red, green, and blue filters. The user looks through this to the Vectrex screen. The Vectrex synchronizes the rotation of the disk to the software frame rate as it draws 6 screens: with the right eye covered: the left eye red image, then green, and then the blue image is drawn, and then, while the left eye is covered by the black 180-degree sector: the right eye red, green, and then the blue image is drawn. Only one eye will see the Vectrex screen and its 3 associated images (or colors) at any one time while the other will be blocked by the 180-degree mask. The prototype was made in the plastic casework of a Viewmaster. The disc spins freely and is driven by a motor. The Vectrex software generates its own frame-rate and compares it to an index signal from the glasses once per revolution. Score is kept of how many wheel rotations are early compared to the software frame rate, and how many are late. The software tries to keep these two trends equal by adjusting the power being delivered to the motor that spins the filter and mask wheel. Pulse Width Modulation (PWM) is used to control the motor speed: the ratio of the "on" time versus the "off" time of a rapid stream of power pulses to the motor. In this way the software synchronizes the wheel rotation to the software's frame rate, or drawing time, for the combined and repeating group of up to 6 evolving images. A single object that does not lie on the plane of the monitor (i.e., in front of or into the monitor) is drawn at least twice to provide information for each eye. The distance between the duplicate images and the angles from which they are drawn will determine where the object will appear to "be" in 3-D space. The 3-D illusion is also enhanced by adjusting the brightness of the object (dimming objects in the background). Spinning the disk at a high enough speed will fool the viewer's eyes/brain into thinking that the multiple images it is seeing are two different views of the same object due to the persistence of vision. This creates the impression of 3-D and color. The same 3-D effect is in fact possible with raster or film-projection images, and the shutter glasses used in some 3-D theaters and virtual reality theme park rides