# [Gmsh] The Control of OCC BSpline points on the intersection edge

Jun Fang hapfang at gmail.com
Tue Mar 10 23:33:02 CET 2020

```Hi Christophe,

I do appreciate the quick response. I am using GMSH a lot to make pure-hex
meshes for geometries of low to intermediate complexity.
Thanks to the Transfinite capabilities GMSH provides, I can generally
achieve the goal by dividing the geometry into appropriate 3D "boxes".
To that end, a 4-point curve loop is way more desirable than a 5-point one
(in designing 3D mesh blocks).

I've tried the `Compound Curve` tutorial example (t12.geo), but  with no
success in making a good structured mesh.
Here are my questions:
1. Is it doable?
2. If so, how can I refer the the combined curve in Transfinite Curve
function? I notice there is no new tag being created for the compound curve
group.

Jun

On Tue, Mar 10, 2020 at 4:14 PM Christophe Geuzaine <cgeuzaine at uliege.be>
wrote:

>
>
> > On 10 Mar 2020, at 17:55, Jun Fang <hapfang at gmail.com> wrote:
> >
> > Dear GMSH users,
> >
> > By applying the BooleanIntersection between a volume and a curved
> surface, I can get a decent BSpline intersection curve loop on the surface.
> The problem I am facing is that the  BooleanIntersection by default gives
> me an “extra” point on the intersection edge, which I don’t want (for good
> reasons).
>
> Can you elaborate?
>
> > A screenshot is posted below. The ‘Cylinder 23(OCC)’ is the outcome of
> Boolean intersection. What I need are the four points (22, 23, 24, 25) and
> four associated BSpline curves on the Surface. Somehow, I couldn’t get rid
> of the extra point (26), which is located at the z max position of Surface
> 23(OCC). My guess is that the additional point is kinda of a reference
> point created by the  BooleanIntersection function.
> >
> > Is it possible to workaround this issue? I know Compound BSpline could
> potentially merge curves (i.e. connecting BSplines 27 and 28 here into
> one), but it only works for the built-in CAD kernel. Is there an equivalent
> function for OCC kernel? Any comments/suggestions are appreciated.
> >
>
> If you don't want the point in the 1D mesh, can't you just use the
> "Compound Curve" meshing constraint?
>
> Christophe
>
>
> > <image.png>
> >
> > Thanks very much,
> >
> > Jun
> >
> > --
> > Jun Fang, Ph.D. - Argonne National Laboratory - (630)252-4561
> > _______________________________________________
> > gmsh mailing list
> > gmsh at onelab.info
> > http://onelab.info/mailman/listinfo/gmsh
>
> —
> Prof. Christophe Geuzaine
> University of Liege, Electrical Engineering and Computer Science
> http://www.montefiore.ulg.ac.be/~geuzaine
>
>
>
>

--
*Jun Fang, Ph.D. - **Argonne **National Laboratory **- **(630)252-4561*
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