Can't get intersection point using Entity.IntersectWith sometimes

[tom.zhengGNS4G]

Hi! I'm new to AutoCAD .NET and trying to get the intersection point of 2 lines using Entity.IntersectWith method lately. Usually, it can find the intersection points. but sometimes it doesn't get it when there should be intersection point between 2 lines.  Can' t figured it out why. can someone help or give some suggestions? thx!

Below is my code for test. I iterate each 2 lines in all lines selected by user and draw a circle on every intersection point. I write in c#:

```

[CommandMethod("Test")]
public void Test()
{
    Active.UsingTransaction(tr =>
    {

        var lineResult = Selector.GetSelection("\nselect lines: ");  //get user selection
        if (lineResult.Status != PromptStatus.OK) return;

        var lines = new List<Line>();
        foreach (var objId in lineResult.Value.GetObjectIds())
            lines.Add(objId.GetObject(OpenMode.ForRead) as Line);

        for (var i = 0; i < lines.Count; i++)
        {
            if (i + 1 == lines.Count) break;

            for (var j = i + 1; j < lines.Count; j++)
            {
                Line line1 = lines[i], line2 = lines[j];
                var interPts = new Point3dCollection();

                line1.IntersectWith(line2, Intersect.OnBothOperands, interPts, IntPtr.Zero, IntPtr.Zero);

                if (interPts.Count > 0) Draw.DrawCircle(interPts[0], 1);  //draw a circle with radius 1 on intersection point
            }
        }
    });
}

```

[_gile]

Hi,

Try with LineSegment3d.IntersectWith instead.

        [CommandMethod("Test")]
        public void Test()
        {
            Active.UsingTransaction(tr =>
            {
                var lineResult = Selector.GetSelection("\nselect lines: ");  //get user selection
                if (lineResult.Status != PromptStatus.OK) return;

                var lineSegments = lineResult.Value
                    .GetObjectIds()
                    .Select(id => (Line)id.GetObject(OpenMode.ForRead))
                    .Select(line => new LineSegment3d(line.StartPoint, line.EndPoint))
                    .ToArray();

                for (var i = 0; i < lineSegments.Length - 1; i++)
                {
                    for (var j = i + 1; j < lineSegments.Length; j++)
                    {
                        var line1 = lineSegments[i];
                        var line2 = lineSegments[j];
                        var interPts = line1.IntersectWith(line2);
                        if (interPts != null) 
                            Draw.DrawCircle(interPts[0], 1);  //draw a circle with radius 1 on intersection point
                    }
                }
            });
        }

When you post some code:

• Use the 'Insert/Edit code sample' button.
• If the code calls method from external libraries, provide the libraries source or a link to them.

[_gile]

I should have started there.

Are you sure there is an intersection between these lines (i.e. they are on the same plane)?

[tom.zhengGNS4G]

I checked all the 'Z' values of line's StartPoint/EndPoint are '0'. Does this mean they are on the same plane?  sorry I'm not familiar with AutoCAD.

[_gile]

---

@tom.zhengGNS4G  a écrit :

I checked all the 'Z' values of line's StartPoint/EndPoint are '0'. Does this mean they are on the same plane?  sorry I'm not familiar with AutoCAD.

---

Yes, that means all the lines lies on the XY plane.

Another thing, if the lines are far away from the origin (about x0,000,000 units for x and/or y) there may be an accuracy issue. A workaround is moving all lines closer to the origin, get the intersections and move back the lines (and intsection points).

[tom.zhengGNS4G]

Thanks for your suggestion! I found that some lines are not on the same plane. The property panel in AutoCAD shows that all lines are on the Z=0 plane. But when I get the position by code it shows that some line's Z value is not 0.

in AutoCAD it shows that all the Z-value is 0.

get the position of same line by code.

so I use a method to transform line to Z=0 plane if it is not.

void transformZAxis(Line line)
{
    line.UpgradeOpen();

    if (line.StartPoint.Z != 0)
        line.StartPoint = line.StartPoint.TransformBy(Matrix3d.Displacement(new Vector3d(0, 0, -line.StartPoint.Z)));
    if (line.EndPoint.Z != 0)
        line.EndPoint = line.EndPoint.TransformBy(Matrix3d.Displacement(new Vector3d(0, 0, -line.EndPoint.Z)));

    line.DowngradeOpen();
}

After all the lines transform to the same plane. it successfully get all the intersection points. Thanks again for your advice.

[_gile · Accepted solution]

You could simply do:

void transformZAxis(Line line)
{
    line.UpgradeOpen();

    if (line.StartPoint.Z != 0)
        line.StartPoint = new Point3d(line.StartPoint.X, line.StartPoint.Y, 0.0);
    if (line.EndPoint.Z != 0)
        line.EndPoint = new Point3d(line.EndPoint.X, line.EndPoint.Y, 0.0);

    line.DowngradeOpen();
}