Principles of Animation

Just about every object you can create in Sketchpad can be animated. To animate an object, you can choose Animate from the Display menu, use the Motion Controller, or create an Animation action button. Of these three methods, action buttons give you the most control of the details of motion.

The only objects you can’t animate are captions, calculations, functions, action buttons, measurements, and pictures.

Different objects in Sketchpad move in different ways.

· An independent point moves freely in the plane.

· A point on path moves along its path. If the path is an interior (polygon interior, circle interior, or arc interior), the point moves along the perimeter of the interior.

The default direction for points on most paths is bidirectional. For circle paths, however, the default direction is counter-clockwise—the direction of an opening angle.

· A parameter changes its value.

· Any other object moves by moving its parent objects.

When you animate a geometric object that isn’t a point, Sketchpad animates that object by animating its parents. For instance, if you animate a triangle interior, Sketchpad animates it by animating the vertices of the triangle. If one vertex is a point on path, it’s moved along its path. If another is an independent point, it moves randomly on the plane. And if the third point is an object such as an intersection that isn’t free to move, it’s moved by moving its parents in turn. Thus, your ability to animate any conceivable geometric object is ultimately based on the animation of independent points and of points constructed on paths.

Most objects that display text (action buttons, measurements, calculations, functions, and captions) cannot be animated. The one exception is parameters. A parameter is like an independent point or a point on path in the sense that the value of a parameter, like the position of an independent point, does not depend on other objects. This means Sketchpad can animate the parameter by changing its value.

Because independent points, points on paths, and parameters are the only objects that can be animated independently of their parents, these are the only objects that Sketchpad directly animates, and they are the only objects that appear in the Motion Controller’s Target pop-up menu. Other objects animate indirectly—by animating their parents.

Other objects can be listed as the Motion Controller target if they are selected. For example, if you select the interior of D ABC, the triangle will be listed as the target. Even though the triangle is listed as the target, any motion changes you make will directly affect points A, B, and C and will affect the triangle indirectly.

Subtopics:
Animation of an Independent Point
Animation of a Point on Path
Animation of a Parameter
Motion Direction
Motion Speed
Once-Only Motion