From 3530fdc78ed8ca6f246dcaeb761bea0f5b76fc44 Mon Sep 17 00:00:00 2001
From: Paul B Mahol <onemda@gmail.com>
Date: Tue, 17 Dec 2019 13:28:27 +0100
Subject: [PATCH] avfilter/vf_stack: set framerate to VFR when inputs frame
 rates differs

---
 libavfilter/vf_stack.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/libavfilter/vf_stack.c b/libavfilter/vf_stack.c
index b9e87a4429..5bc4ccf6ed 100644
--- a/libavfilter/vf_stack.c
+++ b/libavfilter/vf_stack.c
@@ -310,6 +310,17 @@ static int config_output(AVFilterLink *outlink)
     outlink->frame_rate = frame_rate;
     outlink->sample_aspect_ratio = sar;
 
+    for (i = 1; i < s->nb_inputs; i++) {
+        AVFilterLink *inlink = ctx->inputs[i];
+        if (outlink->frame_rate.num != inlink->frame_rate.num ||
+            outlink->frame_rate.den != inlink->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;
+        }
+    }
+
     if ((ret = ff_framesync_init(&s->fs, ctx, s->nb_inputs)) < 0)
         return ret;