Allow setting video rate after adding stream

Synchronize the stream average frame rate from the codec context before
writing the output header. Closes #2301.

Author: WyattBlue

av/video/stream.py

@@ -63,6 +63,8 @@ def decode(self, packet: Packet | None = None):
     @cython.cfunc
     def _finalize_for_output(self):
         Stream._finalize_for_output(self)
+        if self.codec_context is not None:
+            self.ptr.avg_frame_rate = self.codec_context.ptr.framerate
         # avcodec_parameters_from_context() overwrites codecpar.coded_side_data,
         # so inject the display matrix after it, before avformat_write_header().
         if self.codec_context is not None and self._has_display_matrix:

tests/test_encode.py

@@ -162,6 +162,26 @@ def test_encoding_with_pts(self) -> None:
                 assert packet.time_base == Fraction(1, 24)
                 output.mux(packet)
 
+    def test_set_rate_after_add_stream(self) -> None:
+        path = self.sandboxed("deferred_rate.mp4")
+
+        with av.open(path, "w") as output:
+            stream = output.add_stream("mpeg4")
+            stream.codec_context.framerate = Fraction(30, 1)
+            stream.width = 16
+            stream.height = 16
+
+            for i in range(30):
+                frame = VideoFrame(16, 16, "yuv420p")
+                frame.pts = i
+                frame.time_base = Fraction(1, 30)
+                output.mux(stream.encode(frame))
+
+            output.mux(stream.encode(None))
+
+        with av.open(path) as input_:
+            assert input_.streams.video[0].average_rate == 30
+
     def test_encoding_with_unicode_filename(self) -> None:
         path = self.sandboxed("¢∞§¶•ªº.mov")