Finally got a 4000 x 4000 heightfield deterraced, it took 54 minutes. Screenshot showing rectangular subarea deterraced: http://www.daylongraphics.com/products/leveller/gallery/deterrace2_sub2.png Same thing, but with a 10px Guassian blur afterwards: http://www.daylongraphics.com/products/leveller/gallery/deterrace2_sub3.png Ray