mac下ffmpeg录屏怎么偏移啊？

Question

看了官方文档

ffmpeg -f x11grab -video_size cif -framerate 25 -i :0.0+10,20 /tmp/out.mpg

0.0 is display.screen number of your X11 server, same as the DISPLAY environment variable. 10 is the x-offset and 20 the y-offset for the grabbing.

我也是这样指定的，没有效果