https://gist.github.com/dergachev/4627207

1. 用 ScreenShot 或 Quicktime Player 錄製視訊, 儲存位 in.mov
2. ffmpeg -i in.mov -s 600x400 -pix_fmt rgb24 -r 10 -f gif - | gifsicle --optimize=3 --delay=3 > out.gif

Notes on the arguments:

• -r 10 tells ffmpeg to reduce the frame rate from 25 fps to 10
• -s 600x400 tells ffmpeg the max-width and max-height
• --delay=3 tells gifsicle to delay 30ms between each gif
• --optimize=3 requests that gifsicle use the slowest/most file-size optimization