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vp9: Adjust qp_thresh on slide change overshoot detection

For real-time screen-content mode: increase the
qp_thresh for max_Q setting on slide changes.
This will make bitrate spikes less likely on slide changes.

Change-Id: Ie13524a06490214456b1c9c042a864ea0d0750c5

--- a/vp9/encoder/vp9_ratectrl.c

+++ b/vp9/encoder/vp9_ratectrl.c

@@ -2820,7 +2820,7 @@

   // If this decision is not based on an encoded frame size but just on

   // scene/slide change detection (i.e., re_encode_overshoot_rt = 0), adjust the

   // qp_thresh and skip the (frame_size > thresh_rate) condition in this case.

-  if (!sf->re_encode_overshoot_rt) thresh_qp = rc->worst_quality >> 1;

+  if (!sf->re_encode_overshoot_rt) thresh_qp = 3 * (rc->worst_quality >> 2);

   if ((!sf->re_encode_overshoot_rt || frame_size > thresh_rate) &&

       cm->base_qindex < thresh_qp) {

     double rate_correction_factor =