Generating an image involves two steps: computing that image and displaying it. Graphics lists provide a mechanism for saving the computed image. They are useful when images that are computationally expensive to generate are frequently reimaged Another use may be to use a list to drive a hardcopy generator. A list can be generated in one succession of "graphics calls" or piecemeal.

This document is intended for people or automata familiar with Cedar Graphics.

To display an image in Cedar Graphics, one calls one of a number of procedures to control imaging, say to set clipping windows and to actually draw objects. A graphics list is generated with exactly the same sequence of Cedar Graphics calls as if the image were being displayed to the screen. In the latter case, a regular graphics context is used; in the former a special context is used.

The user obtains a graphics list context from the graphics list package using the Start function. Having called that, the user can call procedures in the Cedar Graphics package with the new context, and the calls are saved on the list. When the user has created as much of the image he cares to for that list, he may call End. End returns a list handle. Thus, one can have a list that creates the body of a person, and another list that creates a hat. Then, the person with a hat can be created on the screen by calling GL.Draw twice, once for each graphics list. You can call Start with a list you created earlier: this causes subsequent objects to be appended to the list.

As mentioned in the Introduction, a nice feature of graphics lists is their device independence. Therefore the same list can be output to a variety of devices.

It should be noted that, as an image is being built in a graphics list, it is not displayed onto the output device. Also, none of the information in the context is saved, such as the fat flag, the current transformation, and the clipper. The information is available and consistent as you build the list. We also don't do anything with DrawBits.

So, there are three procedures in the interface: Start, End and Draw.

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