我有一个包含500张图片的文件夹，格式为image0.png，image1.png，...，image499.png。我想使用ffmpeg将这些图片转换为帧率为50的视频。这意味着总视频时长为10秒（= 500/50）。
为此，我使用了以下命令：
ffmpeg -r 50 -f image2 -i image%1d.png -vcodec libx264 -crf 15 -pix_fmt yuv420p video.mp4
但是，创建的视频的持续时间为2.5秒。否则，视频会正确显示图像，并按预期从开始到结束运行。
为什么视频长度不像我预期的那样？
控制台的完整输出是：
ffmpeg version 2.7.6-0ubuntu0.15.10.1 Copyright (c) 2000-2016 the FFmpeg developers
built with gcc 5.2.1 (Ubuntu 5.2.1-22ubuntu2) 20151010
configuration: --prefix=/usr --extra-version=0ubuntu0.15.10.1 --build-suffix=-ffmpeg --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --enable-gpl --enable-shared --disable-stripping --enable-avresample --enable-avisynth --enable-frei0r --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopenjpeg --enable-openal --enable-libopus --enable-libpulse --enable-librtmp --enable-libschroedinger --enable-libshine --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libxvid --enable-libzvbi --enable-opengl --enable-x11grab --enable-libdc1394 --enable-libiec61883 --enable-libzmq --enable-libssh --enable-libsoxr --enable-libx264 --enable-libopencv --enable-libx265
libavutil 54. 27.100 / 54. 27.100
libavcodec 56. 41.100 / 56. 41.100
libavformat 56. 36.100 / 56. 36.100
libavdevice 56. 4.100 / 56. 4.100
libavfilter 5. 16.101 / 5. 16.101
libavresample 2. 1. 0 / 2. 1. 0
libswscale 3. 1.101 / 3. 1.101
libswresample 1. 2.100 / 1. 2.100
libpostproc 53. 3.100 / 53. 3.100
Input #0, image2, from 'image%1d.png':
Duration: 00:00:10.00, start: 0.000000, bitrate: N/A
Stream #0:0: Video: png, rgb24(pc), 1600x600, 50 fps, 50 tbr, 50 tbn, 50 tbc
[libx264 @ 0x122c6c0] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 AVX2 LZCNT BMI2
[libx264 @ 0x122c6c0] profile High, level 3.2
[libx264 @ 0x122c6c0] 264 - core 146 r2555 0c21480 - H.264/MPEG-4 AVC codec - Copyleft 2003-2015 - 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=12 lookahead_threads=2 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=15.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'video.mp4':
Metadata:
encoder : Lavf56.36.100
Stream #0:0: Video: h264 (libx264) ([33][0][0][0] / 0x0021), yuv420p, 1600x600, q=-1--1, 50 fps, 12800 tbn, 50 tbc
Metadata:
encoder : Lavc56.41.100 libx264
Stream mapping:
Stream #0:0 -> #0:0 (png (native) -> h264 (libx264))
Press [q] to stop, [?] for help
frame= 84 fps=0.0 q=23.0 size= 61kB time=00:00:00.52 bitrate= 962.5kbits/frame= 161 fps=160 q=23.0 size= 216kB time=00:00:02.06 bitrate= 860.6kbits/frame= 236 fps=157 q=23.0 size= 438kB time=00:00:03.56 bitrate=1008.2kbits/frame= 315 fps=157 q=23.0 size= 658kB time=00:00:05.14 bitrate=1048.8kbits/frame= 390 fps=155 q=23.0 size= 850kB time=00:00:06.64 bitrate=1048.7kbits/frame= 461 fps=153 q=23.0 size= 1059kB time=00:00:08.06 bitrate=1076.2kbits/frame= 500 fps=145 q=-1.0 Lsize= 1292kB time=00:00:09.96 bitrate=1062.3kbits/s
video:1285kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.521096%
[libx264 @ 0x122c6c0] frame I:2 Avg QP: 9.48 size: 11840
[libx264 @ 0x122c6c0] frame P:223 Avg QP:16.74 size: 3729
[libx264 @ 0x122c6c0] frame B:275 Avg QP:21.86 size: 1672
[libx264 @ 0x122c6c0] consecutive B-frames: 1.0% 73.6% 10.2% 15.2%
[libx264 @ 0x122c6c0] mb I I16..4: 51.2% 40.0% 8.7%
[libx264 @ 0x122c6c0] mb P I16..4: 0.9% 1.7% 0.4% P16..4: 6.2% 3.6% 2.1% 0.0% 0.0% skip:85.0%
[libx264 @ 0x122c6c0] mb B I16..4: 0.2% 0.3% 0.0% B16..8: 7.8% 2.9% 0.7% direct: 0.5% skip:87.5% L0:44.4% L1:50.3% BI: 5.3%
[libx264 @ 0x122c6c0] 8x8 transform intra:52.1% inter:38.1%
[libx264 @ 0x122c6c0] coded y,uvDC,uvAC intra: 9.7% 0.0% 0.0% inter: 1.8% 0.0% 0.0%
[libx264 @ 0x122c6c0] i16 v,h,dc,p: 50% 41% 3% 6%
[libx264 @ 0x122c6c0] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 44% 12% 36% 2% 1% 1% 1% 4% 0%
[libx264 @ 0x122c6c0] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 41% 25% 21% 3% 1% 2% 1% 5% 1%
[libx264 @ 0x122c6c0] i8c dc,h,v,p: 100% 0% 0% 0%
[libx264 @ 0x122c6c0] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x122c6c0] ref P L0: 69.2% 7.5% 16.7% 6.6%
[libx264 @ 0x122c6c0] ref B L0: 78.0% 20.6% 1.4%
[libx264 @ 0x122c6c0] ref B L1: 96.8% 3.2%
[libx264 @ 0x122c6c0] kb/s:1052.00