IGES TYPE QUERY

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
108?

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.

thanks

julia

 

For one thing, different systems have different capabilities
and use different internal representations of the data.
I expect that some that don't even use NURBS are still
out there. And some that represent everything as NURBS.
IF I understood your questions at all ...
Some may also do a bit of tweaking & adjustment to
imprecise or unsupported data based on it's type so adding a
bit more information may aid in that too.

 

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.

julia