Form from Motion

Background:

Motion can help us to determine the shape and form of an object. Look at the group of dots in the image below. The dots have been randomly placed onto the surface of three dimensional object. When the object and its dots are stationary, it can be difficult to determine the shape of the object. If you rotate the object and its dots, the shape of the object should become readily apparent.

Why does motion aid form perception? Part of the explanation is that motion can give us a very powerful depth cue call motion parallax. When you are moving relative to an object (in this case, the object is moving while you are stationary), points farther than the axis of rotation will move in one direction (from left to right in this demonstration if the object rotate clockwise; otherwise right to left) while points closer than axis of rotation will move in the opposite direction. The farther the point is from the axis of rotation, the more rapidly it will move across the screen. A point on the axis of rotation will not move at all.

The visual system can use the information about direction of motion and rate of motion to determine how far away the point is from the axis of rotation. If we know how far away enough points are, the visual system can extrapolate the overall shape of the object.

The Activity:

In this activity, 150 points have been randomly placed on the surface of an invisible three dimensional object. When the points are stationary, it can be difficult to determine the shape of the object. Click on the Animate (X and Y) button to start rotating the object and the dots on its surface. As the object rotates, its three dimensional form (or shape) should become apparent almost instantly.

Clicking on the "Next Object" button will stop any rotation and create a new three dimensional object. Can you identify the object's shape? Start it rotating. Can you identify its shape now?

To help you see the motion parallax, stop all rotation and then click on the Rotate (X Only) button. Search through the dots until you find the red dot. Notice its movement across the screen. When it is primarily moving in one direction (left to right for clockwise rotation), the point will seem to be farther away. When it is primarily moving in the other direction, the point will seem to be closer to you. Note: it is easier to see this on some objects than on other objects. If you are having a hard time seeing the relation between perceived depth and direction of motion, try clicking on the Next Object button and re-start the rotation on the X axis.

How many points do you need in order to be able to determine the shape of the object while it is rotating? Slide the number of points slider at the bottom left and right to decrease and increase the number of points that are painted onto the surface of the object. Drag the slider all the way to the left and then click the Next Object button. Start it rotating. Gradually increase the number of points until you can determine the shape of the object.