Fix Video Stream Time Base Causing Frame Duration Jitter

Util/FFmpeg/FFmpegNativeEncoder.cs

@@ -723,6 +723,16 @@ namespace ADOFAIRenderer.Util.FFmpeg
                     if (ffmpeg.avcodec_parameters_from_context(_pVideoStream->codecpar, _pCodecContext) < 0)
                         throw new Exception("Failed to copy codec parameters to stream.");
 
+                    // Stream time base: unit of the timestamps stored in the container.
+                    // Set to the codec time base (1 / framerate) so that every frame interval is a whole number of ticks.
+                    // When it is left unset, the muxer uses 1/90000, in which the interval of rates such as
+                    // 59.94 fps (1501.5 ticks), 23.976 fps (3753.75 ticks) or 165 fps (545.45 ticks) is not an integer,
+                    // and the frame durations alternate between two values.
+                    // The muxer may still adjust the value in avformat_write_header (e.g. mp4 scales it to at least 10000);
+                    // packet timestamps are rescaled to the final value in SendFrameAndWritePackets.
+                    // Reference: https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/mux.c (init_muxer: default 1/90000)
+                    _pVideoStream->time_base = _pCodecContext->time_base;
+
                     // ── Step 4.5 (Optional): Initialize audio stream ──
                     // If EncoderSettings.Audio is set, add audio stream.
                     if (_settings.Audio.HasValue)