Modified test for auto key frame detection.

The existing test was triggering a lot of false positives on some types
of animated material with very plain backgrounds. These were triggering
code designed to catch key frames in letter box format clips.

This patch tightens up the criteria and imposes a minimum requirement
on the % blocks coded intra in the first pass and the ratio between the
% coded intra and the modified inter % after discounting neutral (flat)
blocks that are coded equally well either way.

On a particular problem animation clip this change eliminated a large
number of false positives including some cases where the old code
selected kf several times in a row. Marginal false negatives are less
damaging typically to compression and in the problem clip there are now
a couple of cases where "visual" scene cuts are ignored because of well
correlated content across the scene cut.

Replaced some magic numbers related to this with #defines and added
explanatory comments.

Change-Id: Ia3d304ac60eb7e4323e3817eaf83b4752cd63ecf

1 files changed, 42 insertions, 10 deletions

diff --git a/vp9/encoder/vp9_firstpass.c b/vp9/encoder/vp9_firstpass.c
index 8103abc83..456dabe75 100644
--- a/vp9/encoder/vp9_firstpass.c
+++ b/vp9/encoder/vp9_firstpass.c
@@ -2049,29 +2049,61 @@ static void define_gf_group(VP9_COMP *cpi, FIRSTPASS_STATS *this_frame) {
   }
 }
 
-// TODO(PGW) Re-examine the use of II ration in this code in the light of#
-// changes elsewhere
+// Threshold for use of the lagging second reference frame. High second ref
+// usage may point to a transient event like a flash or occlusion rather than
+// a real scene cut.
+#define SECOND_REF_USEAGE_THRESH 0.1
+// Minimum % intra coding observed in first pass (1.0 = 100%)
+#define MIN_INTRA_LEVEL 0.25
+// Minimum ratio between the % of intra coding and inter coding in the first
+// pass after discounting neutral blocks (discounting neutral blocks in this
+// way helps catch scene cuts in clips with very flat areas or letter box
+// format clips with image padding.
+#define INTRA_VS_INTER_THRESH 2.0
+// Hard threshold where the first pass chooses intra for almost all blocks.
+// In such a case even if the frame is not a scene cut coding a key frame
+// may be a good option.
+#define VERY_LOW_INTER_THRESH 0.05
+// Maximum threshold for the relative ratio of intra error score vs best
+// inter error score.
+#define KF_II_ERR_THRESHOLD 2.5
+// In real scene cuts there is almost always a sharp change in the intra
+// or inter error score.
+#define ERR_CHANGE_THRESHOLD 0.4
+// For real scene cuts we expect an improvment in the intra inter error
+// ratio in the next frame.
+#define II_IMPROVEMENT_THRESHOLD 3.5
 #define KF_II_MAX 128.0
+
 static int test_candidate_kf(TWO_PASS *twopass,
                              const FIRSTPASS_STATS *last_frame,
                              const FIRSTPASS_STATS *this_frame,
                              const FIRSTPASS_STATS *next_frame) {
   int is_viable_kf = 0;
+  double pcnt_intra = 1.0 - this_frame->pcnt_inter;
+  double modified_pcnt_inter =
+    this_frame->pcnt_inter - this_frame->pcnt_neutral;
 
   // Does the frame satisfy the primary criteria of a key frame?
+  // See above for an explanation of the test criteria.
   // If so, then examine how well it predicts subsequent frames.
-  if ((this_frame->pcnt_second_ref < 0.10) &&
-      (next_frame->pcnt_second_ref < 0.10) &&
-      ((this_frame->pcnt_inter < 0.05) ||
-       (((this_frame->pcnt_inter - this_frame->pcnt_neutral) < 0.35) &&
+  if ((this_frame->pcnt_second_ref < SECOND_REF_USEAGE_THRESH) &&
+      (next_frame->pcnt_second_ref < SECOND_REF_USEAGE_THRESH) &&
+      ((this_frame->pcnt_inter < VERY_LOW_INTER_THRESH) ||
+       ((pcnt_intra > MIN_INTRA_LEVEL) &&
+        (pcnt_intra > (INTRA_VS_INTER_THRESH * modified_pcnt_inter)) &&
         ((this_frame->intra_error /
-          DOUBLE_DIVIDE_CHECK(this_frame->coded_error)) < 2.5) &&
+          DOUBLE_DIVIDE_CHECK(this_frame->coded_error)) <
+          KF_II_ERR_THRESHOLD) &&
         ((fabs(last_frame->coded_error - this_frame->coded_error) /
-              DOUBLE_DIVIDE_CHECK(this_frame->coded_error) > 0.40) ||
+          DOUBLE_DIVIDE_CHECK(this_frame->coded_error) >
+          ERR_CHANGE_THRESHOLD) ||
          (fabs(last_frame->intra_error - this_frame->intra_error) /
-              DOUBLE_DIVIDE_CHECK(this_frame->intra_error) > 0.40) ||
+          DOUBLE_DIVIDE_CHECK(this_frame->intra_error) >
+          ERR_CHANGE_THRESHOLD) ||
          ((next_frame->intra_error /
-           DOUBLE_DIVIDE_CHECK(next_frame->coded_error)) > 3.5))))) {
+          DOUBLE_DIVIDE_CHECK(next_frame->coded_error)) >
+          II_IMPROVEMENT_THRESHOLD))))) {
     int i;
     const FIRSTPASS_STATS *start_pos = twopass->stats_in;
     FIRSTPASS_STATS local_next_frame = *next_frame;