rtc-vp9: Fix to overshoot for 1 pass VBR no lookahead

Use the buffer_level to monitor the long-term bitrate
over/undershoot and avoid going to very low QP for
overshoot. This also required adding the
vp9_update_buffer_level_preencode() for the 1 pass VBR
encoding (as is done for CBR).

This reduces the overshoot in the issue below.

Bug: 376707227
Change-Id: Idae2bb0934543063e09ba7c32f80806cc9494f13

vp9/encoder/vp9_ratectrl.c

@@ -1299,6 +1299,27 @@ static int rc_pick_q_and_bounds_one_pass_vbr(const VP9_COMP *cpi,
   } else {
     q = vp9_rc_regulate_q(cpi, rc->this_frame_target, active_best_quality,
                           active_worst_quality);
+
+    // For no lookahead: if buffer_level indicates overshoot, then avoid going
+    // to very low QP. This reduces overshoot observed in Issue: 376707227.
+    // Note the buffer_level is updated for every encoded frame as:
+    // buffer_level - starting_buffer_level += (avg_frame_bandwidth -
+    // encoded_frame_size). So normalizing this with framerate and #encoded
+    // frames (current_video_frame) gives the difference/error between target
+    // and encoding bitrate. The additional avg_frame_bandwidth term is to
+    // compensate for the pre-encoded buffer update (in
+    // vp9_rc_get_one_pass_vbr_params).
+    const int bitrate_err =
+        (int)(cpi->framerate *
+              (rc->buffer_level - rc->starting_buffer_level -
+               rc->avg_frame_bandwidth) /
+              (cm->current_video_frame + 1));
+    // Threshold may be tuned, but for now condition this on low QP.
+    if (cpi->oxcf.lag_in_frames == 0 && bitrate_err / 1000 < -10 && q < 24) {
+      q = 24;
+      *top_index = VPXMAX(*top_index, q);
+    }
+
     if (q > *top_index) {
       // Special case when we are targeting the max allowed rate
       if (rc->this_frame_target >= rc->max_frame_bandwidth)
@@ -2126,6 +2147,7 @@ void vp9_rc_get_one_pass_vbr_params(VP9_COMP *cpi) {
   else
     target = vp9_calc_pframe_target_size_one_pass_vbr(cpi);
   vp9_rc_set_frame_target(cpi, target);
+  if (cm->show_frame) vp9_update_buffer_level_preencode(cpi);
   if (cpi->oxcf.aq_mode == CYCLIC_REFRESH_AQ && cpi->oxcf.pass == 0)
     vp9_cyclic_refresh_update_parameters(cpi);
 }