【解決方法】STL オブジェクトの非表示部分を変更するには?

What happens in this code is that the selected object is shown according to the value entered in the textbox and the rest of the object is hidden.

Example :- 
The selected object is 90mm in total. Now if I enter 50 in the textbox, the 1mm to 50mm object is shown and the 51mm to 90mm object is hidden. Now what I want to do is that the object from 51mm to 90mm should be removed.

私が試したこと：

private void viewPort3d_MouseDown(object sender, MouseButtonEventArgs e)
{
    if (e.ClickCount == 2 && e.ChangedButton == MouseButton.Left)
	{
		// Store the original material of the newly selected object
        var group2 = selectedModel.Content as Model3DGroup;
        if (group2 != null && group2.Children.Count > 0)
        {
            var model = group2.Children[0] as GeometryModel3D;
            if (model != null)
            {
                STL.UpdateMaterial = model.Material;

                var bounds = model.Bounds;
                double width = bounds.SizeX * 1000; // convert to millimeters
                double height = bounds.SizeY * 1000; // convert to millimeters
                double depth = bounds.SizeZ * 1000; // convert to millimeters

                double hideDepth = 0.0;
                var Z = depth / 1000.0;
							
                double.TryParse(depthTextBox.Text, out hideDepth);
                // Calculate the percentage of the object to be hidden
                double hidePercent = hideDepth / Z;

                // Iterate through all the mesh triangles and hide those above the hidePercent
                var meshGeometry = model.Geometry as MeshGeometry3D;
                if (meshGeometry != null)
                {
                    for (int i = 0; i < meshGeometry.TriangleIndices.Count; i += 3)
                    {
                       Point3D p1 = meshGeometry.Positions[meshGeometry.TriangleIndices[i]];
                       Point3D p2 = meshGeometry.Positions[meshGeometry.TriangleIndices[i + 1]];
                       Point3D p3 = meshGeometry.Positions[meshGeometry.TriangleIndices[i + 2]];

                       double minZ = Math.Min(Math.Min(p1.Z, p2.Z), p3.Z);
                       double maxZ = Math.Max(Math.Max(p1.Z, p2.Z), p3.Z);

                       if (maxZ >= bounds.SizeZ * hidePercent)
                       {
                          // Hide the triangle by setting its normal to (0,0,-1)
                          meshGeometry.Normals[meshGeometry.TriangleIndices[i]] = new Vector3D(0, 0, -1);
                          meshGeometry.Normals[meshGeometry.TriangleIndices[i + 1]] = new Vector3D(0, 0, -1);
                          meshGeometry.Normals[meshGeometry.TriangleIndices[i + 2]] = new Vector3D(0, 0, -1);
                       }
                    }
                    // Invalidate the visual to refresh the changes
                    model.Geometry = meshGeometry;
                    viewPort3d.InvalidateVisual();
                  }
			  }
		}
    }
}