avfilter/vf_minterpolate: change the default threshold to get better scene change detect result

 ./ffmpeg -loglevel debug -i ../fate-suite/svq3/Vertical400kbit.sorenson3.mov -vf
 minterpolate=fps=60:mi_mode=blend -an -f null -
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 1600
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 4120
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 5780
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 6700
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 8140
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 9740
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 14060
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 15680
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 18480
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 20020
 [Parsed_minterpolate_0 @ 0x7fe7f3e193c0] scene changed, input pts 21740

 The results are consistent with tests/ref/fate/filter-metadata-scenedetect

 For the master, it'll detect more than 20 scene change for the same source.
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>

doc/filters.texi

@@ -13640,7 +13640,7 @@ Frame difference. Corresponding pixel values are compared and if it satisfies @v
 Default method is @samp{fdiff}.
 
 @item scd_threshold
-Scene change detection threshold. Default is @code{5.0}.
+Scene change detection threshold. Default is @code{10.}.
 @end table
 
 @section mix

libavfilter/vf_minterpolate.c

@@ -230,7 +230,7 @@ static const AVOption minterpolate_options[] = {
     { "scd", "scene change detection method", OFFSET(scd_method), AV_OPT_TYPE_INT, {.i64 = SCD_METHOD_FDIFF}, SCD_METHOD_NONE, SCD_METHOD_FDIFF, FLAGS, "scene" },
         CONST("none",   "disable detection",                    SCD_METHOD_NONE,        "scene"),
         CONST("fdiff",  "frame difference",                     SCD_METHOD_FDIFF,       "scene"),
-    { "scd_threshold", "scene change threshold", OFFSET(scd_threshold), AV_OPT_TYPE_DOUBLE, {.dbl = 5.0}, 0, 100.0, FLAGS },
+    { "scd_threshold", "scene change threshold", OFFSET(scd_threshold), AV_OPT_TYPE_DOUBLE, {.dbl = 10.}, 0, 100.0, FLAGS },
     { NULL }
 };
 
@@ -1105,6 +1105,7 @@ static void interpolate(AVFilterLink *inlink, AVFrame *avf_out)
     }
 
     if (mi_ctx->scene_changed) {
+        av_log(ctx, AV_LOG_DEBUG, "scene changed, input pts %"PRId64"\n", mi_ctx->frames[1].avf->pts);
         /* duplicate frame */
         av_frame_copy(avf_out, alpha > ALPHA_MAX / 2 ? mi_ctx->frames[2].avf : mi_ctx->frames[1].avf);
         return;