diff --git a/meson.build b/meson.build
index be5caac..d5f4cca 100644
--- a/meson.build
+++ b/meson.build
@@ -53,10 +53,12 @@ if host_machine.cpu_family().startswith('x86')
     )
 endif
 
+avutil_dep = dependency('libavutil', version: '>=59.39.0')
+
 deps = [
+    avutil_dep,
     dependency('libavcodec', version: '>=61.19.0'),
     dependency('libavformat', version: '>=61.7.0'),
-    dependency('libavutil', version: '>=59.39.0'),
     dependency('libxxhash'),
 ]
 
@@ -100,7 +102,7 @@ if get_option('enable_plugin')
     endif
 
     shared_module('bestsource', plugin_sources,
-        dependencies: [bestsource_dep, vapoursynth_dep],
+        dependencies: [bestsource_dep, vapoursynth_dep, avutil_dep.partial_dependency(compile_args: true, includes: true)],
         gnu_symbol_visibility: 'hidden',
         install: true,
         install_dir: vapoursynth_dep.get_variable(pkgconfig: 'libdir') / 'vapoursynth',
diff --git a/src/avisynth.cpp b/src/avisynth.cpp
index 64e99d4..3925e21 100644
--- a/src/avisynth.cpp
+++ b/src/avisynth.cpp
@@ -255,10 +255,9 @@ class AvisynthVideoSource : public IClip {
             Env->ThrowError("BestVideoSource: %s", e.what());
         }
 
-        const BSVideoProperties &VP = V->GetVideoProperties();
         AVSMap *Props = Env->getFramePropsRW(Dst);
 
-        SetSynthFrameProperties(Src, VP, RFF, V->GetFrameIsTFF(n, RFF),
+        SetSynthFrameProperties(n, Src, *V, RFF, V->GetFrameIsTFF(n, RFF),
             [Props, Env](const char *Name, int64_t V) { Env->propSetInt(Props, Name, V, 1); },
             [Props, Env](const char *Name, double V) { Env->propSetFloat(Props, Name, V, 1); },
             [Props, Env](const char *Name, const char *V, int Size, bool Utf8) { Env->propSetData(Props, Name, V, Size, 1); });
diff --git a/src/synthshared.cpp b/src/synthshared.cpp
index 852d92a..1382885 100644
--- a/src/synthshared.cpp
+++ b/src/synthshared.cpp
@@ -21,7 +21,13 @@
 #include "synthshared.h"
 #include "VSHelper4.h"
 
-void SetSynthFrameProperties(const std::unique_ptr<BestVideoFrame> &Src, const BSVideoProperties &VP, bool RFF, bool TFF, const std::function<void(const char *, int64_t)> &mapSetInt, const std::function<void(const char *, double)> &mapSetFloat, const std::function<void(const char *, const char *, int, bool)> &mapSetData) {
+extern "C" {
+#include <libavutil/avutil.h>
+}
+
+void SetSynthFrameProperties(int n, const std::unique_ptr<BestVideoFrame> &Src, const BestVideoSource &VS, bool RFF, bool TFF, const std::function<void(const char *, int64_t)> &mapSetInt, const std::function<void(const char *, double)> &mapSetFloat, const std::function<void(const char *, const char *, int, bool)> &mapSetData) {
+    const BSVideoProperties VP = VS.GetVideoProperties();
+
     // Set AR variables
     if (VP.SAR.Num > 0 && VP.SAR.Den > 0) {
         mapSetInt("_SARNum", VP.SAR.Num);
@@ -49,14 +55,22 @@ void SetSynthFrameProperties(const std::unique_ptr<BestVideoFrame> &Src, const B
         mapSetInt("_FieldBased", FieldBased);
         mapSetInt("RepeatField", Src->RepeatPict);
 
-        // FIXME, use PTS difference between frames instead?
-        if (Src->Duration > 0) {
-            int64_t DurNum = VP.TimeBase.Num;
-            int64_t DurDen = VP.TimeBase.Den;
-            vsh::muldivRational(&DurNum, &DurDen, Src->Duration, 1);
-            mapSetInt("_DurationNum", DurNum);
-            mapSetInt("_DurationDen", DurDen);
+        if (n < VP.NumFrames - 1) {
+            int64_t NextPTS = VS.GetFrameInfo(n + 1).PTS;
+
+            // Leave _Duration unset when it can't be computed reliably, let callers decide
+            // whether or how to guess values.
+            if (Src->PTS != AV_NOPTS_VALUE && NextPTS != AV_NOPTS_VALUE && NextPTS > Src->PTS) {
+                int64_t DurNum = VP.TimeBase.Num;
+                int64_t DurDen = VP.TimeBase.Den;
+
+                vsh::muldivRational(&DurNum, &DurDen, NextPTS - Src->PTS, 1);
+                mapSetInt("_DurationNum", DurNum);
+                mapSetInt("_DurationDen", DurDen);
+            }
         }
+        // FIXME Use Src->Duration or the track's duration for the last frame?
+        // These are how you'd compute the last frame's duration, but they're not always accurate.
     }
 
     mapSetInt("TopFieldFirst", TFF);
diff --git a/src/synthshared.h b/src/synthshared.h
index 15ac94b..5fc0107 100644
--- a/src/synthshared.h
+++ b/src/synthshared.h
@@ -26,6 +26,6 @@
 #include <functional>
 #include <cstdint>
 
-void SetSynthFrameProperties(const std::unique_ptr<BestVideoFrame> &Src, const BSVideoProperties &VP, bool RFF, bool TFF, const std::function<void(const char *, int64_t)> &mapSetInt, const std::function<void(const char *, double)> &mapSetFloat, const std::function<void(const char *, const char *, int, bool)> &mapSetData);
+void SetSynthFrameProperties(int n, const std::unique_ptr<BestVideoFrame> &Src, const BestVideoSource &VS, bool RFF, bool TFF, const std::function<void(const char *, int64_t)> &mapSetInt, const std::function<void(const char *, double)> &mapSetFloat, const std::function<void(const char *, const char *, int, bool)> &mapSetData);
 
 #endif
\ No newline at end of file
diff --git a/src/vapoursynth.cpp b/src/vapoursynth.cpp
index 6d3fafb..9b8b606 100644
--- a/src/vapoursynth.cpp
+++ b/src/vapoursynth.cpp
@@ -108,12 +108,11 @@ static const VSFrame *VS_CC BestVideoSourceGetFrame(int n, int ActivationReason,
             return nullptr;
         }
 
-        const BSVideoProperties &VP = D->V->GetVideoProperties();
         VSMap *Props = vsapi->getFramePropertiesRW(Dst);
         if (AlphaDst)
             vsapi->mapConsumeFrame(Props, "_Alpha", AlphaDst, maAppend);
 
-        SetSynthFrameProperties(Src, VP, D->RFF, D->V->GetFrameIsTFF(n, D->RFF),
+        SetSynthFrameProperties(n, Src, *D->V, D->RFF, D->V->GetFrameIsTFF(n, D->RFF),
             [Props, vsapi](const char *Name, int64_t V) { vsapi->mapSetInt(Props, Name, V, maAppend); },
             [Props, vsapi](const char *Name, double V) { vsapi->mapSetFloat(Props, Name, V, maAppend); },
             [Props, vsapi](const char *Name, const char *V, int Size, bool Utf8) { vsapi->mapSetData(Props, Name, V, Size, Utf8 ? dtUtf8 : dtBinary, maAppend); });