Adjustment to prediction decay.

Adjustment to stop excessive prediction decay triggered by blocks
or frames with extremely low spatial complexity which rendered the
comparison of intra and inter coded errors meaningless.

This was causing much shorter than expected groups on some 4k
test content.

Change-Id: I3f2c64200ef6dcef4721fc9f2ec09e480056ffc2

vp9/encoder/vp9_firstpass.c

diff --git a/vp9/encoder/vp9_firstpass.c b/vp9/encoder/vp9_firstpass.c
index b5f52ad..f91a9a8 100644
--- a/vp9/encoder/vp9_firstpass.c
+++ b/vp9/encoder/vp9_firstpass.c
@@ -65,7 +65,7 @@
 
 #define NCOUNT_INTRA_THRESH 8192
 #define NCOUNT_INTRA_FACTOR 3
-#define NCOUNT_FRAME_II_THRESH 5.0
+
 
 #define DOUBLE_DIVIDE_CHECK(x) ((x) < 0 ? (x) - 0.000001 : (x) + 0.000001)
 
@@ -1382,6 +1382,8 @@
 #define DEFAULT_DECAY_LIMIT 0.75
 #define LOW_SR_DIFF_TRHESH 0.1
 #define SR_DIFF_MAX 128.0
+#define LOW_CODED_ERR_PER_MB 10.0
+#define NCOUNT_FRAME_II_THRESH 6.0
 
 static double get_sr_decay_rate(const VP9_COMP *cpi,
                                 const FIRSTPASS_STATS *frame) {
@@ -1398,8 +1400,9 @@
        (cpi->initial_height + cpi->initial_width));
 
   modified_pct_inter = frame->pcnt_inter;
-  if ((frame->intra_error / DOUBLE_DIVIDE_CHECK(frame->coded_error)) <
-      (double)NCOUNT_FRAME_II_THRESH) {
+  if (((frame->coded_error / num_mbs) > LOW_CODED_ERR_PER_MB) &&
+      ((frame->intra_error / DOUBLE_DIVIDE_CHECK(frame->coded_error)) <
+       (double)NCOUNT_FRAME_II_THRESH)) {
     modified_pct_inter = frame->pcnt_inter - frame->pcnt_neutral;
   }
   modified_pcnt_intra = 100 * (1.0 - modified_pct_inter);