我想用 ffmpeg 按帧剪切视频。到目前为止，我想到的是
ffmpeg -i P2.mp4 -vf "fps,setpts='PTS-STARTPTS',select='gte(n\,20)*lte(n\,30)',crop='600:430:100:650',scale='280:-1'" P2o.avi
这些参数用于测试，以查看一切是否按要求执行。
我得到的是一段视频，其中有 20 个黑帧，后面跟着 10 个内容帧。虽然我想要这 10 帧，但我不知道这 20 个黑帧是怎么来的。- selectfilter 的文档明确指出，只选择表达式求值为非 0 的帧。上面的gte(n,20)应该删除它们。
如果有帮助的话，这里是 ffmpeg 的输出：
G:\>ffmpeg -i P2.mp4 -vf "fps,setpts='PTS-STARTPTS',select='gte(n\,20)*lte(n\,30)',crop='600:430:100:650',scale='280:-1'" P2o.avi
ffmpeg version N-50911-g9efcfbe Copyright (c) 2000-2013 the FFmpeg developers
built on Mar 13 2013 21:26:48 with gcc 4.7.2 (GCC)
configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-libass --enable-libbluray --enable-libcaca --enable-libfreetype --enable-libgsm --enable-libilbc --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --enable-zlib
libavutil 52. 19.100 / 52. 19.100
libavcodec 55. 0.100 / 55. 0.100
libavformat 55. 0.100 / 55. 0.100
libavdevice 54. 4.100 / 54. 4.100
libavfilter 3. 45.103 / 3. 45.103
libswscale 2. 2.100 / 2. 2.100
libswresample 0. 17.102 / 0. 17.102
libpostproc 52. 2.100 / 52. 2.100
[h264 @ 039a1ae0] mmco: unref short failure
Last message repeated 5 times
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'P2.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf55.0.100
Duration: 01:51:58.78, start: 0.000000, bitrate: 8479 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080
[SAR 1:1 DAR 16:9], 8084 kb/s, 49.96 fps, 50 tbr, 90k tbn, 50 tbc
Metadata:
handler_name : VideoHandler
Stream #0:1(eng): Audio: ac3 (ac-3 / 0x332D6361), 48000 Hz, stereo, fltp, 384 kb/s
Metadata:
handler_name : SoundHandler
File 'P2o.avi' already exists. Overwrite ? [y/N] y
Invalid pixel aspect ratio 603/602, limit is 255/255 reducing
Output #0, avi, to 'P2o.avi':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
ISFT : Lavf55.0.100
Stream #0:0(und): Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 280x201 [SAR 1:1 DAR 280:201], q=2-31, 200 kb/s, SAR 603:602 DAR 60:43, 25 tbn, 25 tbc
Metadata:
handler_name : VideoHandler
Stream #0:1(eng): Audio: mp3 (U[0][0][0] / 0x0055), 48000 Hz, stereo, fltp
Metadata:
handler_name : SoundHandler
Stream mapping:
Stream #0:0 -> #0:0 (h264 -> mpeg4)
Stream #0:1 -> #0:1 (ac3 -> libmp3lame)
Press [q] to stop, [?] for help
[h264 @ 046f1680] mmco: unref short failure
Last message repeated 1 times
[h264 @ 05e4a900] mmco: unref short failure
Last message repeated 3 times
[swscaler @ 039b63e0] Warning: data is not aligned! This can lead to a speedloss
frame= 1 fps=0.0 q=4.2 size= 29kB time=00:00:01.82 bitrate= 131.8kbits/s
frame= 11 fps= 11 q=7.4 size= 71kB time=00:00:03.16 bitrate= 184.4kbits/s
frame= 11 fps=7.2 q=7.4 size= 71kB time=00:00:04.60 bitrate= 126.8kbits/s
frame= 11 fps=5.4 q=7.4 size= 71kB time=00:00:06.19 bitrate= 94.3kbits/s
frame= 11 fps=4.3 q=7.4 size= 71kB time=00:00:07.48 bitrate= 78.0kbits/s
frame= 11 fps=3.6 q=7.4 size= 71kB time=00:00:08.76 bitrate= 66.7kbits/s
frame= 11 fps=3.1 q=7.4 size= 71kB time=00:00:10.08 bitrate= 58.0kbits/s
frame= 11 fps=2.7 q=7.4 size= 71kB time=00:00:11.42 bitrate= 51.1kbits/s
frame= 11 fps=2.6 q=7.4 Lsize= 253kB time=00:00:12.04 bitrate= 171.8kbits/s
video:42kB audio:189kB subtitle:0 global headers:0kB muxing overhead 9.670474%
G:\>
有人能建议我如何去掉这 20 帧吗？我觉得在这里用不了-ss。
答案1
尝试放置setpts过滤器后濾器select。
setpts='PTS-STARTPTS'重置每个帧的显示时间戳（因此第一帧为 0.0 秒，并且它们正确递增） - 选择过滤器会将选定的帧发送到 ffmpeg 的其余部分，但它本身不会改变显示时间戳，这可能会导致各种各样的问题 - 正如您所发现的。