12.3 LINE DRAWING

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A wireframe object is simply rendered using the lines (straight, and curved) in a geometric model, then it is converted to screen coordinates using the Perspective Transformation.

The process is as pictured below,

If the geometric model is in some other form, such as CSG (Constructive Solids Geometry), it may have to be converted to lines before it may be used.

Advantages,

- Very fast, allows real time manipulation

- Easy to see features normally hidden

- Supported by almost all computer technology

- This display method works with most displays

Disadvantages,

- Drawings can become very crowded, very fast

- Hard to visualize solid properties

Almost all computer graphics packages offer a wireframe display option.

12.3.1 Hidden Lines

While the results seem similar, this method requires more sophisticated algorithms.

Polygons must be used for surface representation.

The diagram below shows the basic steps in the method of rendering

Advantages,

- similar advantages of wireframe

- overcomes the drawing crowding

- can be simplified if polygons do not overlap

Disadvantages,

- Consumes more computer time

- Does not support simple elemental geometric models

Clipping algorithms may be difficult, and often use tricks like subdividing polygons into triangles. (There are only 5 cases to consider with overlapping triangles)