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1.3 LINE DRAWING

• 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.

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1.3.1 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)

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