1.4 EDITING AND CREATING
• Each CAD package allows us to manipulate the geometrical model using various interactive techniques.
• Editing Geometry depends upon the representation the geometry is stored with.
• If an elemental (remember: lines, circles, arcs, etc.) geometric model is used, then the methods are much different than a B-Rep model.
• The major editing methods used are for Elemental, Surfaces, and CSG.
1.4.1 2D Curves and Lines
• A number of functions must be provided to allow editing of 2D geometrical entities, such as lines, circles, arcs.
• Some of the basic editing functions are listed below,
- Basic Entity Creation (lines/ circles/ arcs/ etc)
- create using exact coordinates
- two screen points for line ends, circle radius/centre/diameter/etc
- Line Trimming
- trim lines back to intersection
- extend lines to intersection
- trim line to perpendicular point
- cut a circle/arc on one side of an intersection
- Point Creation
- screen position
- exact numerical coordinate
- nearest tangent of line to an arc
- nearest end of a line
- midpoint of nearest line
- centre of nearest arc
- nearest grid point
- Arc Creation
- intersection of circle with another line
- Special Techniques
- offset of a line
- extend lines to intersection
- trim line to perpendicular point
- delete entities
- etc
• There are a number of ways (philosophies) for creating drawings using the basic elemental editing techniques. A few popular methods are listed below.
- Construction Lines - A set of construction lines are set up, then segments of the lines are selected for the actual drawing
- Trimming - The construction lines are all drawn, then the unwanted parts are trimmed off
- Navigation - A line figure is built up using successive line segments.
- Parametrization - Objects such as rectangles, circles, arcs, etc. are created using their dimensions, then positioned with traditional methods.
1.4.2 Surfaces
• Most surface modelling packages rely on the elemental definition of lines, and points.
• There are a number of basic philosophies for creating surfaces,
- Swept profiles - a profile, and a path in space are used to sweep out a surface.
- Rotated Profiles - a Profile is created then swept about an arbitrary axis
- Extruded Profiles - a profile is created, then grown in one direction.
- Skins (Splines) - a direct creation of points, then the splines that connect them
- Polygon Approximation - polygons are defined which join up to define a surface
- Sections - sections are defined for different points along a path, which then allow generation of complex transition geometries.
• Once surfaces have been created, they may be operated on by boolean operations.
• This method is often used as a preliminary stage to CSG editing.
1.4.3 CSG
• This is by far the simplest method
• Solid Primitives are progressively cut and joined to form new shapes.
• Primitives may come from,
- Traditional Sources - Blocks, Spheres, Wedges, etc.
- Surfaces - A Volume is assigned to a surface model
- Previous operations
- others ?
• CSG editing requires storage of the results of operations. This is because a part may be used many times to cut another part, for example a chamfered hole for a sunken screw.
• The fundamental CSG operations are,
Union - both parts joined as one
Intersection - Only where two parts overlap
Subtraction - only where parts do not overlap, One of the parts is typically discarded.
