【Unity】为网格生成顶点法线（Mesh.RecalculateNormals计算异常的解决方案）-Toy模板网

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背景

我们通过代码动态创建的网格，因为没有法线，不会接收到光照。

正常情况下调用Mesh.RecalculateNormals方法，重新生成法线即可。

但特定情况下通过此方法计算出的顶点发现都是（0, 0，0），这种情况下只能手动生成法线了

如下图，左边物体有正确的法线，可以接收光照信息，右侧物体无法线，无法接收光照。
unity顶点法线,Unity,unity,游戏引擎

RecalculateNormals计算异常的原因

经测试发现导致Mesh.RecalculateNormals计算异常的情况：
如果Mesh中的某个顶点，在三角形标号数组中，即画了正面的网格，又画了反面的网格，则会导致RecalculateNormals计算错误，该点法线计算结果为（0, 0，0）。

解决方法1

对于双面网格，将顶点数组增加一倍，用于绘制反面的网格，分别放入Mesh的三角形标号数组中，以保证每个点法线计算正确。

解决方法2

用下方的生成顶点法线算法，手动计算期望的法线数组。

原理

法线：法线是指始终垂直于某平面的直线。在几何学中，法线指平面上垂直于曲线在某点的切线的一条线。

网格的法线数组：法线数组存放Mesh每个顶点的法线，数组大小与顶点坐标对应，normals[i]对应顶点vertices[i]的法线。

在 Max Wagner 的《Generating Vertex Normals》文章中写到——严格意义上讲，点是没有法线的。点的法线是在使用Phone或Gouraud模型时计算光照使用。如果一个面上的所有法线都一样，他们的光照也就一样，就会产生 flatness 效果；而如果把每个顶点的法向设置不同，则更平滑。

我们通过代码创建的网格一般都是多边形，所以给每个顶点赋值他所在的平面的法线即可。

生成顶点法线算法

unity顶点法线,Unity,unity,游戏引擎
创建网格，并设定如上图的八个点位，三角形序列，uv

 		Mesh mesh = new Mesh();
        Vector3[] vertices = new Vector3[8];
        vertices[0] = new Vector3(0,0,0);
        vertices[1] = new Vector3(0,1,0);
        vertices[2] = new Vector3(1,0,0);
        vertices[3] = new Vector3(1,1,0);
        
        vertices[4] = new Vector3(1,0,0);
        vertices[5] = new Vector3(1,1,0);
        vertices[6] = new Vector3(2,0,1);
        vertices[7] = new Vector3(2,1,1);
        mesh.vertices = vertices;
        
        int[] triangles = new int[12];
        triangles[0] = 0;
        triangles[1] = 1;
        triangles[2] = 3;
        
        triangles[3] = 2;
        triangles[4] = 0;
        triangles[5] = 3;
        
        triangles[6] = 4;
        triangles[7] = 5;
        triangles[8] = 7;

        triangles[9] = 6;
        triangles[10] = 4;
        triangles[11] = 7;
        
        mesh.triangles = triangles;

        Vector2[] uv = new Vector2[8];
        uv[0] = new Vector2(0,0);
        uv[1] = new Vector2(0,1);
        uv[2] = new Vector2(1,0);
        uv[3] = new Vector2(1,1);
        
        uv[4] = new Vector2(0,0);
        uv[5] = new Vector2(0,1);
        uv[6] = new Vector2(1,0);
        uv[7] = new Vector2(1,1);
        mesh.uv = uv;

生成法线数组

 Vector3[] normals = new Vector3[8];
        Vector3 v1 = Vector3.Cross(vertices[0] - vertices[1], vertices[0] - vertices[3]);
        normals[0] = v1;
        normals[1] = v1;
        normals[2] = v1;
        normals[3] = v1;

        Vector3 v2 = Vector3.Cross(vertices[4] - vertices[5], vertices[4] - vertices[7]);
        normals[4] = v2;
        normals[5] = v2;
        normals[6] = v2;
        normals[7] = v2;
        mesh.normals = normals;
        //mesh.RecalculateNormals();

最终效果

unity顶点法线,Unity,unity,游戏引擎

（此案例中，使用Mesh.RecalculateNormal,也可生成正确法线。）文章来源地址https://www.toymoban.com/news/detail-639941.html

完整代码，增加了点位，注释部分为增加了背部网格

   public Mesh CreatMeshTest()
    {
        Mesh mesh = new Mesh();
        Vector3[] vertices = new Vector3[12];
        vertices[0] = new Vector3(0,0,0);
        vertices[1] = new Vector3(0,1,0);
        vertices[2] = new Vector3(1,0,0);
        vertices[3] = new Vector3(1,1,0);
        
        vertices[4] = new Vector3(1,0,0);
        vertices[5] = new Vector3(1,1,0);
        vertices[6] = new Vector3(0.5f,0,1);
        vertices[7] = new Vector3(0.5f,1,1);
        
        vertices[8] = new Vector3(0.5f,0,1);
        vertices[9] = new Vector3(0.5f,1,1);
        vertices[10] = new Vector3(0,0,0);
        vertices[11] = new Vector3(0,1,0);
        mesh.vertices = vertices;
        
        int[] triangles = new int[18];
        triangles[0] = 0;
        triangles[1] = 1;
        triangles[2] = 3;
        
        triangles[3] = 2;
        triangles[4] = 0;
        triangles[5] = 3;
        
        triangles[6] = 4;
        triangles[7] = 5;
        triangles[8] = 7;

        triangles[9] = 6;
        triangles[10] = 4;
        triangles[11] = 7;
        
        triangles[12] = 8;
        triangles[13] = 9;
        triangles[14] = 11;

        triangles[15] = 10;
        triangles[16] = 8;
        triangles[17] = 11;
        
        // triangles[18] = 0;
        // triangles[19] = 3;
        // triangles[20] = 1;
        //
        // triangles[21] = 2;
        // triangles[22] = 3;
        // triangles[23] = 0;
        //
        // triangles[24] = 4;
        // triangles[25] = 7;
        // triangles[26] = 5;
        //
        // triangles[27] = 6;
        // triangles[28] = 7;
        // triangles[29] = 4;
        //
        // triangles[30] = 8;
        // triangles[31] = 11;
        // triangles[32] = 9;
        //
        // triangles[33] = 10;
        // triangles[34] = 11;
        // triangles[35] = 8;
        
        mesh.triangles = triangles;

        Vector2[] uv = new Vector2[12];
        uv[0] = new Vector2(0,0);
        uv[1] = new Vector2(0,1);
        uv[2] = new Vector2(1,0);
        uv[3] = new Vector2(1,1);
        
        uv[4] = new Vector2(0,0);
        uv[5] = new Vector2(0,1);
        uv[6] = new Vector2(1,0);
        uv[7] = new Vector2(1,1);
        
        uv[8] = new Vector2(0,0);
        uv[9] = new Vector2(0,1);
        uv[10] = new Vector2(1,0);
        uv[11] = new Vector2(1,1);
        mesh.uv = uv;
        
        Vector3[] normals = new Vector3[12];
        Vector3 v1 = Vector3.Cross(vertices[0] - vertices[1], vertices[0] - vertices[3]);
        normals[0] = v1;
        normals[1] = v1;
        normals[2] = v1;
        normals[3] = v1;

        Vector3 v2 = Vector3.Cross(vertices[4] - vertices[5], vertices[4] - vertices[7]);
        normals[4] = v2;
        normals[5] = v2;
        normals[6] = v2;
        normals[7] = v2;
        
        Vector3 v3 = Vector3.Cross(vertices[8] - vertices[9], vertices[8] - vertices[11]);
        normals[8] = v3;
        normals[9] = v3;
        normals[10] = v3;
        normals[11] = v3;
        mesh.normals = normals;
        //mesh.RecalculateNormals();
        return mesh;

    }