topic Re: How to get Normal vector of a Line? in .NET Forum

How to get Normal vector of a Line?

nur91m — Fri, 18 Aug 2017 03:21:27 GMT

Hi all. When I try to get the normal vector of a line I get (0,0,1) vector though I tried any rotation with my line. How should I get the normal vector of a line?

Line line = new Line(pt1,pt2);
Vector3d v3d = line.Normal;
ed.WriteMessage(line.Normal.ToString());

Re: How to get Normal vector of a Line?

ActivistInvestor — Fri, 18 Aug 2017 05:10:29 GMT

Lines drawn while the UCS is set to WORLD have no normal vector.

So, the Normal property returns the 'default' normal vector, which is the WCS Z axis.

If you draw a line in a non-world UCS, it will have an explicit normal vector and Normal returns that.

Re: How to get Normal vector of a Line?

nur91m — Fri, 18 Aug 2017 09:57:07 GMT

Thanks for information. I will just calculate normal with formula

Re: How to get Normal vector of a Line?

_gile — Fri, 18 Aug 2017 10:56:09 GMT

nur91m a écrit :

Thanks for information. I will just calculate normal with formula

Geometrically, you can not calculate the normal of a line because it does not define a plane (a line belongs to an infinity of planes).

As @ActivistInvestor said, AutoCAD defines the "extrusion direction" of a line (Normal property or DXF code 210) with the Z axis of the coordinate system that was active when the line was drawn.
But a line can be drawn not parallel to the current XY plane.
This data is therefore not reliably linked to the geometry of the line.

For example, in the WCS, you can draw a line from (10, 10, 0) to (10, 10, 20).
The direction of extrusion of the line * would be identical to the axis WCS Z (0 0 1) despite the fact that this line is parallel to this direction.

* You can get it by evaluating the expression LISP: (cdr (assoc 210 (entget (entlast))) after drawing the line.

Re: How to get Normal vector of a Line?

_gile — Fri, 18 Aug 2017 11:18:54 GMT

Perhaps I misundertood the request.

If you want the unit vector (normalized) of the line direction, simply do:

line.StartPoint.GetVectorTo(line.Endpoint).GetNormal();

(line.EndPoint - line.StartPoint).GetNormal();

If I'm still wrong, please clarify what you're trying to get.

Re: How to get Normal vector of a Line?

ActivistInvestor — Fri, 18 Aug 2017 18:16:01 GMT

Just to clarify, as @_gile mentioned, a line has no normal. The Normal property returns a value that is used by AutoCAD to extrude the line if you give it a thickness.

If the line was drawn in the XY plane of the current UCS (and wasn't edited afterwards), the value returned by Normal will be the Z-axis of the UCS that was current when the line was drawn. But, you can't rely on that vector to be perpendicular to the line, because it may not be.

Re: How to get Normal vector of a Line?

_gile — Fri, 18 Aug 2017 19:02:54 GMT

Activist_Investor a écrit :

Just to clarify, as @_gile mentioned, a line has no normal. The Normal property returns a value that is used by AutoCAD to extrude the line if you give it a thickness.

If the line was drawn in the XY plane of the current UCS (and wasn't edited afterwards), the value returned by Normal will be the Z-axis of the UCS that was current when the line was drawn. But, you can't rely on that vector to be perpendicular to the line, because it may not be.

Thanks for clarifying this in good English.