1.  Field of the Invention
The present invention relates to computer graphics displays and more particularly to a method and apparatus for precisely varying the display rate of computer generated graphic images. 
2.  Art Background
In the field of computer graphics, it is often desirable to have a means for displaying animated images.  With an animated display, the graphic image which is generated by the computer varies over time.  Animated displays are useful in many different fields.  For example, they allow a computer user to visually see how a particular object will change in response to an external stimulus.  Particular applications can be found in the field of medicine where an animated display can be used to dynamically "watch" the operation of internal organs over time with the use of a device such as a CAT scanner. 
Typically, an animated image is made up of a series of individual static pictures known as cells.  Each cell is a rendering of the object "frozen" at a given point in time.  The individual cells are sequentially flashed on the computer's video display.  Each image is displayed for only a short period of time.  When the entire sequence of images is viewed in real time, it appears that the object is moving.  The process is similar to the manner in which animated cartoons are created. 
In prior art devices, the animation sequence could only be displayed at a single fixed rate or at a very restricted set of variable rates.  This limitation arose because of the hardware design of the graphics devices.  The amount of time in which the images were displayed was fixed.  Thus, a user could not arbitrarily slow down or speed up the animated sequence as he desired.  In many instances, it is desirable to be able to change this display rate in a precise smooth and arbitrary manner.  For example, if an object changes very quickly over time, a fast display rate may prevent all the details of the object from being observed.  With a slower display rate, more details can be seen when using prior art devices, although a user may miss important changes in the object due to the limitations of the display device.  If there was a method for varying the display rate smoothly and arbitrarily, the user would be able to slow down the animated sequence and properly observe the object. 
As will be described, the present invention overcomes the limitations found in the prior art, and provides a method for precisely, smoothly, and arbitrarily varying the display rate of computer generated graphic images.