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Fix kf detection in some slide shows.

This fix improves detection of key frames in slide shows.

In particular it helps if the slides are pictures of varying formats
as in a sample provided by yclin@.

This change does not impact any of the clips in our standard tests
but for the example slide show test clip helped global psnr by
several db and resolved a serious visual quality issue.

Change-Id: Iaeeeed55dc0bb50aeacd4996ed660ced06374603

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

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

@@ -2701,6 +2701,15 @@

 #endif

 }

+// Slide show transition detection.

+// Tests for case where there is very low error either side of the current frame

+// but much higher just for this frame. This can help detect key frames in

+// slide shows even where the slides are pictures of different sizes.

+// It will not help if the transition is a fade or other multi-frame effect.

+static int slide_transition(double this_err, double last_err, double next_err) {

+  return (this_err > (last_err * 5.0)) && (this_err > (next_err * 5.0));

+}

+

 // 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.

@@ -2745,6 +2754,8 @@

   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) ||

+       (slide_transition(this_frame->coded_error, last_frame->coded_error,

+                         next_frame->coded_error)) ||

        ((pcnt_intra > MIN_INTRA_LEVEL) &&

         (pcnt_intra > (INTRA_VS_INTER_THRESH * modified_pcnt_inter)) &&

         ((this_frame->intra_error /