Ripples radiate from the center.
This is the whole thing. Phluxel runs it on every cell, every step — the animation is what happens when you repeat it.
R = ((N.R + S.R + E.R + W.R) * 0.5 - G) * 0.98; G = R
Paste it into the Rule Editor, or pick Wave Propagation from the template list and skip the typing.
Phluxel runs in the browser as a demo, and the free desktop edition has every feature with a watermark on exports.
Open the browser demo Download free (Windows) Wishlist on Steam