HiddenLineDrawing: Clipping Planes are ignored in Make2D calculation (C#)

[Ali_Javaheri1 (Ali Javaheri)]

I am developing a custom Make2D command using Rhino.Geometry.HiddenLineDrawing in C#. I have successfully implemented the standard hidden line drawing (Visible/Hidden curves), but I am stuck on getting Clipping Planes to work.

The Goal: I want to generate a 2D drawing of selected objects that respects the active Clipping Planes in the view, exactly like the native Rhino Make2D command does with “Clipping Plane Intersections” enabled.

The Issue: Even though I am adding the active clipping planes to the HiddenLineDrawingParameters and enabling OccludingSectionOption, the Compute method returns the full, uncut geometry. It acts as if the clipping planes do not exist.

What I have tried:

1. Checking that the Clipping Plane is active in the specific Viewport ID.

2. Setting hldParams.OccludingSectionOption = true.

3. Adding the plane using hldParams.AddClippingPlane(plane).

4. I have tried Global Clipping (implicit) and Selective Clipping using hldParams.AddGeometryAndPlanes(...).

In all cases, the output is the complete object, ignoring the cut.

Code Snippet: Here is the core logic I am using to set up the parameters. Am I missing a step to “link” the planes to the view or the geometry?

public void CalculateMake2D(Rhino.Display.RhinoViewport viewport, List objects)
{
var hldParams = new HiddenLineDrawingParameters
{
AbsoluteTolerance = RhinoDoc.ActiveDoc.ModelAbsoluteTolerance,
IncludeTangentEdges = false,
IncludeHiddenCurves = true,
Flatten = true, // We want the 2D projection
OccludingSectionOption = true // Enabled for solid caps
};

hldParams.SetViewport(new ViewportInfo(viewport));

// 1. Find and Add Active Clipping Planes
List<Plane> activePlanes = new List<Plane>();
foreach (var obj in RhinoDoc.ActiveDoc.Objects.FindByObjectType(ObjectType.ClipPlane))
{
    if (obj is ClippingPlaneObject cp && cp.ClippingPlaneGeometry.ViewportIds().Contains(viewport.Id))
    {
        // The plane is active in this view
        var plane = cp.ClippingPlaneGeometry.Plane;
        activePlanes.Add(plane);
        
        // Register the plane
        hldParams.AddClippingPlane(plane);
    }
}

// 2. Add Geometry
foreach (var objRef in objects)
{
    var geo = objRef.Geometry();
    if (geo is Brep || geo is Mesh)
    {
        // Attempt 1: Selective Clipping (Explicitly passing the planes)
        if (activePlanes.Count > 0)
        {
             // Using overload: (geometry, xform, id, occluding_sections, clips)
             hldParams.AddGeometryAndPlanes(geo, Transform.Identity, objRef.ObjectId, true, activePlanes);
        }
        // Attempt 2: Standard Add (hoping for implicit global clipping)
        else 
        {
             hldParams.AddGeometry(geo, Transform.Identity, objRef.ObjectId, true);
        }
    }
}

// 3. Compute
// The result here contains all curves (Visible/Hidden) but NO intersection cuts 
// and the geometry is fully drawn as if uncut.
var hld = HiddenLineDrawing.Compute(hldParams, true);

// ... Processing hld.Segments ...

}