Surface / Curve > Difference (A=A-B) (Trim Menu)

This command can be used to combine existing flat objects into one surface which covers all parts of the drawing previously covered only by the first object and not by the second object. This is the same as the difference of two surfaces according to the group theory.

The following object types are classed as "flat": circles, circle sectors, circle segments, ellipses, ellipse sectors, ellipse segments and surfaces. Ellipses and ellipse parts are converted into polygons and are therefore "only" contained as polygons in the result.

In this command's parameters, you can choose whether the result shall be automatically simplified (see "Simplify Curve / Surface" dialog), and set the corresponding tolerances.

The result in each case is one "surface" object, which may however be made up of more than one surface part. There follows a graphic showing examples of differences:

Note that the order in which the objects are identified is important. The object identified second is always "subtracted" from the object identified first! The resulting object will contain all attributes of the two chosen objects without duplicates.

This command works with all objects which have a closed outline. These are automatically converted to surfaces for the calculation. Many object types can not be converted into a surface without loss. Ellipses and ellipse parts must be converted into polylines, which invariably leads to a loss of quality. The precision of the conversion can be specified using the command Drawing Settings > Precision in the entry "Transform". The original objects (which remain unchanged if the duplicate function is active), are not influenced by this conversion. Surfaces with "holes" can also be used. There can be problems if both objects share (part) of an edge. In this case there can, in spite of calculation to 16 decimal places, be incorrect interpretations. Do not rely on the sides of overlapping objects always or never remaining unaltered. This varies from case to case.