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Fix bug in use of zoom metric as part of arf breakout.

The in/out (or zoom metrics) in accumulate_frame_motion_stats()
are in effect a % of the blocks that have a motion vector pointing
either towards or away from the center. As such they are already
normalized in terms of image size and the thresholds against which
these are tested should be image size independent.

In practice a zoom either in or out is an indicator for a shorter group
length so the abs value is more important as a breakout clause.

This patch fixes the threshold test. Clips without noticeable zoom show
no effect but some  with strong zooms such as "station" show a big
gain (5-10%). Average psnr-hvs gain on hdres set was 0.292%

Change-Id: I4f97a72b0e273e4e844ade15285749c32cd81c1c
(cherry picked from commit 0226ce79e9389ccf7d10ed7acacba6840ad911c9)

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

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

@@ -44,7 +44,6 @@

 #define COMPLEXITY_STATS_OUTPUT 0

 #define FIRST_PASS_Q 10.0

-#define GF_MAX_BOOST 96.0

 #define INTRA_MODE_PENALTY 1024

 #define MIN_ARF_GF_BOOST 240

 #define MIN_DECAY_FACTOR 0.01

@@ -1949,6 +1948,7 @@

 }

 #define BASELINE_ERR_PER_MB 12500.0

+#define GF_MAX_BOOST 96.0

 static double calc_frame_boost(VP9_COMP *cpi, const FIRSTPASS_STATS *this_frame,

                                double this_frame_mv_in_out) {

   double frame_boost;

@@ -2383,6 +2383,8 @@

 // Analyse and define a gf/arf group.

 #define ARF_DECAY_BREAKOUT 0.10

+#define ARF_ABS_ZOOM_THRESH 4.0

+

 static void define_gf_group(VP9_COMP *cpi, FIRSTPASS_STATS *this_frame) {

   VP9_COMMON *const cm = &cpi->common;

   RATE_CONTROL *const rc = &cpi->rc;

@@ -2411,7 +2413,6 @@

   double mv_in_out_accumulator = 0.0;

   double abs_mv_in_out_accumulator = 0.0;

   double mv_ratio_accumulator_thresh;

-  double mv_in_out_thresh;

   double abs_mv_in_out_thresh;

   double sr_accumulator = 0.0;

   const double av_err = get_distribution_av_err(cpi, twopass);

@@ -2457,8 +2458,7 @@

   // Motion breakout threshold for loop below depends on image size.

   mv_ratio_accumulator_thresh =

       (cpi->initial_height + cpi->initial_width) / 4.0;

-  mv_in_out_thresh = (cpi->initial_height + cpi->initial_width) / 300.0;

-  abs_mv_in_out_thresh = (cpi->initial_height + cpi->initial_width) / 200.0;

+  abs_mv_in_out_thresh = ARF_ABS_ZOOM_THRESH;

   // Set a maximum and minimum interval for the GF group.

   // If the image appears almost completely static we can extend beyond this.

@@ -2562,7 +2562,6 @@

             (!flash_detected) &&

             ((mv_ratio_accumulator > mv_ratio_accumulator_thresh) ||

              (abs_mv_in_out_accumulator > abs_mv_in_out_thresh) ||

-             (mv_in_out_accumulator < -mv_in_out_thresh) ||

              (sr_accumulator > next_frame.intra_error)))) {

       break;

     }