C# Transform - Line vs. Curve?

Grasshopper Developer
1

I’m confused about how transforms work. These seem like they should work the same, but they don’t. Or maybe it has something to do with the type?
Transforms.gh (10.9 KB)

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image1574×635 50.7 KB

input: Line
output: Rotated Lines

List < Line > geoList = new List< Line >();
List<Transform> transList = new List<Transform>();

Line geo = line;

Point3d geoCenter = geo.BoundingBox.Center;

// rotation = new Transform();


for (int i = 0; i < steps; i++)
{
  Transform rotation = Transform.Rotation(Angle, geoCenter);

  geo.Transform(rotation);

  geoList.Add(geo);
}

A = geoList;

input: Curve
output: Multiple curves at same location

List < Curve > geoList = new List< Curve >();
List<Transform> transList = new List<Transform>();

Curve geo = curve;

Point3d geoCenter = geo.GetBoundingBox(true).Center;

for (int i = 0; i < steps; i++)
{
  Transform rotation = Transform.Rotation(Angle, geoCenter);

  geo.Transform(rotation);

  geoList.Add(geo);
}

A = geoList;
2

Hello,

yes it has something to do with the type. But probaly not like you expect.A Curve is a class, whereas a Line object is a struct. Since classes pass by reference and structs by value (google for differences of structs and classes), there is different behaviour expected. Changing the transform does not copy the object, but because a line is a struct, you duplicate when adding to a list.

If you replace geoList.Add(geo) in the Curve part with geoList.Add(geo.Duplicate() as Curve); you will get the correct result in return.

Hope it helps…

3

Hi Tom,

That makes sense. It also answers a related question doing the same operation using the GeometryBase Class instead of Curve.

Thanks!

1 Like
4

As a sidenote, Visual Studio 2019 differentiates structs and classes by using different colors for them. This is quite handy, because otherwise its not obvious that some objects are structs, since the common C# coding style (if there is one) usually does not differentiate here.

5

If you don’t want to think about any of that you can use IGH_GeometricGoo, GH_Transform, and ITransform from the Grasshopper API.

6

Hi Michael,

Thanks for the suggestion, though I’m not sure quite how to do that.

I’ve replaced the references to GeometryBase and Rhino.Transform with IGH_GeometricGoo and GH_Transform, but I’m not clear on how to apply the GH_Transform to the GeometricGoo object.

On a related note, could you help me understand why these (and other) functions are duplicated in the Grasshopper API? I have a vague sense of why it would be useful, but I’m not clear on when I should use one instead of the other. Thanks!

 //old
List < GeometryBase > geoList = new List< GeometryBase >();
GeometryBase geo = Geo;
Point3d geoCenter = geo.GetBoundingBox(true).Center;
Transform rotation = new Transform();

//new
List<IGH_GeometricGoo> gooList = new List<IGH_GeometricGoo>();
IGH_GeometricGoo goo = GH_Convert.ToGeometricGoo(Geo);
Point3d gooCenter = goo.GetBoundingBox(Transform.Identity).Center;
Grasshopper.Kernel.Types.Transforms.Rotation gooRotation = new Grasshopper.Kernel.Types.Transforms.Rotation();

for (int i = 0; i < steps; i++)
{
  //old
  rotation = Transform.Rotation(angle, geoCenter);
  geo.Transform(rotation);
  geoList.Add(geo.Duplicate() as GeometryBase);

  //new
  gooRotation = new Grasshopper.Kernel.Types.Transforms.Rotation(gooCenter, Vector3d.ZAxis, angle);
  
  //confused here
  goo.Transform(?); //??? Takes Rhino.Geometry.Transform

  gooList.Add(goo.DuplicateGeometry());
  
}
7

goo.Transform() returns the transformed geometry, it doesn’t change the value inside the goo.