SparkClient/Views/UserControl/ViewportData/Helper/ObjExporter.cs

using System.IO;
using System.Text;
using HelixToolkit.SharpDX.Core;
using SharpDX;
using SparkClient.Views.UserControl.ViewportData.Entity;
using SparkClient.Views.UserControl.ViewportData.Enum;

namespace SparkClient.Views.UserControl.ViewportData.Helper;

public class ObjExporter
{
    public static void ExportToObj2(List<Viewport3DTriangleEntity> entities, string outputPath)
{
    // 分组：按面代码 PlaneCode 分组
    var faceGroups = entities
        .GroupBy(e => e.PlaneCode)
        .ToDictionary(g => g.Key, g => g.ToList());

    // 存储每个面的顶点和法线
    var faceData = new Dictionary<string, FaceData>();

    // Step 1: 处理每个面，生成顶点列表并计算法线
    foreach (var group in faceGroups)
    {
        // 合并所有顶点并去重
        var vertices = group.Value
            .SelectMany(e => new[] { e.Point1, e.Point2, e.Point3 })
            .Distinct()
            .ToList();

        // 按凸包算法排序顶点（避免交叉）
        var sortedVertices = ConvexHullSort(vertices);

        // 计算面的法线（基于排序后的顶点）
        var normal = CalculateFaceNormal(sortedVertices, ViewportManager.CenterVector);

        // 存储面数据
        faceData[group.Key] = new FaceData
        {
            Vertices = sortedVertices,
            Normal = normal
        };
    }

    // Step 2: 写入 OBJ 文件
    using (var writer = new StreamWriter(outputPath))
    {
        // 写入顶点
        var vertexIndexMap = new Dictionary<Vector3, int>();
        int index = 1;
        foreach (var face in faceData.Values)
        {
            foreach (var vertex in face.Vertices)
            {
                if (!vertexIndexMap.ContainsKey(vertex))
                {
                    vertexIndexMap[vertex] = index++;
                    writer.WriteLine($"v {vertex.X} {vertex.Y} {vertex.Z}");
                }
            }
        }

        // 写入法线（每个面一个法线）
        var normalIndexMap = new Dictionary<Vector3, int>();
        index = 1;
        foreach (var face in faceData.Values)
        {
            if (!normalIndexMap.ContainsKey(face.Normal))
            {
                normalIndexMap[face.Normal] = index++;
                // writer.WriteLine($"vn {face.Normal.X} {face.Normal.Y} {face.Normal.Z}");
            }
        }

        // 写入面
        foreach (var faceEntry in faceData)
        {
            var face = faceEntry.Value;
            var normalIndex = normalIndexMap[face.Normal];

            // 正面（顺时针）
            writer.Write("f ");
            foreach (var vertex in face.Vertices)
            {
                writer.Write($"{vertexIndexMap[vertex]}//{normalIndex} ");
            }
            writer.WriteLine();

            // 反面（逆时针）
            writer.Write("f ");
            foreach (var vertex in face.Vertices.AsEnumerable().Reverse())
            {
                writer.Write($"{vertexIndexMap[vertex]}//{normalIndex} ");
            }
            writer.WriteLine();
        }
    }
}

    /// <summary>
    /// 凸包排序（Andrew's Monotone Chain 算法）
    /// </summary>
    private static List<Vector3> ConvexHullSort(List<Vector3> points)
    {
        if (points.Count <= 3) return points;

        // 按 X 坐标排序，若相同则按 Y 排序
        var sorted = points.OrderBy(p => p.X).ThenBy(p => p.Y).ToList();

        // 构建下凸包和上凸包
        var lower = new List<Vector3>();
        foreach (var p in sorted)
        {
            while (lower.Count >= 2 && Cross(lower[lower.Count - 2], lower[lower.Count - 1], p) <= 0)
                lower.RemoveAt(lower.Count - 1);
            lower.Add(p);
        }

        var upper = new List<Vector3>();
        foreach (var p in sorted.AsEnumerable().Reverse())
        {
            while (upper.Count >= 2 && Cross(upper[upper.Count - 2], upper[upper.Count - 1], p) <= 0)
                upper.RemoveAt(upper.Count - 1);
            upper.Add(p);
        }

        // 合并凸包（移除重复点）
        lower.RemoveAt(lower.Count - 1);
        upper.RemoveAt(upper.Count - 1);
        return lower.Concat(upper).ToList();
    }

    /// <summary>
    /// 计算叉积 (p2-p0) × (p1-p0)
    /// </summary>
    private static float Cross(Vector3 p0, Vector3 p1, Vector3 p2)
    {
        return (p1.X - p0.X) * (p2.Z - p0.Z) - (p1.Z - p0.Z) * (p2.X - p0.X);
    }
    /// <summary>
    /// 计算面的法线（基于顶点顺序）
    /// </summary>
    private static Vector3 CalculateFaceNormal(List<Vector3> vertices, Vector3 modelCenter)
    {
        // 使用前三个顶点计算初始法线
        var p0 = vertices[0];
        var p1 = vertices[1];
        var p2 = vertices[2];
        var edge1 = p1 - p0;
        var edge2 = p2 - p0;
        var normal = Vector3.Cross(edge1, edge2).Normalized();

        // 确保法线朝外（面中心到模型中心的向量与法线方向一致）
        var faceCenter = GetCenter(vertices);
        var toModelCenter = modelCenter - faceCenter;
        if (Vector3.Dot(normal, toModelCenter) > 0)
        {
            normal = -normal;
        }

        return normal;
    }

    /// <summary>
    /// 计算顶点列表的中心点
    /// </summary>
    private static Vector3 GetCenter(List<Vector3> vertices)
    {
        var center = Vector3.Zero;
        foreach (var v in vertices) center += v;
        return center / vertices.Count;
    }
}

class FaceData
{
    public List<Vector3> Vertices { get; set; }
    public Vector3 Normal { get; set; }
}