Program.cs

using GShark.Geometry;

namespace example;

// example-0022
// Nurb surface join on two tubes
//
// - use 'space' application key to toggle joint visiblity

class Program
{

    static void Main(string[] args)
    {
        InitAvalonia();

        GLPointFigure? profilePtsFig = null;

        var w = GLWindow.Create(

            // customize ControlFigureVisible to not show green profile dots on "RENDER" titled views
            onGLControlCreated: (avaloniaGLControl) =>
            {
                avaloniaGLControl.GLControl.ControlFigureVisible = (glControl, glFigure) =>
                {
                    if (glControl.Title.StartsWith("RENDER"))
                        return glFigure != profilePtsFig;

                    return true;
                };
            },

            onFocusedControlChanged: (split, AvaloniaGLControl, isInitial) =>
            {
                if (isInitial)
                    split.LoadViewLayout();
            }
        );

        GLTriangleFigure? joinFig = null;

        w.GLModel.BuildModel = (glCtl, isInitial) =>
        {
            if (!isInitial) return;

            var glModel = glCtl.GLModel;

            const int JOINT_DIV = 8;
            const int TUBE_DIV = JOINT_DIV;
            const int RAIL_DIVS = 4;

            glModel.Clear();

            glModel.AddFigure(MakeWCSFigure());

            var tube1 = new Cone(
                baseCS: (WCS * Matrix4x4.CreateRotationX((float)(PI / 2)) * Matrix4x4.CreateRotationZ(-(float)(PI / 4)))
                    .Move((float)(-3 * Cos(45d.ToRad())), (float)(-3 * Sin(45d.ToRad())), 0),
                baseRadius: 2, topRadius: 2, height: 6,
                bottomCap: false, topCap: false).Figure(JOINT_DIV);

            // mirror tube1 respect the YZ plane
            var tube2 = tube1.Mirror(YZCS)!;

            glModel.AddFigure(tube1);
            glModel.AddFigure(tube2);

            // grab tube vertexes near join as profile to sweep

            var _profilePts = tube1
                .BuildVertexPosDict(tol: 1e-5f)
                .Select(w => w.Value.First())
                .Where(r => r.Position.Length() < 5).Select(w => new
                {
                    vtx = w,
                    sig = w.PositionSignature(1e-5f),
                    pos = w.Position
                }).ToList();
            var profilePts = _profilePts.Select(w => w.pos).ToList();

            var profilePtsMean = profilePts.Mean();
            
            var profileCenter = profilePtsMean;
            var profilePlane = MakeCS(profilePtsMean,
                profilePts[JOINT_DIV / 4] - profilePtsMean,
                profilePts[0] - profilePtsMean,
                makeOrthonormalization: true);

            Debug.WriteLine($"profilePlane START: {profilePlane.BaseZ()}");

            profilePts.Add(profilePts[0]);

            profilePtsFig = new GLPointFigure(profilePts).SetColor(Color.Green);
            glModel.AddFigure(profilePtsFig);

            var railPts = new List<Vector3>();
            {
                var N = RAIL_DIVS;
                var alpha = 0f;
                var alphaStep = (float)(PI / 2 / N);
                var rotCenter = new Vector3(0, (float)(-3 * Sqrt(2)), 0);
                for (int i = 0; i < N + 1; ++i)
                {
                    railPts.Add(Vector3.Transform(profileCenter, Matrix4x4.CreateRotationZ(-alpha, rotCenter)));

                    alpha += alphaStep;
                }
            }

            for (int i = 0; i < railPts.Count; ++i)
            {
                var railPt = railPts[i];
                System.Console.WriteLine($"rail [{i}]: {railPt}");
            }

            System.Console.WriteLine($"profile CENTER : {profileCenter}");

            for (int i = 0; i < profilePts.Count; ++i)
            {
                var profilePt = profilePts[i];
                System.Console.WriteLine($"profile [{i}]: {profilePt}");
            }

            var sweepNurb = NurbsSurface.FromSweep(
                new NurbsCurve(railPts.ToPoint3().ToList(), 1),
                new NurbsCurve(profilePts.ToPoint3().ToList(), 1),
                startTangent: new GShark.Geometry.Vector3(0.7071068, 0.70710677, 0),
                endTangent: new GShark.Geometry.Vector3(0.7071068, -0.70710677, 0));

            var railFig = new GLPointFigure(railPts).SetColor(Color.Yellow);
            glModel.AddFigure(railFig);

            joinFig = sweepNurb.NurbToGL(Color.Red, N: TUBE_DIV).ToFigure();
            glModel.AddFigure(joinFig);
        };

        w.KeyDown += (sender, e) =>
        {
            if (e.Key == Key.Space)
            {
                if (joinFig is not null) joinFig.Visible = !joinFig.Visible;

                w.Invalidate();
            }
        };

        w.ShowSync();
    }

}