Fastest method for screen capturing on Linux

Question

This question is similar to this one

Fastest method of screen capturing

but for linux/X11.

To be more specific, i need a method to capture the pixel

Answer 1

You can use xwd(1) to dump a X11 window (and xwud(1) to display it). ImageMagick and the Netpbm tools know how to deal with the XWD format. But these are not especially fast. And they will not reveal hidden portions of a window.