Killer Game Programming in Java

Picking with a Mouse Click

Picking projects a pick shape (usually a line or ray) into the scene from the user's viewpoint through the mouse pointer position on screen until it intersects with a shape in the scene (see Figure 23-3).

Java 3D's PickCanvas class is used to turn a mouse click into a ray (a PickShape object). Java 3D finds the pickable shapes that intersect with the PickShape object, returning them as a list of PickResult objects. Returning the PickResult object closest to the viewer is possible.

A single PickResult may contain many PickIntersection objects, which hold the data for each intersection of the shape (e.g., the ray may go through the front and back face of a shape, leading to two intersection points).

The complexity of the picking coding is somewhat alleviated by using Java 3D's PickMouseBehavior utility class, a subclass of Behaviour, which hides much of the picking mechanism. The general format for a subclass of PickMouseBehavior is given in Example 23-1.

Example 23-1. A typical PickMouseBehavior subclass

 import javax.media.j3d.*; import com.sun.j3d.utils.picking.PickTool; import com.sun.j3d.utils.picking.PickResult; import com.sun.j3d.utils.picking.behaviors.PickMouseBehavior; // other imports as necessary... public class ExamplePickBehavior extends PickMouseBehavior { public PickHighlightBehavior(Canvas3D canvas, BranchGroup bg, Bounds bounds) { super(canvas, bg, bounds); setSchedulingBounds(bounds); pickCanvas.setMode(PickTool.GEOMETRY_INTERSECT_INFO); ...