Geraldo de Barros
Senhor de Barros is a Brazilian artist who does a quite a bit of wonderful geometric abstract work. See: http://www.geraldodebarros.com/main/?page_id=582 for more.
This sketch is trying to generalize the first de Barros painting I saw, via tophat77: http://peano.tumblr.com/post/126411650460/topcat77-geraldo-de-barros
Some very interesting (to me) visual fractions form in these digital images.
More GeoGebra at bit.ly/mh-ggb