In the top left box, you draw (by clicking) the corners of the shape you are interested in. The convex hull is shown in the bottom left box. In the big box on the right you can draw anything you like. The little red squares show the corners of the shape you drew.
If you press <space>, you play one move of the easy solitaire of independence with respect to that shape. If you press <a>, a line is drawn in the direction perpendicular to the coordinates you've put in the two textboxes and with the correct width for the shape to get determinism in that direction. If you press <e>, all is erased. If you press <r>, you toggle random application of the rules.