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Relax maximum Q for extreme overshoot.

Added code to relax the active maximum Q in response
to extreme local overshoot to reduce bandwidth peaks.

The impact is small in metrics terms, but it this helps reduce
bandwidth spikes and overall overshoot in a number of
clips in our tests sets (especially the YT test set).

In particular this should help prevent very big spikes where a clip
is mainly easy but has a short hard section. In such a case a choice
of maximum Q for the clip as a whole may allow us to hit the overall
target rate but give some extreme spikes. The chunked encoding in YT
mitigates this problem but it can show up where a longer clip is
coded as a single chunk.

Change-Id: I213d09950ccb8489d10adf00fda1e53235b39203

--- a/vp9/encoder/vp9_firstpass.c

+++ b/vp9/encoder/vp9_firstpass.c

@@ -2478,7 +2478,6 @@

 }

 #define MINQ_ADJ_LIMIT 32

-#define Q_LIMIT_STEP 1

 void vp9_twopass_postencode_update(VP9_COMP *cpi) {

   TWO_PASS *const twopass = &cpi->twopass;

   RATE_CONTROL *const rc = &cpi->rc;

@@ -2523,16 +2522,22 @@

     if (rc->rate_error_estimate > cpi->oxcf.under_shoot_pct) {

       --twopass->extend_maxq;

       if (rc->rolling_target_bits >= rc->rolling_actual_bits)

-        twopass->extend_minq += Q_LIMIT_STEP;

+        ++twopass->extend_minq;

     // Overshoot.

     } else if (rc->rate_error_estimate < -cpi->oxcf.over_shoot_pct) {

       --twopass->extend_minq;

       if (rc->rolling_target_bits < rc->rolling_actual_bits)

-        twopass->extend_maxq += Q_LIMIT_STEP;

+        ++twopass->extend_maxq;

     } else {

+      // Adjustment for extreme local overshoot.

+      if (rc->projected_frame_size > (2 * rc->base_frame_target) &&

+          rc->projected_frame_size > (2 * rc->avg_frame_bandwidth))

+        ++twopass->extend_maxq;

+

+      // Unwind undershoot or overshoot adjustment.

       if (rc->rolling_target_bits < rc->rolling_actual_bits)

         --twopass->extend_minq;

-      if (rc->rolling_target_bits > rc->rolling_actual_bits)

+      else if (rc->rolling_target_bits > rc->rolling_actual_bits)

         --twopass->extend_maxq;

     }

     twopass->extend_minq = clamp(twopass->extend_minq, 0, MINQ_ADJ_LIMIT);