vfr: improve duplicate frame drop detection

More strictly limit the range of frames inspected when choosing a frame
to drop.

We were getting too many false positives which resulted in dropping
good frames. Limiting the range results in emitting good frames
before they can become the worst of a collection of other good
frames, and therefore be evaluated as droppable.

1 files changed, 47 insertions, 19 deletions

diff --git a/libhb/vfr.c b/libhb/vfr.c
index 0629b90fa..460f5eb14 100644
--- a/libhb/vfr.c
+++ b/libhb/vfr.c
@@ -34,6 +34,7 @@ struct hb_filter_private_s
 
     // Duplicate frame detection members
     int             frame_analysis_depth;
+    int64_t         frame_analysis_duration;
     hb_list_t     * frame_rate_list;
     double        * frame_metric;
 
@@ -138,19 +139,55 @@ static void delete_metric(double * metrics, int pos, int size)
     memmove(dst, src, msize);
 }
 
-static int find_drop_frame(double *metrics, int count)
+static int find_drop_frame(hb_filter_private_t * pv, int count)
 {
-    int ii, min;
+    int           ii, min;
+    double      * metrics = pv->frame_metric;
+    hb_buffer_t * buf, * first;
+    double        cfr_stop;
 
-    min = 1;
-    for (ii = 2; ii < count - 1; ii++)
+    // compute where the second to last frame in the frame_rate_list would
+    // stop if the frame rate were constant.
+    //
+    // this is our target stopping time for CFR and earliest possible
+    // stopping time for PFR.
+    cfr_stop = pv->out_last_stop + pv->frame_duration * (count - 1);
+
+    // If the last frame's stop timestamp is before the calculated
+    // CFR stop time of the second to last frame, then we need to drop a frame.
+    buf   = hb_list_item(pv->frame_rate_list, count - 1);
+
+    // Shortcut exit when entire list is CFR OK
+    if (buf->s.stop >= (int64_t)cfr_stop)
     {
-        if (metrics[ii] < metrics[min])
+        return -1;
+    }
+
+    first = hb_list_item(pv->frame_rate_list, 0);
+    min = 0;
+    for (ii = 1; ii < count; ii++)
+    {
+        hb_buffer_t * buf = hb_list_item(pv->frame_rate_list, ii);
+        // Don't check buffers outside analysis window
+        if (buf->s.stop - first->s.start > pv->frame_analysis_duration)
+        {
+            break;
+        }
+        if (min < 0 || metrics[ii] < metrics[min])
         {
             min = ii;
         }
     }
 
+    cfr_stop = pv->out_last_stop + pv->frame_duration * (ii - 1);
+    buf      = hb_list_item(pv->frame_rate_list, ii - 1);
+
+    // Don't drop buffers if frames fit in CRF time bounds
+    if (buf->s.stop >= (int64_t)cfr_stop)
+    {
+        return -1;
+    }
+
     return min;
 }
 
@@ -220,24 +257,15 @@ static hb_buffer_t * adjust_frame_rate( hb_filter_private_t * pv,
     }
 
     hb_buffer_list_t   list;
-    hb_buffer_t      * out, * last;
+    hb_buffer_t      * out;
     double             cfr_stop;
+    int                drop_frame;
 
     hb_buffer_list_clear(&list);
 
-    // compute where the second to last frame in the frame_rate_list would
-    // stop if the frame rate were constant.
-    //
-    // this is our target stopping time for CFR and earliest possible
-    // stopping time for PFR.
-    cfr_stop = pv->out_last_stop + pv->frame_duration * (count - 1);
-
-    // If the last frame's stop timestamp is before the calculated
-    // CFR stop time of the second to last frame, then we need to drop a frame.
-    last = hb_list_item(pv->frame_rate_list, count - 1);
-    if (last->s.stop < cfr_stop)
+    drop_frame = find_drop_frame(pv, count);
+    if (drop_frame >= 0)
     {
-        int drop_frame;
 
         // We may have to drop multiple frames.  Pick frames to drop
         // that appear to have minimum motion.
@@ -246,7 +274,6 @@ static hb_buffer_t * adjust_frame_rate( hb_filter_private_t * pv,
         // "progressive telecine" where there is a repeating pattern
         // of a new frame followed by some number of repeated frames.
         // We want to keep the "new frames" and drop the repeates.
-        drop_frame = find_drop_frame(pv->frame_metric, count);
         out = hb_list_item(pv->frame_rate_list, drop_frame);
 
 #if defined(HB_DEBUG_CFR_DROPS)
@@ -383,6 +410,7 @@ static int hb_vfr_init(hb_filter_object_t *filter, hb_filter_init_t *init)
             pv->frame_analysis_depth = MAX_FRAME_ANALYSIS_DEPTH;
         }
     }
+    pv->frame_analysis_duration = pv->frame_analysis_depth * 90000 / in_vrate;
     pv->frame_metric = calloc(pv->frame_analysis_depth, sizeof(double));
     pv->frame_metric[0] = INT_MAX;