This applet illustrates several pieces of code from Computational Geometry in C by Joseph O'Rourke . The code illustrated includes orientation.c, area.c, centroid.c, inpoly.c, and tri.c. Code from the forthcoming second edition of the book is used, but this is similar to the code shown in the first edition (and distributed via ftp).

The applet includes polygon entering and editing.

• Start by drawing a polygon. Click on the position of the vertices on the canvas with your mouse in order, and then push the "Finish Polygon" button. If you click within the disk of the first point, the polygon is automatically closed. Immediately after closing, the orientation of the polygon is computed, and if you entered it clockwise, it is reversed to be counterclockwise. This is done because some of the code assumes the polygon has been input counterclockwise.
• After a polygon is created, it can be modified: vertices can be added, deleted, or moved. For each operation, press the corresponding button and then click with the mouse on the area or vertex where the change should occur. Click on a vertex to delete it; click near an edge to add a new point; click on a vertex to move it. The current mode (add, delete, move) is indicated by italics in the corresponding button.
• To find the area, the center of gravity, or a triangulation of the polygon, press the corresponding button. Note: the triangulation code does not work for non-simple polygons.
• To exercise the point-in-polygon code, press the "In Poly?" button, and then click a query point on the canvas. The rays used in the code are displayed. (This is one instance where the code differs substantially between the First and Second editions.)
• To draw a new polygon, clear the screen with the "To Clear" button.

The Java code was written by Lilla Zollei and Irena Pashchenko. Questions to orourke@cs.smith.edu