About the algorithm:
Two horizontally oriented curves are created from randomly chosen points fit with a 3rd degree spline function. One curve is oriented the same in both drawings having the space below it filled with parallel vertical lines and the space above with parallel horizontal lines. The second curve has only parallel vertical lines in the space above it in the first drawing and is flipped in the second drawing so that the parallel vertical lines are on the bottom.

First published:
Catalog Manfred Mohr, "Drawings Dessins Zeichnungen Dibujos", Galerie Weiller, Paris and Galerie Gilles Gheerbrant, Montreal, 1974