I have not, but i can explane why it looks square.
here is a stroke of halftone, that has the same effect
the second pic is zoomed in extremely and explanes why it looks like squares, its because the dots are overlapping and the holes created by it look a bit square.
the more overlap, the smaller the square becomes
In the lighterpart, it looks like there is more spacing between the dots, but what actually is happening, is that the diameter of the dots is getting smaller and leaves more of the background color "shining through"
