I want to cut a video into many segments and then stitch them back together. To account for videos that have a framerate of 29.97 I cut them into 100-second videos via ```ffmpeg -ss 0 -t 100``` which works well for the most part and outputs videos that have a duration of 100 seconds and 2997 frames.
I've noticed that for some video sources with a (constant) frame-rate of 29.97 it'll output a video with a duration of 100.034 and 2998 frames instead of the expected 100 and 2997. For subsequent segments (e.g. ```ffmpeg -ss 100 -t 100```) it will output a video with a duration of 100.67 and a framerate of 2999.
The issue is that this results with segments that have overlapping frames and when I stitch the segments back together I have a video that is longer than its source and will be out of sync with the audio.
I noticed that for those problematic videos if I use ```ffmpeg -ss 0 -t 99.999``` and ```ffmpeg -ss 99.999 -t 99.999``` etc. it'll output videos of precisely 100 seconds and 2997 frames which solves the problem.
My question is why am I seeing these inconsistencies and how can I approach this in a reliable way that will work for all videos.
Full command:
```
ffmpeg -ss 0 -t 100 -i "./input.mov" -map V:0 -c:v libx264 -vf scale=-2:240 out.mp4
```
Logs:
```
ffmpeg version 2021-01-12-git-ca21cb1e36-full_build-www.gyan.dev Copyright (c) 2000-2021 the FFmpeg developers
built with gcc 10.2.0 (Rev6, Built by MSYS2 project)
configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-lzma --enable-libsnappy --enable-zlib --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libglslang --enable-vulkan --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
libavutil 56. 63.100 / 56. 63.100
libavcodec 58.116.100 / 58.116.100
libavformat 58. 65.101 / 58. 65.101
libavdevice 58. 11.103 / 58. 11.103
libavfilter 7. 95.100 / 7. 95.100
libswscale 5. 8.100 / 5. 8.100
libswresample 3. 8.100 / 3. 8.100
libpostproc 55. 8.100 / 55. 8.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from './input.mov':
Metadata:
major_brand : qt
minor_version : 512
compatible_brands: qt
encoder : Lavf58.35.100
Duration: 00:07:09.83, start: 0.000000, bitrate: 37022 kb/s
Stream #0:0: Audio: pcm_s16le (sowt / 0x74776F73), 44100 Hz, stereo, s16, 1411 kb/s (default)
Metadata:
handler_name : SoundHandler
vendor_id : [0][0][0][0]
Stream #0:1: Audio: pcm_s16le (sowt / 0x74776F73), 48000 Hz, stereo, s16, 1536 kb/s
Metadata:
handler_name : SoundHandler
vendor_id : [0][0][0][0]
Stream #0:2: Audio: pcm_s16le (sowt / 0x74776F73), 48000 Hz, stereo, s16, 1536 kb/s
Metadata:
handler_name : SoundHandler
vendor_id : [0][0][0][0]
Stream #0:3: Video: prores (HQ) (apch / 0x68637061), yuv422p10le(tv, bt709, progressive), 1920x1080, 32531 kb/s, SAR 1:1 DAR 16:9, 29.97 fps, 29.97 tbr, 30k tbn, 30k tbc (default)
Metadata:
handler_name : VideoHandler
vendor_id : FFMP
encoder : Lavc58.64.101 prores_ks
Stream mapping:
Stream #0:3 -> #0:0 (prores (native) -> h264 (libx264))
Press [q] to stop, [?] for help
[libx264 @ 000001a481da7080] using SAR=640/639
[libx264 @ 000001a481da7080] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 000001a481da7080] profile High 4:2:2, level 2.1, 4:2:2, 10-bit
[libx264 @ 000001a481da7080] 264 - core 161 r3033 0d754ec - H.264/MPEG-4 AVC codec - Copyleft 2003-2020 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=7 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=81 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'out.mp4':
Metadata:
major_brand : qt
minor_version : 512
compatible_brands: qt
encoder : Lavf58.65.101
Stream #0:0: Video: h264 (avc1 / 0x31637661), yuv422p10le(tv, bt709, progressive), 426x240 [SAR 640:639 DAR 16:9], q=2-31, 29.97 fps, 30k tbn (default)
Metadata:
handler_name : VideoHandler
vendor_id : FFMP
encoder : Lavc58.116.100 libx264
Side data:
cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
frame= 1 fps=0.0 q=0.0 size= 0kB time=00:00:00.00 bitrate=N/A speed= 0x
frame= 140 fps=0.0 q=41.0 size= 0kB time=00:00:02.86 bitrate= 0.1kbits/s speed=5.72x
frame= 273 fps=272 q=41.0 size= 0kB time=00:00:07.30 bitrate= 0.1kbits/s speed=7.28x
frame= 423 fps=281 q=41.0 size= 256kB time=00:00:12.31 bitrate= 170.4kbits/s speed=8.17x
frame= 571 fps=284 q=41.0 size= 512kB time=00:00:17.25 bitrate= 243.2kbits/s speed=8.58x
frame= 715 fps=284 q=41.0 size= 768kB time=00:00:22.05 bitrate= 285.3kbits/s speed=8.77x
frame= 853 fps=283 q=41.0 size= 768kB time=00:00:26.66 bitrate= 236.0kbits/s speed=8.84x
frame= 986 fps=280 q=41.0 size= 1024kB time=00:00:31.09 bitrate= 269.8kbits/s speed=8.84x
frame= 1117 fps=278 q=41.0 size= 1280kB time=00:00:35.46 bitrate= 295.6kbits/s speed=8.82x
frame= 1263 fps=279 q=41.0 size= 1536kB time=00:00:40.34 bitrate= 311.9kbits/s speed=8.93x
frame= 1435 fps=286 q=41.0 size= 1536kB time=00:00:46.07 bitrate= 273.1kbits/s speed=9.18x
frame= 1584 fps=287 q=41.0 size= 1536kB time=00:00:51.05 bitrate= 246.5kbits/s speed=9.25x
frame= 1734 fps=288 q=41.0 size= 1536kB time=00:00:56.05 bitrate= 224.5kbits/s speed= 9.3x
frame= 1880 fps=288 q=41.0 size= 1792kB time=00:01:00.92 bitrate= 240.9kbits/s speed=9.34x
frame= 2013 fps=286 q=41.0 size= 1792kB time=00:01:05.36 bitrate= 224.6kbits/s speed= 9.3x
frame= 2178 fps=289 q=41.0 size= 1792kB time=00:01:10.87 bitrate= 207.1kbits/s speed=9.41x
frame= 2338 fps=291 q=41.0 size= 2048kB time=00:01:16.20 bitrate= 220.2kbits/s speed=9.49x
frame= 2484 fps=291 q=41.0 size= 2048kB time=00:01:21.08 bitrate= 206.9kbits/s speed= 9.5x
frame= 2646 fps=293 q=41.0 size= 2048kB time=00:01:26.48 bitrate= 194.0kbits/s speed=9.57x
frame= 2814 fps=295 q=38.0 size= 2304kB time=00:01:32.09 bitrate= 205.0kbits/s speed=9.65x
frame= 2981 fps=297 q=41.0 size= 2304kB time=00:01:37.66 bitrate= 193.3kbits/s speed=9.72x
frame= 2998 fps=296 q=-1.0 Lsize= 2533kB time=00:01:39.93 bitrate= 207.7kbits/s speed=9.86x
video:2498kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.416280%
[libx264 @ 000001a481da7080] frame I:17 Avg QP:28.16 size: 6543
[libx264 @ 000001a481da7080] frame P:849 Avg QP:35.09 size: 2215
[libx264 @ 000001a481da7080] frame B:2132 Avg QP:37.45 size: 265
[libx264 @ 000001a481da7080] consecutive B-frames: 3.6% 3.8% 2.5% 90.1%
[libx264 @ 000001a481da7080] mb I I16..4: 34.4% 48.6% 17.1%
[libx264 @ 000001a481da7080] mb P I16..4: 1.5% 4.4% 0.9% P16..4: 32.5% 14.7% 9.3% 0.0% 0.0% skip:36.8%
[libx264 @ 000001a481da7080] mb B I16..4: 0.1% 0.1% 0.0% B16..8: 18.5% 2.0% 0.4% direct: 1.3% skip:77.6% L0:46.9% L1:42.6% BI:10.5%
[libx264 @ 000001a481da7080] 8x8 transform intra:60.5% inter:61.4%
[libx264 @ 000001a481da7080] coded y,uvDC,uvAC intra: 40.1% 47.4% 14.3% inter: 11.4% 6.1% 0.6%
[libx264 @ 000001a481da7080] i16 v,h,dc,p: 52% 15% 8% 24%
[libx264 @ 000001a481da7080] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 19% 16% 36% 3% 5% 5% 6% 5% 4%
[libx264 @ 000001a481da7080] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 28% 21% 14% 4% 7% 7% 9% 5% 5%
[libx264 @ 000001a481da7080] i8c dc,h,v,p: 66% 11% 15% 8%
[libx264 @ 000001a481da7080] Weighted P-Frames: Y:25.8% UV:12.1%
[libx264 @ 000001a481da7080] ref P L0: 59.9% 23.7% 14.8% 1.5%
[libx264 @ 000001a481da7080] ref B L0: 69.1% 19.4% 11.5%
[libx264 @ 000001a481da7080] ref B L1: 94.4% 5.6%
[libx264 @ 000001a481da7080] kb/s:204.50
```