Trimmed Surfaces and the IGES entity #142

A trimmed surface in the IGES format is an entity #144. An entity #142 — Curve on parametric surface — is associated with each entity #144 — Trimmed surface.

The entity #142 contains all the information about how the surface is to be trimmed. A parameter section for an entity #142 is like the following:



A curve on a parametric surface is given by:

  1. The mapping C and an indication that the curve lies on on the surface S(u,v)
  2. The mappings B and S whose composition gives the curve C

The parametric data section contains three pointers:

  1. A pointer to the surface S(u,v)
  2. A pointer to the u,v space curve (isoparametric curve). A pointer to the curve from which B(t) is derived
  3. A pointer to model space curve (3D curve). A pointer to the mapping C(t)
    It also contains two flags:
  4. A flag to indicate how the curve was created
  5. A flag to indicate which of the two alternative representations was preferred by the sending system.

The meaning of the second flag is:
0 unspecified
1 The isoparametric curve is preferred (S or B)
2 The 3D curve is preferred (C)
3 The isoparametric and the 3D curves are equally preferred.
 

See also Initial Graphics Exchange Specification (IGES) Version 4.0, NBSIR 88-3813, June 1988, page 3.