据我所知，使用 ffmpeg 完全解码视频应该给我一个准确的持续时间。如果我使用：
ffmpeg -i my_vid.mov -f null -
对于特定视频，我得到的输出：
...
frame= 901 fps=0.0 q=-0.0 Lsize=N/A time=00:00:30.52 bitrate=N/A speed= 247x
...
但是当我运行以下命令来提取接近末尾的帧时，它失败了：
ffmpeg -i my_vid.mov -ss 00:00:30.450 -q:v 2 -vframes 1 test_frame.jpg
输出：
Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)
使用 ffmpeg 命令备份到 00:00:30.000 确实 会给我一个帧，但是，在此之前的任何时间也可以按预期工作。不过，看起来我应该能够获取 00:00:30.520 之前的帧。这里可能出了什么问题？我是否误读了持续时间输出？
这是我从帧提取命令获得的完整输出：
> ffmpeg -ss 00:00:30.450 -i file_example_MOV_640_800kB.mov -q:v 2 -vframes 1 test_frame.jpg
ffmpeg version 2021-04-20-git-718e03e5f2-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-librist --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. 73.100 / 56. 73.100
libavcodec 58.136.101 / 58.136.101
libavformat 58. 78.100 / 58. 78.100
libavdevice 58. 14.100 / 58. 14.100
libavfilter 7.111.100 / 7.111.100
libswscale 5. 10.100 / 5. 10.100
libswresample 3. 10.100 / 3. 10.100
libpostproc 55. 10.100 / 55. 10.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'file_example_MOV_640_800kB.mov':
Metadata:
major_brand : qt
minor_version : 512
compatible_brands: qt
encoder : Lavf57.19.100
Duration: 00:00:30.57, start: 0.000000, bitrate: 208 kb/s
Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 640x360 [SAR 1:1 DAR 16:9], 60 kb/s, 30 fps, 30 tbr, 15360 tbn, 60 tbc (default)
Metadata:
handler_name : VideoHandler
vendor_id : FFMP
encoder : Lavc57.16.101 libx264
Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 139 kb/s (default)
Metadata:
handler_name : SoundHandler
vendor_id : [0][0][0][0]
Stream mapping:
Stream #0:0 -> #0:0 (h264 (native) -> mjpeg (native))
Press [q] to stop, [?] for help
[swscaler @ 000001acc1d28580] deprecated pixel format used, make sure you did set range correctly
Output #0, image2, to 'test_frame.jpg':
Metadata:
major_brand : qt
minor_version : 512
compatible_brands: qt
encoder : Lavf58.78.100
Stream #0:0(eng): Video: mjpeg, yuvj420p(pc), 640x360 [SAR 1:1 DAR 16:9], q=2-31, 200 kb/s, 30 fps, 30 tbn (default)
Metadata:
handler_name : VideoHandler
vendor_id : FFMP
encoder : Lavc58.136.101 mjpeg
Side data:
cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: N/A
frame= 0 fps=0.0 q=0.0 Lsize=N/A time=00:00:00.00 bitrate=N/A speed= 0x
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)
这是带有副本的完整输出：
> ffmpeg -copyts -sseof -3 -i file_example_MOV_640_800kB.mov -q:v 2 -update 1 test_frame.jpg
ffmpeg version 2021-04-20-git-718e03e5f2-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-librist --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. 73.100 / 56. 73.100
libavcodec 58.136.101 / 58.136.101
libavformat 58. 78.100 / 58. 78.100
libavdevice 58. 14.100 / 58. 14.100
libavfilter 7.111.100 / 7.111.100
libswscale 5. 10.100 / 5. 10.100
libswresample 3. 10.100 / 3. 10.100
libpostproc 55. 10.100 / 55. 10.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'file_example_MOV_640_800kB.mov':
Metadata:
major_brand : qt
minor_version : 512
compatible_brands: qt
encoder : Lavf57.19.100
Duration: 00:00:30.57, start: 0.000000, bitrate: 208 kb/s
Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 640x360 [SAR 1:1 DAR 16:9], 60 kb/s, 30 fps, 30 tbr, 15360 tbn, 60 tbc (default)
Metadata:
handler_name : VideoHandler
vendor_id : FFMP
encoder : Lavc57.16.101 libx264
Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 139 kb/s (default)
Metadata:
handler_name : SoundHandler
vendor_id : [0][0][0][0]
File 'test_frame.jpg' already exists. Overwrite? [y/N] y
Stream mapping:
Stream #0:0 -> #0:0 (h264 (native) -> mjpeg (native))
Press [q] to stop, [?] for help
[swscaler @ 0000022d7e964480] deprecated pixel format used, make sure you did set range correctly
Output #0, image2, to 'test_frame.jpg':
Metadata:
major_brand : qt
minor_version : 512
compatible_brands: qt
encoder : Lavf58.78.100
Stream #0:0(eng): Video: mjpeg, yuvj420p(pc, progressive), 640x360 [SAR 1:1 DAR 16:9], q=2-31, 200 kb/s, 30 fps, 30 tbn (default)
Metadata:
handler_name : VideoHandler
vendor_id : FFMP
encoder : Lavc58.136.101 mjpeg
Side data:
cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: N/A
frame= 73 fps=0.0 q=2.0 Lsize=N/A time=00:00:00.00 bitrate=N/A speed= 0x
video:859kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
通过输出搜索，FFmpeg 等待搜索点之后的第一个关键帧开始发出帧。
使用输入搜索，
ffmpeg -ss 00:00:30.450 -i my_vid.mov -q:v 2 -vframes 1 test_frame.jpg
要更通用地从 MOV/MP4 获取最后一帧，请使用
ffmpeg -sseof -3 -i my_vid.mov -q:v 2 -update 1 test_frame.jpg