# DiskFlow Applet

In geometry and its applications, there are many situations where one wants to evolve a curve by a process that shortens it while reducing self-intersections.
1. Finding geodesics, or shortest curves.
2. Smoothing rough edges into smooth curves.
3. Finding the smooth boundary of a pixelated region in a picture.
4. Predicting how ink seeps into paper.

Many processes have been proposed for such a curve evolution. See references below. It is difficult to define a process that shortens curves without introducing singularities or new intersections. The applet below implements an approach introduced in the paper Shortening Curves on Surfaces by Joel Hass and Peter Scott, Topology 33, (1994) 25-43. For curves in the plane, the process shrinks a curve to a point. while monotonically reducing the number of self-intersections.

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To use this applet, drag your mouse in the panel to draw an initial curve. Press the "complete" button to close the curve. Press the "start disk flow" button to initiate the flow. The colors change at random as the curve evolves. If you press the "clear" button you can start again.

If you uncheck the button labeled "leave a trace", the curve will erase and redraw each time it evolves.

This applet was programmed in Java by Richard Vaughn and Joel Hass.

The basic idea of the algorithm is simple:
1. Cover the screen with squares.
2. Replace the intersection of a curve with a given square with straight arcs having the same boundary points on the square's boundary.
3. Offset the squares by 1/3 of their widths and heights. Repeat step 2.