Adjust cq_level in constrained quality mode

If we are already saving a lot in bits from the target (maximum)
bitrate in the constrained quality mode, allow the quantizer
to go lower than the cq level. This hopefully will solve issues
with getting too low a bitrate and consequently poor quality for
certain videos in cq mode.

Change-Id: I1c4e8b0171fcf58f95198b3add85eea5f3c8f19f

vp9/encoder/vp9_ratectrl.c

@@ -607,13 +607,26 @@ static int rc_pick_q_and_bounds_one_pass_cbr(const VP9_COMP *cpi,
   return q;
 }
 
+static int get_active_cq_level(const RATE_CONTROL *rc,
+                               const VP9EncoderConfig *const oxcf) {
+  static const double cq_adjust_threshold = 0.5;
+  int active_cq_level = oxcf->cq_level;
+  if (oxcf->rc_mode == RC_MODE_CONSTRAINED_QUALITY) {
+    const double x = rc->total_actual_bits / rc->total_target_bits;
+    if (x < cq_adjust_threshold) {
+      active_cq_level = active_cq_level * x / cq_adjust_threshold;
+    }
+  }
+  return active_cq_level;
+}
+
 static int rc_pick_q_and_bounds_one_pass_vbr(const VP9_COMP *cpi,
                                              int *bottom_index,
                                              int *top_index) {
   const VP9_COMMON *const cm = &cpi->common;
   const RATE_CONTROL *const rc = &cpi->rc;
   const VP9EncoderConfig *const oxcf = &cpi->oxcf;
-  const int cq_level = oxcf->cq_level;
+  const int cq_level = get_active_cq_level(rc, oxcf);
   int active_best_quality;
   int active_worst_quality = calc_active_worst_quality_one_pass_vbr(cpi);
   int q;
@@ -791,7 +804,7 @@ static int rc_pick_q_and_bounds_two_pass(const VP9_COMP *cpi,
   const VP9_COMMON *const cm = &cpi->common;
   const RATE_CONTROL *const rc = &cpi->rc;
   const VP9EncoderConfig *const oxcf = &cpi->oxcf;
-  const int cq_level = oxcf->cq_level;
+  const int cq_level = get_active_cq_level(rc, oxcf);
   int active_best_quality;
   int active_worst_quality = cpi->twopass.active_worst_quality;
   int q;