CG Modeling 2: NURBS - An Introduction to Computer Graphics for Artists - page 262

Fig. 14.24 Crossed curve direction

Fig. 14.25 The chain ring ( a ) is built from sections ( b ) that are trim surfaces ( c )

14.4.7 Trims

A trim is a section of a surface defi ned by either a closed projected curve or a group
of open projected curves that together completely encloses an area. The area within
the defi ned region may be defi ned as either cut away , or remaining . The renderer will
then render either the area outside of the trim curve(s) or within them (Fig. 14.25 ).
Trim surfaces effectively double the amount of geometry for an object, because
the original untrimmed surface is always retained as a reference for the renderer.