• 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.