From 553bcdc324ee9f08357463e26ce479846a65206c Mon Sep 17 00:00:00 2001
From: Ari Koivula <ari@koivu.la>
Date: Wed, 18 Dec 2013 12:48:20 +0200
Subject: [PATCH] Rewrite intra prediction mode coding to be simpler.

Should work exactly the same, but with the prediction mode selection done
separately from the binarization it's easier to see that the implementation
is correct.
---
 src/encoder.c | 63 +++++++++++++++++++++++++++------------------------
 1 file changed, 33 insertions(+), 30 deletions(-)

diff --git a/src/encoder.c b/src/encoder.c
index 0e363bd8..a823443b 100644
--- a/src/encoder.c
+++ b/src/encoder.c
@@ -1241,41 +1241,44 @@ void encode_coding_tree(encoder_control *encoder, uint16_t x_ctb,
       CABAC_BINS_EP(&cabac, tmp_pred, 5, "rem_intra_luma_pred_mode");
     }
 
-    // Chroma intra prediction
-    /** Table 9-35 – Binarization for intra_chroma_pred_mode
-     * intra_chroma_pred_mode  bin_string
-     *                      4           0
-     *                      0         100
-     *                      1         101
-     *                      2         110
-     *                      3         111
-     */
-    cabac.ctx = &g_chroma_pred_model[0];
-    CABAC_BIN(&cabac, ((intra_pred_mode_chroma != 36) ? 1 : 0), "intra_chroma_pred_mode");
+    {  // start intra chroma pred mode coding
+      unsigned pred_mode = 5;
+      unsigned chroma_pred_modes[4] = {0, 26, 10, 1};
 
-    // If not copied from luma, signal it
-    if (intra_pred_mode_chroma != 36) {
-      int8_t tmp_pred = intra_pred_mode_chroma;
-      // Default chroma predictors
-      uint32_t allowed_chroma_dir[5] = { 0, 26, 10, 1, 36 };
-          
-      // If intra is the same as one of the default predictors, replace it
-      for (i = 0; i < 4; i++) {
-        if (intra_pred_mode == allowed_chroma_dir[i]) {
-          allowed_chroma_dir[i] = 34;  // VER+8 mode
-          break;
+      if (intra_pred_mode_chroma == 36) {
+        pred_mode = 4;
+      } else if (intra_pred_mode_chroma == 34) {
+        // Angular 34 mode is possible only if intra pred mode is one of the
+        // possible chroma pred modes, in which case it is signaled with that
+        // duplicate mode.
+        for (i = 0; i < 4; ++i) {
+          if (intra_pred_mode == chroma_pred_modes[i]) pred_mode = i;
+        }
+      } else {
+        for (i = 0; i < 4; ++i) {
+          if (intra_pred_mode_chroma == chroma_pred_modes[i]) pred_mode = i;
         }
       }
 
-      for (i = 0; i < 4; i++) {
-        if (tmp_pred == allowed_chroma_dir[i]) {
-          tmp_pred = i;
-          break;
-        }
+      /**
+       * Table 9-35 – Binarization for intra_chroma_pred_mode
+       *   intra_chroma_pred_mode  bin_string
+       *                        4           0
+       *                        0         100
+       *                        1         101
+       *                        2         110
+       *                        3         111
+       * Table 9-37 - Assignment of ctxInc to syntax elements with context coded bins
+       *   intra_chroma_pred_mode[][] = 0, bypass, bypass
+       */
+      cabac.ctx = &g_chroma_pred_model[0];
+      if (pred_mode == 4) {
+        CABAC_BIN(&cabac, 0, "intra_chroma_pred_mode");
+      } else {
+        CABAC_BIN(&cabac, 1, "intra_chroma_pred_mode");
+        CABAC_BINS_EP(&cabac, pred_mode, 2, "intra_chroma_pred_mode");
       }
-
-      CABAC_BINS_EP(&cabac, tmp_pred, 2, "intra_chroma_pred_mode");
-    }
+    }  // end intra chroma pred mode coding
 
     encode_transform_coeff(encoder, x_ctb, y_ctb, depth, 0, 0, 0);
   }