Can’t write packet with unknown timestamp

FFmpeg报错：

[matroska @ 0x7f9206809800] Timestamps are unset in a packet for stream 0. This is deprecated and will stop working in the future. Fix your code to set the timestamps properly
[matroska @ 0x7f9206809800] Can't write packet with unknown timestamp
av_interleaved_write_frame(): Invalid argument

这个问题出在avi格式的视频上，解决：

在ffmpeg后面加上：-fflags +genpts