acdream/src/AcDream.App/Rendering/Wb/WbFrustum.cs

// Ported from references/WorldBuilder/Chorizite.OpenGLSDLBackend/Frustum.cs
// Phase A8 extraction (2026-05-28). Verbatim algorithm; adaptations:
//   - Namespace: AcDream.App.Rendering.Wb
//   - Class renamed Frustum -> WbFrustum
//   - BoundingBox -> WbBoundingBox (inlined below; no new project dep)
//   - FrustumTestResult kept as-is (inlined below)
//   - WbPlane inlined (was an inner type of Frustum.cs in WB)

using System.Numerics;

namespace AcDream.App.Rendering.Wb;

public struct WbBoundingBox
{
    public Vector3 Min;
    public Vector3 Max;

    public WbBoundingBox(Vector3 min, Vector3 max)
    {
        Min = min;
        Max = max;
    }

    public static WbBoundingBox Union(WbBoundingBox a, WbBoundingBox b) =>
        new WbBoundingBox(
            Vector3.Min(a.Min, b.Min),
            Vector3.Max(a.Max, b.Max));
}

public enum FrustumTestResult
{
    Outside,
    Inside,
    Intersecting
}

/// <summary>
/// View-frustum helper extracted from WorldBuilder's Frustum class.
/// Source: references/WorldBuilder/Chorizite.OpenGLSDLBackend/Frustum.cs
/// Phase A8 extraction (2026-05-28).
/// </summary>
public sealed class WbFrustum
{
    private struct Plane
    {
        public Vector3 Normal;
        public float D;

        public Plane(float a, float b, float c, float d)
        {
            Normal = new Vector3(a, b, c);
            float length = Normal.Length();
            Normal /= length;
            D = d / length;
        }

        public float Dot(Vector3 point) => Vector3.Dot(Normal, point) + D;
    }

    private readonly Plane[] _planes = new Plane[6];
    private readonly object _lock = new();

    public void Update(Matrix4x4 matrix)
    {
        lock (_lock)
        {
            // Left plane
            _planes[0] = new Plane(matrix.M14 + matrix.M11, matrix.M24 + matrix.M21, matrix.M34 + matrix.M31, matrix.M44 + matrix.M41);
            // Right plane
            _planes[1] = new Plane(matrix.M14 - matrix.M11, matrix.M24 - matrix.M21, matrix.M34 - matrix.M31, matrix.M44 - matrix.M41);
            // Bottom plane
            _planes[2] = new Plane(matrix.M14 + matrix.M12, matrix.M24 + matrix.M22, matrix.M34 + matrix.M32, matrix.M44 + matrix.M42);
            // Top plane
            _planes[3] = new Plane(matrix.M14 - matrix.M12, matrix.M24 - matrix.M22, matrix.M34 - matrix.M32, matrix.M44 - matrix.M42);
            // Near plane
            _planes[4] = new Plane(matrix.M14 + matrix.M13, matrix.M24 + matrix.M23, matrix.M34 + matrix.M33, matrix.M44 + matrix.M43);
            // Far plane
            _planes[5] = new Plane(matrix.M14 - matrix.M13, matrix.M24 - matrix.M23, matrix.M34 - matrix.M33, matrix.M44 - matrix.M43);
        }
    }

    public bool Intersects(WbBoundingBox box, bool ignoreNearPlane = false)
    {
        lock (_lock)
        {
            for (int i = 0; i < 6; i++)
            {
                if (ignoreNearPlane && i == 4) continue;

                Vector3 positive = box.Min;
                if (_planes[i].Normal.X >= 0) positive.X = box.Max.X;
                if (_planes[i].Normal.Y >= 0) positive.Y = box.Max.Y;
                if (_planes[i].Normal.Z >= 0) positive.Z = box.Max.Z;

                if (_planes[i].Dot(positive) < 0)
                {
                    return false;
                }
            }
        }
        return true;
    }

    public FrustumTestResult TestBox(WbBoundingBox box, bool ignoreNearPlane = false)
    {
        var result = FrustumTestResult.Inside;
        lock (_lock)
        {
            for (int i = 0; i < 6; i++)
            {
                if (ignoreNearPlane && i == 4) continue;

                Vector3 positive = box.Min;
                Vector3 negative = box.Max;
                if (_planes[i].Normal.X >= 0)
                {
                    positive.X = box.Max.X;
                    negative.X = box.Min.X;
                }
                if (_planes[i].Normal.Y >= 0)
                {
                    positive.Y = box.Max.Y;
                    negative.Y = box.Min.Y;
                }
                if (_planes[i].Normal.Z >= 0)
                {
                    positive.Z = box.Max.Z;
                    negative.Z = box.Min.Z;
                }

                if (_planes[i].Dot(positive) < 0)
                {
                    return FrustumTestResult.Outside;
                }
                if (_planes[i].Dot(negative) < 0)
                {
                    result = FrustumTestResult.Intersecting;
                }
            }
        }
        return result;
    }
}