Overview | Statement List

Extract

Synopsis

obj = BSURF/SURFC,obj[,APPROX][,TOLER,dtol,atol][,IFERR,label:]

Description

Creates a new sheet body with an underlying B-surface face type by extracting a face from an existing body. You can either extract the face into a cubic polynomial B-surface type sheet, or a more general B-surface type sheet.

Parameters

Parameter	Description
SURFC	Minor word denoting a sheet extraction.
obj	The existing face to be extracted.
APPROX	Optional minor word indicating that the resulting sheet is to be a polynomial cubic. The system almost always approximates the sheet, so the original surface type may not be replicated exactly. If APPROX is not specified, the system attempts to convert the surface type exactly. But, if the surface is of a type that cannot be converted exactly, then the system approximates it. See the table on page for a breakdown of each instance and whether the system performs an approximation or exact replication.
TOLER	Optional minor word indicating that two tolerance values are being specified for the extraction. The tighter (smaller) the tolerances, the more patches the system creates in the resulting B-surface type sheet. If none is specified, the default is the value set in the Unigraphics option Preferences—>Modeling. You can also read and set these tolerances using the GPAs &DISTOL and &ANGTOL.
dtol	The distance tolerance to be used during the approximation. The maximum allowable distance between corresponding points on the original body and the new sheet.
atol	The angle tolerance to be used during the approximation. The maximum allowable angle between the face normals at corresponding points.
IFERR,label:	Specifies a label to which program execution jumps if an error occurs; for example, if the body cannot be approximated within the input tolerances, or when the extraction is to be done exactly and the resulting sheet exceeds the maximum size allowed. In these cases, the following error message is returned: Unable To Define Surface, No Surface Created

As mentioned previously, the system may sometimes replicate the original surface type exactly, but other times may have to approximate them, depending on the original surface type and the method of conversion that you specify. The following table illustrates each instance. The bracketed numbers refer to the notes listed below.

Original Surface Type	Approximation Specified	Approximation Not Specified
Cylinder, cone, sphere	Approximate	Exact
Tabulated cylinder	Approximate	Exact
Surface of revolution	Approximate	Exact
Ruled surface	Approximate	Exact [1]
Fillet surface	Approximate	Approximate [2]
Sculptured surface	Approximate	Approximate [2]
B-surface	Approximate [3]	Exact
Offset surface	Approximate	Approximate [2]
Bounded planes	Exact	Exact

Notes:

1. This conversion of a ruled surface type sheet is an approximation if either or both generating curves are an arc or conic.

2. In these cases, the general B-surface option produces the same result as the polynomial cubic option.

3. If the selected sheet is already a polynomial cubic, no conversion is necessary.