Hi there,

I wonder why we have some many form of curves in IGES? Can't type 126 more than enough to create a curve? Why we still need the 100, 104, or


Form type in field 15 in the directory entry(DE) for type 126 has options:

0 - form of curve must be determined from the rational b spline parameters 1 - line 2 - circular arc 3 - elliptical arc 4 - parabolic arc 5 - hyperbolic arc

In order words, why iges provide the option in type 142 to have BPTR - pointer to the DE of the entity that contain the definition of the curve B in the parametric space (u,v) of the surface CPTR - pointer to the DE of the curve C

If we select 0 in type 126, then CPTR is NULL and type 100, 104 or 108 wont be necessary anymore.

I wonder if it is not always possible to intepret a curve using

126(rational b spline entity)?

what kind of curve in general not doing well if we use tpe 126?

please correct me if I am wrong.



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thanks. It kind of strange to me that some iges file(simply these type

144, 142, 126, 102, 128) can be read in some software(like solidworks) and some can't.


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