我想使用ffmpeg将视频的前几秒（比如说5秒）转换为多种输出格式。
当我使用此语法转换整个视频时，一切正常：
（简单来说，我把所有选项都保留为默认值）
ffmpeg -i input.flv output1.mp4 output2.avi
但是使用这种语法尝试前5秒：
ffmpeg -t 5 -i input.flv output1.mp4 output2.avi
第一个输出文件（即ouptput1.mp4）正常，长度为5秒，但第二个（和下一个输出，如果存在）的大小为原始文件。
这就是ffmpeg在屏幕上打印出来的内容。
D:\ffmpeg\bin>ffmpeg -t 5 -i input.flv output1.mp4 output2.avi
ffmpeg version N-40301-gc1fe2db Copyright (c) 2000-2012 the FFmpeg developers
built on May 3 2012 11:40:38 with gcc 4.6.3
configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-ru
ntime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libass
--enable-libcelt --enable-libopencore-amrnb --enable-libopencore-amrwb --enable
-libfreetype --enable-libgsm --enable-libmp3lame --enable-libnut --enable-libope
njpeg --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libth
eora --enable-libutvideo --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-
libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --e
nable-zlib
libavutil 51. 49.100 / 51. 49.100
libavcodec 54. 17.101 / 54. 17.101
libavformat 54. 3.100 / 54. 3.100
libavdevice 53. 4.100 / 53. 4.100
libavfilter 2. 72.104 / 2. 72.104
libswscale 2. 1.100 / 2. 1.100
libswresample 0. 11.100 / 0. 11.100
libpostproc 52. 0.100 / 52. 0.100
Input #0, flv, from 'input.flv':
Duration: 00:00:37.00, start: 0.000000, bitrate: 366 kb/s
Stream #0:0: Video: flv1, yuv420p, 320x240, 300 kb/s, 29.97 tbr, 1k tbn, 1k
tbc
Stream #0:1: Audio: mp3, 22050 Hz, mono, s16, 56 kb/s
[buffer @ 01de98c0] w:320 h:240 pixfmt:yuv420p tb:1/1000000 sar:0/1 sws_param:fl
ags=2
[buffer @ 01dea4e0] w:320 h:240 pixfmt:yuv420p tb:1/1000000 sar:0/1 sws_param:fl
ags=2
[libx264 @ 02b067a0] using cpu capabilities: MMX2 Cache64
[libx264 @ 02b067a0] profile High, level 1.3
[libx264 @ 02b067a0] 264 - core 120 r2164 da19765 - H.264/MPEG-4 AVC codec - Cop
yleft 2003-2012 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deb
lock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 m
e_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chro
ma_qp_offset=-2 threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_c
ompat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 we
ightb=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=69 qpstep=4
ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'output1.mp4':
Metadata:
encoder : Lavf54.3.100
Stream #0:0: Video: h264 (![0][0][0] / 0x0021), yuv420p, 320x240, q=-1--1, 3
0k tbn, 29.97 tbc
Stream #0:1: Audio: aac (@[0][0][0] / 0x0040), 22050 Hz, mono, s16, 128 kb/s
Output #1, avi, to 'output2.avi':
Metadata:
ISFT : Lavf54.3.100
Stream #1:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 320x240, q=2-31, 200
kb/s, 29.97 tbn, 29.97 tbc
Stream #1:1: Audio: mp3 (U[0][0][0] / 0x0055), 22050 Hz, mono, s16
Stream mapping:
Stream #0:0 -> #0:0 (flv -> libx264)
Stream #0:1 -> #0:1 (mp3 -> libvo_aacenc)
Stream #0:0 -> #1:0 (flv -> mpeg4)
Stream #0:1 -> #1:1 (mp3 -> libmp3lame)
Press [q] to stop, [?] for help
frame= 48 fps=0.0 q=29.0 q=2.4 size= 8kB time=00:00:00.16 bitrate= 399.6
frame= 62 fps= 59 q=29.0 q=2.4 size= 32kB time=00:00:00.63 bitrate= 408.8
frame= 73 fps= 46 q=29.0 q=3.5 size= 57kB time=00:00:01.00 bitrate= 466.1
frame= 104 fps= 50 q=29.0 q=3.5 size= 87kB time=00:00:02.03 bitrate= 349.7
frame= 134 fps= 52 q=29.0 q=3.6 size= 113kB time=00:00:03.03 bitrate= 303.9
***frame= 150 fps= 49 q=29.0 q=2.4 size= 126kB time=00:00:03.57 bitrate= 289.3
frame= 150 fps= 42 q=29.0 q=8.8 size= 126kB time=00:00:03.57 bitrate= 289.3
frame= 150 fps= 37 q=29.0 q=6.9 size= 126kB time=00:00:03.57 bitrate= 289.3
frame= 150 fps= 33 q=29.0 q=9.7 size= 126kB time=00:00:03.57 bitrate= 289.3
frame= 150 fps= 29 q=29.0 q=6.0 size= 126kB time=00:00:03.57 bitrate= 289.3
frame= 150 fps= 27 q=29.0 q=11.2 size= 126kB time=00:00:03.57 bitrate= 289.***
frame= 150 fps= 24 q=29.0 Lq=11.0 size= 171kB time=00:00:04.93 bitrate= 283
.7kbits/s
video:1282kB audio:225kB global headers:0kB muxing overhead -88.650217%
[libx264 @ 02b067a0] frame I:1 Avg QP:10.27 size: 74
[libx264 @ 02b067a0] frame P:114 Avg QP:23.49 size: 727
[libx264 @ 02b067a0] frame B:35 Avg QP:29.30 size: 124
[libx264 @ 02b067a0] consecutive B-frames: 68.0% 2.7% 0.0% 29.3%
[libx264 @ 02b067a0] mb I I16..4: 100.0% 0.0% 0.0%
[libx264 @ 02b067a0] mb P I16..4: 2.1% 3.2% 0.1% P16..4: 19.2% 5.1% 3.6%
0.0% 0.0% skip:66.6%
[libx264 @ 02b067a0] mb B I16..4: 0.2% 0.5% 0.0% B16..8: 9.6% 1.2% 0.4%
direct: 0.6% skip:87.4% L0:46.1% L1:42.5% BI:11.4%
[libx264 @ 02b067a0] 8x8 transform intra:51.7% inter:78.4%
[libx264 @ 02b067a0] coded y,uvDC,uvAC intra: 29.8% 44.5% 10.3% inter: 8.9% 10.3
% 2.2%
[libx264 @ 02b067a0] i16 v,h,dc,p: 52% 31% 16% 1%
[libx264 @ 02b067a0] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 24% 25% 45% 1% 0% 1% 0%
1% 3%
[libx264 @ 02b067a0] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 17% 48% 23% 2% 1% 2% 3%
2% 1%
[libx264 @ 02b067a0] i8c dc,h,v,p: 54% 26% 16% 3%
[libx264 @ 02b067a0] Weighted P-Frames: Y:2.6% UV:0.9%
[libx264 @ 02b067a0] ref P L0: 79.8% 7.0% 9.6% 3.6% 0.0%
[libx264 @ 02b067a0] ref B L0: 92.5% 6.7% 0.8%
[libx264 @ 02b067a0] ref B L1: 96.8% 3.2%
[libx264 @ 02b067a0] kb/s:139.51
D:\ffmpeg\bin>
如何使所有输出文件的大小相同？
这不是ffmpeg的错误吗？ （注意转换进度的最后几行，所有时间戳看起来都相同）
提前感谢。
答案 0 :(得分：2)
您需要为每个输出添加输出选项：
ffmpeg -i input.flv -t 5 output1.mp4 -t 5 output2.avi
考虑为输出质量添加一个额外的选项 - 特别是对于avi，因为libx264（如果可用）是mp4的默认选项，而libx264的默认选项是理智的。
ffmpeg -i input.flv -t 5 output1.mp4 -t 5 -qscale 3 output2.avi
有关-qscale（也称为-q:v）的详情，请参阅FFmpeg: The ultimate Video and Audio Manipulation Tool。