ffmpeg keep aspect ratio

Keep aspect ratio from source video:

-vf "scale=iw*sar:ih , pad=max(iw\,ih*

(16/9)):ow/(16/9):(ow-iw)/2:(oh-ih)/2" -s 640x360 -aspect 16:9

REF: https://lists.ffmpeg.org/pipermail/ffmpeg-user/2011-July/001746.html