6.3.2 Tracking Guns

In this example, we will show you how to use "Links" to use same objects to define motions for different methods. Any parameter object of a method can be replaced by a link object which points to the actual parameter. The parameter can be everywhere in the object hierarchy and more than one link can point to it.

Lets consider the following situation: a sphere follows a path and no matter where it goes, gun always points towards it. To do this:

1. Create a sphere and define a path for it using Animate/Create/Path.

2. Create a level object and name it as "Gun", at the same level with the moving sphere object. Make it the current level.

3. Create a gun object under the level, pointing to the sphere.

4. Select Create/Structure/Method and create a SWEEP method. Create it at the same level where the gun shape is; the purpose is to animate the gun. Make the method level the current level.

5. Define a center of rotation for the gun by creating an offset primitive under the method level; this is the first sweep parameter.

6. Select the B-Spline curve used for defining a motion for the sphere and select the function Create/Structure/Link. Select the "link" and Modify/Structure/Cut it. Then make the gun sweep method the current level and Modify/Structure/Paste the link under the method. The link will act as the second sweep parameter.

Play the animation, and your gun is rotated so it always points to the sphere.

Create another gun by duplicating the original gun (the whole sweeping gun), move it to another place and rotate it so it still points to the sphere, and when you play the animation, both guns follow the sphere.

The rotations for all guns as well as the motion for the sphere are all defined by the same curve.

Now, modify the sphere path using Modify/Freeform/Move Knotpoint function and play the animation. All guns have automatically changed their behavior so they still keep pointing to the sphere!