[MSTN V8i] [C#]Judge four points on a plane？

Hi Alisa,

Alisa LI said:

If there is any way

I recommend to search existing discussions, this issue has been discussed many times already.

There are many ways how to implement such test, I recall e.g.

• Create a shape and test, whether it's planar or not. For this solution, I also remember another discussion that IsPlanar COM method provides in some situations different results from planar test in MDL.
• Create a plane from 3 points and calculate a distance between 4th point and this plane.
• For different sets of 3 points (at least 2 sets), create planes, calculate their normal vectors and compare them. They have to be the same.
• Check / use e.g. NetTopologySuite for such functionality.

What solution is the best for you and how it should be implemented depends solely on your own conditions and requirements: How far the points are, required precision and what difference is still counted planar and what not.

With regards,

  Jan

Thank you for your answer, I want to use the second method,

• Create a plane from 3 points and calculate a distance between 4th point and this plane.

can you give me some help for this method? I didn't find a way to create a plane with three points in the VBA documentation. I would be grateful if you could provide it to me.

Hi Alisa,

there are several examples how to work with plane in VBA documentation available, but to create a plane is not always straightforward (define base point and normal vector).

I remember (again, search this forum, all these topics were discussed in the past) this problem (plane + point distance) can be transformed to situation, when 3 points are used to define transformation, so they define custom coordinate system. The distance is Z coordinate in this system. I like this approach, because it's mostly based on transformations (rotation matrices), which is elegant solution.

With regards,

  Jan

Alisa LI said:

I didn't find a way to create a plane with three points in the VBA documentation

Three points define a triangle, which is always planar...

• Construct a ShapeElement from your points
• Get its normal (ShapeElement.Normal property)
• Create a plane (VBA Plane3d) from that normal and one of your points

Thank you for your answer, it gave me a lot of help.

Alisa LI said:

it gave me a lot of help

You might like to read this article: Determine Co-Planar Shapes .

Alisa LI said:

it gave me a lot of help

You might like to read this article: Determine Co-Planar Shapes .