一、演算法

畫線可以簡化為正八棱柱

演算法為：

兩點A，B 向量AB 求向量AB的任意垂直向量，旋轉垂直向量獲得正半邊形的頂點，通過頂點繪制三角形網格

二、實作

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

// 畫線
public class MeshLine 
{
    private Material material; // 材質
    private RepeatSegmentMesh segment_mesh;
    private RenderObject render_object;
    private MeshEnableFlag flag;

    public MeshLine()
    {
        this.material = GlobalAppData.Instance.Data.line_material;
        render_object = RenderObject.Create(null, null, Vector3.zero, Quaternion.identity);
        render_object.mesh_render.material = material;
        render_object.mesh_render.castShadows = true;
        render_object.mesh_render.receiveShadows = true;
        render_object.go.name = "MeshLine";
        
        //Light light = render_object.mesh_filter.GetComponent<Lighting>();
        flag = MeshEnableFlag.Vertice | MeshEnableFlag.Normal | MeshEnableFlag.Triangle | MeshEnableFlag.Color | MeshEnableFlag.UV;
        segment_mesh = new RepeatSegmentMesh(3,3, 100, flag);

        segment_mesh.SegmentData.MarkTriangleTopology();       
    }
    
    public static Vector3 GetVerticalDir(Vector3 _dir) // 計算垂直向量
    {
        if (_dir.z == 0)
        {
            return new Vector3(0, 0, -1);
        }
        else
        {
            return new Vector3(-_dir.z / _dir.x, 0, 1).normalized;
        }
    }

    // 增加線段
    public void AddLine(Vector3 start, Vector3 end, Color color , float radius = 0.1f)
    {
        int num = 8;
        int angle = 360 / num;
        Vector3 dir = (end - start).normalized; // 取得向量AB
        var vertical = GetVerticalDir(dir).normalized; // 垂直于AB的向量v
        
        for (int i = 1; i <= 8; i++)
        {
             var r = Quaternion.AngleAxis(i * angle, dir) * vertical; // 繞AB旋轉angle度
             var r1 = Quaternion.AngleAxis((i + 1) * angle, dir) * vertical; // angle * i度

             var p1 = (r * radius) + start;  // 開始正八邊形頂點1
             var p2 = (r1 * radius) + start; // 開始正八邊形頂點2
             var p3 = (r * radius) + end;    // 結束正八邊形頂點1
             var p4 = (r1 * radius) + end;   // 結束正八邊形頂點2
             segment_mesh.SegmentData.SetTriangleVertice(0, start, p2, p1); // 底部三角形
             segment_mesh.SegmentData.SetColor(color);
             segment_mesh.AddSegmentData(flag);
             
             segment_mesh.SegmentData.SetTriangleVertice(0, end, p3, p4); // 頂部三角形
             segment_mesh.SegmentData.SetColor(color);
             segment_mesh.AddSegmentData(flag);
             
             segment_mesh.SegmentData.SetTriangleVertice(0, p1, p2, p4); // 柱狀三角形1
             segment_mesh.SegmentData.SetColor(color);
             segment_mesh.AddSegmentData(flag);

             segment_mesh.SegmentData.SetTriangleVertice(0, p3, p1, p4); // 柱狀三角形2
             segment_mesh.SegmentData.SetColor(color);
             segment_mesh.AddSegmentData(flag);

        }
        
        segment_mesh.CopyToMesh(render_object.mesh_filter.mesh, true);
        render_object.mesh_filter.mesh.RecalculateNormals();
    }

    // 增加多段線
    public void AddLine(List<Vector3> points, Color color , float radius = 0.1f)
    {
        for (int i = 1; i < points.Count; i++)
        {
            AddLine(points[i -1], points[i], color, radius);
        }
    }

    // 清除
    public void Clear()
    {
        segment_mesh.SetSegmentCount(0);
        segment_mesh.CopyToMesh(render_object.mesh_filter.mesh, true);
        render_object.mesh_filter.mesh.RecalculateNormals();   
    }
}

使用：

        MeshLine mesh_line = new MeshLine();
        mesh_line.AddLine(new Vector3(2,2,0), new Vector3(10,10,10), Color.yellow, 0.1f);
        
        List<Vector3> list = new List<Vector3>();
        list.Add(new Vector3(1, 1, 1));
        list.Add(new Vector3(10, 1, 1));
        list.Add(new Vector3(1, 1, 10));
        list.Add(new Vector3(1, 10, 1));
        mesh_line.AddLine(list, Color.yellow);

三、效果