Defining the Model
computer representation of drawings and diagrams
Defining the graphic elements
The user has a variety of different ways to call a particular graphic element and position it on the geometric model. There are several ways of defining points, lines, arcs, and other components of geometry through interaction with the ICG (interactive computer graphics) system. These components are maintained in the database in mathematical form and referenced to a 3D coordinate system.
A component must be modeled before it can be drawn.
Methods of defining elements in interactive computer graphics Points
Methods of defining points in computer graphics include:
*Pointing to the location on the screen by means of cursor control.
*Entering the coordinates via the alphanumeric keyboard.
*Entering the offset (distance in x, y, and z) from a previously defined point.
*The intersection of two points.
*Locating points at fixed intervals along an element.
Methods of defining lines include:
*Using two previously defined points.
*Using one point and specifying the angle of the line with the horizontal.
*Using a point and making the line either normal or tangent to a curve.
*Using a point and making the line either parallel or perpendicular to another line.
*Making the line tangent to two curves.
*Making the line tangent to a curve and parallel or perpendicular to a line.
Arcs and circles
Methods of defining arcs and circles include:
*Specifying the center and the radius.
*Specifying the center and a point on the circle.
*Making the curve pass through three previously defined points.
*Making the curve tangent to three lines.
*Specifying the radius and making the curve tangent to two lines or curves.
Some of the methods for generating surfaces include:
*Using a surface of revolution formed by rotating any lines and/ or curves around a specific axis.
*Using the intersection line or surface of two intersecting surfaces.