使用ffmpeg将swf转成MP4只转出了一帧

Question

本地有个swf文件，然后用ffmpeg转成mp4格式

ffmpeg -i 3.swf 3-1.mp4

然后显示具体显示如下

ffmpeg version N-80408-g39c0b22-tessus Copyright (c) 2000-2016 the FFmpeg developers
  built with Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)
  configuration: --cc=/usr/bin/clang --prefix=/opt/ffmpeg --as=yasm --extra-version=tessus --enable-avisynth --enable-fontconfig --enable-gpl --enable-libass --enable-libbluray --enable-libfreetype --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopus --enable-libschroedinger --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzmq --enable-version3 --disable-ffplay --disable-indev=qtkit --disable-indev=x11grab_xcb
  libavutil      55. 24.100 / 55. 24.100
  libavcodec     57. 44.100 / 57. 44.100
  libavformat    57. 37.101 / 57. 37.101
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 46.100 /  6. 46.100
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
[swf @ 0x7f9154800000] SWF compressed file detected
Input #0, swf, from '3.swf':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: mjpeg, yuvj420p(pc, bt470bg/unknown/unknown), 710x320 [SAR 1:1 DAR 71:32], 30 tbr, 30 tbn
    Stream #0:1: Video: rawvideo (ARGB / 0x42475241), argb, 86x57, 30 tbr, 30 tbn
No pixel format specified, yuvj420p for H.264 encoding chosen.
Use -pix_fmt yuv420p for compatibility with outdated media players.
[libx264 @ 0x7f915500ae00] using SAR=1/1
[libx264 @ 0x7f915500ae00] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 AVX2 LZCNT BMI2
[libx264 @ 0x7f915500ae00] profile High, level 3.0
[libx264 @ 0x7f915500ae00] 264 - core 148 - H.264/MPEG-4 AVC codec - Copyleft 2003-2016 - 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=10 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=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
[mp4 @ 0x7f9155801600] Using AVStream.codec to pass codec parameters to muxers is deprecated, use AVStream.codecpar instead.
Output #0, mp4, to '3-1.mp4':
  Metadata:
    encoder         : Lavf57.37.101
    Stream #0:0: Video: h264 (libx264) ([33][0][0][0] / 0x0021), yuvj420p(pc), 710x320 [SAR 1:1 DAR 71:32], q=-1--1, 30 fps, 15360 tbn, 30 tbc
    Metadata:
      encoder         : Lavc57.44.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
Stream mapping:
  Stream #0:0 -> #0:0 (mjpeg (native) -> h264 (libx264))
Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=29.0 Lsize=      10kB time=00:00:00.00 bitrate=1299692.3kbits/s speed=0.00388x
video:10kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 8.218897%
[libx264 @ 0x7f915500ae00] frame I:1     Avg QP:27.19  size:  9082
[libx264 @ 0x7f915500ae00] mb I  I16..4: 16.3% 81.2%  2.4%
[libx264 @ 0x7f915500ae00] 8x8 transform intra:81.2%
[libx264 @ 0x7f915500ae00] coded y,uvDC,uvAC intra: 67.6% 79.9% 11.2%
[libx264 @ 0x7f915500ae00] i16 v,h,dc,p:  5% 24%  6% 65%
[libx264 @ 0x7f915500ae00] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 15% 28% 27%  5%  4%  3%  6%  4%  8%
[libx264 @ 0x7f915500ae00] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 18% 13% 15%  9% 11%  8%  7%  9%  9%
[libx264 @ 0x7f915500ae00] i8c dc,h,v,p: 41% 32% 16% 11%
[libx264 @ 0x7f915500ae00] kb/s:2179.68

信息中显示转了1帧，结果也是这样。
请问大家知道知道这是什么原因吗？