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vp9: 1 pass VBR: Fix to rate control at low min-q.

Fix to avoid getting stuck at very low Q even
though content is changing, which can happen for --min-q=0.

Fix is to more aggressively increase active_worst_quality
when detecting significant rate_deviation at very low Q.

Change will only affect 1 pass VBR for --min-q < 4, so no
change in ytlive metrics for --min-q >= 4.

Change-Id: I4dd77dd7c08a30a4390da0ff2c8bda6fccfa76d7

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

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

@@ -2172,6 +2172,11 @@

     if (rate_err < 2.0 && !high_content) {

       rc->fac_active_worst_inter = 120;

       rc->fac_active_worst_gf = 90;

+    } else if (rate_err > 8.0 && rc->avg_frame_qindex[INTER_FRAME] < 16) {

+      // Increase active_worst faster at low Q if rate fluctuation is high.

+      rc->fac_active_worst_inter = 200;

+      if (rc->avg_frame_qindex[INTER_FRAME] < 8)

+        rc->fac_active_worst_inter = 400;

     }

     if (low_content && rc->avg_frame_low_motion > 80) {

       rc->af_ratio_onepass_vbr = 15;