lavfi/concat: allow to support inputs with different frame rates

Right now, the concat filter does not set the frame_rate value on any of
the out links. As a result, the default ffmpeg behaviour kicks in - to
copy the framerate from the first input to the outputs.

If a later input is higher framerate, this results in dropped frames; if
a later input is lower framerate it might cause judder.

This patch checks if all of the video inputs have the same framerate, and
if not it sets the out link to use '1/0' as the frame rate, the value
meaning "unknown/vfr".

A test is added to verify the VFR behaviour. The existing test for CFR
behaviour passes unchanged.

libavfilter/avf_concat.c

@@ -131,8 +131,21 @@ static int config_output(AVFilterLink *outlink)
     outlink->h                   = inlink->h;
     outlink->sample_aspect_ratio = inlink->sample_aspect_ratio;
     outlink->format              = inlink->format;
+    outlink->frame_rate          = inlink->frame_rate;
+
+    for (seg = 1; seg < cat->nb_segments; seg++) {
+        inlink = ctx->inputs[in_no + seg * ctx->nb_outputs];
+        if (outlink->frame_rate.num != inlink->frame_rate.num ||
+            outlink->frame_rate.den != outlink->frame_rate.den) {
+            av_log(ctx, AV_LOG_VERBOSE,
+                    "Video inputs have different frame rates, output will be VFR\n");
+            outlink->frame_rate = av_make_q(1, 0);
+            break;
+        }
+    }
+
     for (seg = 1; seg < cat->nb_segments; seg++) {
-        inlink = ctx->inputs[in_no += ctx->nb_outputs];
+        inlink = ctx->inputs[in_no + seg * ctx->nb_outputs];
         if (!outlink->sample_aspect_ratio.num)
             outlink->sample_aspect_ratio = inlink->sample_aspect_ratio;
         /* possible enhancement: unsafe mode, do not check */

tests/fate/filter-video.mak

@@ -424,9 +424,11 @@ FATE_FILTER_SAMPLES-$(call ALLYES, VMD_DEMUXER VMDVIDEO_DECODER FORMAT_FILTER PE
 fate-filter-gradfun-sample: tests/data/filtergraphs/gradfun
 fate-filter-gradfun-sample: CMD = framecrc -i $(TARGET_SAMPLES)/vmd/12.vmd -filter_script $(TARGET_PATH)/tests/data/filtergraphs/gradfun -an -frames:v 20
 
-FATE_FILTER-$(call ALLYES, TESTSRC_FILTER SINE_FILTER CONCAT_FILTER) += fate-filter-concat
+FATE_FILTER-$(call ALLYES, TESTSRC_FILTER SINE_FILTER CONCAT_FILTER) += fate-filter-concat fate-filter-concat-vfr
 fate-filter-concat: tests/data/filtergraphs/concat
 fate-filter-concat: CMD = framecrc -filter_complex_script $(TARGET_PATH)/tests/data/filtergraphs/concat
+fate-filter-concat-vfr: tests/data/filtergraphs/concat-vfr
+fate-filter-concat-vfr: CMD = framecrc -filter_complex_script $(TARGET_PATH)/tests/data/filtergraphs/concat-vfr
 
 FATE_FILTER-$(call ALLYES, TESTSRC2_FILTER FPS_FILTER MPDECIMATE_FILTER) += fate-filter-mpdecimate
 fate-filter-mpdecimate: CMD = framecrc -lavfi testsrc2=r=2:d=10,fps=3,mpdecimate -r 3 -pix_fmt yuv420p

tests/filtergraphs/concat-vfr

+testsrc=r=5:n=1:d=2  [v1];
+sine=440:b=2:d=1     [a1];
+testsrc=r=15:n=1:d=1 [v2];
+sine=622:b=2:d=2     [a2];
+testsrc=r=8:n=1:d=1  [v3];
+sine=880:b=2:d=1     [a3];
+
+[v1][a1][v2][a2][v3][a3] concat=v=1:a=1:n=3

tests/ref/fate/filter-concat-vfr

+#tb 0: 1/1000000
+#media_type 0: video
+#codec_id 0: rawvideo
+#dimensions 0: 320x240
+#sar 0: 1/1
+#tb 1: 1/44100
+#media_type 1: audio
+#codec_id 1: pcm_s16le
+#sample_rate 1: 44100
+#channel_layout 1: 4
+#channel_layout_name 1: mono
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