[FFmpeg-user] How does ffprobe determine 'fps'?

[Mark Filipak]

H:\VIDEO_TS>ffprobe VTS_01_1.VOB
ffprobe version git-2020-04-20-cacdac8 Copyright (c) 2007-2020 the FFmpeg developers
   built with gcc 9.3.1 (GCC) 20200328
   configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls 
--enable-iconv --enable-libass --enable-libdav1d --enable-libbluray --enable-libfreetype 
--enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg 
--enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libsrt 
--enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp 
--enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib 
--enable-gmp --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvo-amrwbenc 
--enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --disable-w32threads 
--enable-libmfx --enable-ffnvcodec --enable-cuda-llvm --enable-cuvid --enable-d3d11va --enable-nvenc 
--enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenmpt --enable-amf
   libavutil      56. 42.102 / 56. 42.102
   libavcodec     58. 80.100 / 58. 80.100
   libavformat    58. 42.100 / 58. 42.100
   libavdevice    58.  9.103 / 58.  9.103
   libavfilter     7. 79.100 /  7. 79.100
   libswscale      5.  6.101 /  5.  6.101
   libswresample   3.  6.100 /  3.  6.100
   libpostproc    55.  6.100 / 55.  6.100
Input #0, mpeg, from 'VTS_01_1.VOB':
   Duration: 00:22:25.82, start: 0.039456, bitrate: 6382 kb/s
     Stream #0:0[0x1bf]: Data: dvd_nav_packet
     Stream #0:1[0x1e0]: Video: ... 29.97 fps, 59.94 tbr, 90k tbn, 59.94 tbc
                                    |||||||||
                                    How does ffprobe determine this?
                                    Can you give me a link to the code?

Thanks,
Mark.