Prevent severe rate control errors in CBR mode

In rare cases, the interaction between rate correction factor and Q
choices may cause severe oscillating frame sizes that are way off
target bandwidth. This commit adds tracking of rate control results
for last two frames, and use the information to prevent oscillating
Q choices.

Change-Id: I9a6d125a15652b9bcac0e1fec6d7a1aedc4ed97e

2 files changed, 28 insertions, 3 deletions

diff --git a/vp9/encoder/vp9_ratectrl.c b/vp9/encoder/vp9_ratectrl.c
index c19602a14..2390e6fb7 100644
--- a/vp9/encoder/vp9_ratectrl.c
+++ b/vp9/encoder/vp9_ratectrl.c
@@ -430,7 +430,8 @@ void vp9_rc_update_rate_correction_factors(VP9_COMP *cpi, int damp_var) {
       adjustment_limit = 0.75;
       break;
     case 1:
-      adjustment_limit = 0.125 + 0.5 * MIN(1, fabs(log10(0.01 * correction_factor)));
+      adjustment_limit = 0.25 +
+          0.5 * MIN(1, fabs(log10(0.01 * correction_factor)));
       break;
     case 2:
     default:
@@ -438,12 +439,21 @@ void vp9_rc_update_rate_correction_factors(VP9_COMP *cpi, int damp_var) {
       break;
   }
 
+  cpi->rc.q_2_frame = cpi->rc.q_1_frame;
+  cpi->rc.q_1_frame = cm->base_qindex;
+  cpi->rc.rc_2_frame = cpi->rc.rc_1_frame;
+  if (correction_factor > 110)
+    cpi->rc.rc_1_frame = -1;
+  else if (correction_factor < 90)
+    cpi->rc.rc_1_frame = 1;
+  else
+    cpi->rc.rc_1_frame = 0;
+
   if (correction_factor > 102) {
     // We are not already at the worst allowable quality
     correction_factor = (int)(100 + ((correction_factor - 100) *
                                   adjustment_limit));
     rate_correction_factor = (rate_correction_factor * correction_factor) / 100;
-
     // Keep rate_correction_factor within limits
     if (rate_correction_factor > MAX_BPB_FACTOR)
       rate_correction_factor = MAX_BPB_FACTOR;
@@ -494,6 +504,14 @@ int vp9_rc_regulate_q(const VP9_COMP *cpi, int target_bits_per_frame,
     }
   } while (++i <= active_worst_quality);
 
+  // In CBR mode, this makes sure q is between oscillating Qs to prevent
+  // resonance.
+  if (cpi->oxcf.rc_mode == VPX_CBR &&
+      (cpi->rc.rc_1_frame * cpi->rc.rc_2_frame == -1) &&
+      cpi->rc.q_1_frame != cpi->rc.q_2_frame) {
+    q = clamp(q, MIN(cpi->rc.q_1_frame, cpi->rc.q_2_frame),
+              MAX(cpi->rc.q_1_frame, cpi->rc.q_2_frame));
+  }
   return q;
 }
 
diff --git a/vp9/encoder/vp9_ratectrl.h b/vp9/encoder/vp9_ratectrl.h
index 2bc5b59f2..6200b396c 100644
--- a/vp9/encoder/vp9_ratectrl.h
+++ b/vp9/encoder/vp9_ratectrl.h
@@ -99,7 +99,14 @@ typedef struct {
   int64_t starting_buffer_level;
   int64_t optimal_buffer_level;
   int64_t maximum_buffer_size;
-  // int active_best_quality;
+  // rate control history for last frame(1) and the frame before(2).
+  // -1: undershot
+  //  1: overshoot
+  //  0: not initialized.
+  int rc_1_frame;
+  int rc_2_frame;
+  int q_1_frame;
+  int q_2_frame;
 } RATE_CONTROL;
 
 struct VP9_COMP;