decoder.c
+++ b/vp10/decoder/decoder.c
@@ -119,7 +119,7 @@ VP10Decoder *vp10_decoder_create(BufferPool *const pool) {
   vp10_loop_restoration_precal();
 #endif  // CONFIG_LOOP_RESTORATION
 #if CONFIG_ANS
-  vp10_build_pareto8_dec_tab(vp10_pareto8_token_probs, pbi->token_tab);
+  vp10_build_pareto8_cdf_tab(vp10_pareto8_token_probs, cm->token_tab);
 #endif  // CONFIG_ANS
 
   cm->error.setjmp = 0;

diff --git a/vp10/decoder/decoder.h b/vp10/decoder/decoder.h
index e590d8be16e5db82c64c7b47061aaefcf0e11d5b..a69d05f07b0e3d8231e763f4887dfa1a3f546303 100644 (file)
--- a/vp10/decoder/decoder.h
+++ b/vp10/decoder/decoder.h
@@ -89,9 +89,6 @@ typedef struct VP10Decoder {
   int inv_tile_order;
   int need_resync;  // wait for key/intra-only frame.
   int hold_ref_buf;  // hold the reference buffer.
-#if CONFIG_ANS
-  rans_dec_lut token_tab[COEFF_PROB_MODELS];
-#endif  // CONFIG_ANS
 } VP10Decoder;
 
 int vp10_receive_compressed_data(struct VP10Decoder *pbi,

diff --git a/vp10/encoder/bitstream.c b/vp10/encoder/bitstream.c
index fa6c2cbe7c6961d396803fe03c84c74e2a7a8d1f..2603b6b48659d9f91a3ea235d944a0edd8b9da72 100644 (file)
--- a/vp10/encoder/bitstream.c
+++ b/vp10/encoder/bitstream.c
@@ -634,6 +634,7 @@ static void pack_mb_tokens(vpx_writer *w,
 // This function serializes the tokens backwards both in token order and
 // bit order in each token.
 static void pack_mb_tokens_ans(struct AnsCoder *const ans,
+                               rans_dec_lut token_tab[COEFF_PROB_MODELS],
                                const TOKENEXTRA *const start,
                                const TOKENEXTRA *const stop,
                                vpx_bit_depth_t bit_depth) {
@@ -676,14 +677,10 @@ static void pack_mb_tokens_ans(struct AnsCoder *const ans,
 
       {
         struct rans_sym s;
-        int j;
-        const vpx_prob *token_probs =
-            vp10_pareto8_token_probs[p->context_tree[PIVOT_NODE] - 1];
-        s.cum_prob = 0;
-        for (j = ONE_TOKEN; j < t; ++j) {
-          s.cum_prob += token_probs[j - ONE_TOKEN];
-        }
-        s.prob = token_probs[t - ONE_TOKEN];
+        const rans_dec_lut *token_cdf =
+            &token_tab[p->context_tree[PIVOT_NODE] - 1];
+        s.cum_prob = (*token_cdf)[t - ONE_TOKEN];
+        s.prob = (*token_cdf)[t - ONE_TOKEN + 1] - s.cum_prob;
         rans_write(ans, &s);
       }
     }
@@ -2200,7 +2197,8 @@ static size_t encode_tiles(VP10_COMP *cpi, uint8_t *data_ptr,
                   NULL, NULL);
       vpx_stop_encode(&mode_bc);
       ans_write_init(&token_ans, mode_data_start + mode_bc.pos);
-      pack_mb_tokens_ans(&token_ans, tok, tok_end, cm->bit_depth);
+      pack_mb_tokens_ans(&token_ans, cm->token_tab, tok, tok_end,
+                         cm->bit_depth);
       token_section_size = ans_write_end(&token_ans);
       if (put_tile_size) {
         // size of this tile

diff --git a/vp10/encoder/encoder.c b/vp10/encoder/encoder.c
index fc9e2e9242a0eabf8c0760e7dfd69c2345998cdc..ac8d2770c9a68896d52c40ba139d5d160cbed0ef 100644 (file)
--- a/vp10/encoder/encoder.c
+++ b/vp10/encoder/encoder.c
@@ -1986,6 +1986,9 @@ VP10_COMP *vp10_create_compressor(VP10EncoderConfig *oxcf,
 #if CONFIG_LOOP_RESTORATION
   vp10_loop_restoration_precal();
 #endif  // CONFIG_LOOP_RESTORATION
+#if CONFIG_ANS
+  vp10_build_pareto8_cdf_tab(vp10_pareto8_token_probs, cm->token_tab);
+#endif  // CONFIG_ANS
 
   cm->error.setjmp = 0;