Zhenyas-MacBook-Pro:ffmpeg lewisou$ cat b2.mp4 | ./ffmpeg -pix_fmt yuv420p -i pipe:0  a.mp4
ffmpeg version N-65526-g52b81ff Copyright (c) 2000-2014 the FFmpeg developers
  built on Aug 13 2014 14:13:48 with Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
  configuration: --enable-libx264 --enable-libass --enable-gpl --disable-yasm
  libavutil      54.  1.100 / 54.  1.100
  libavcodec     56.  0.100 / 56.  0.100
  libavformat    56.  0.100 / 56.  0.100
  libavdevice    56.  0.100 / 56.  0.100
  libavfilter     5.  0.100 /  5.  0.100
  libswscale      3.  0.100 /  3.  0.100
  libswresample   1.  0.100 /  1.  0.100
  libpostproc    53.  0.100 / 53.  0.100
Option pixel_format not found.