关于“Shader wants tangents, but the mesh doesn't have them”的解决

在我用UV代码绘制的物体替换shader时，报出“Shader wants tangents, but the mesh doesn't have them”的警告。

顾名思义，材质需要切线，但是网格中并不存在。

在国外论坛找到如下解决的方法：

private static void TangentSolver(Mesh theMesh)
{
int vertexCount = theMesh.vertexCount;
Vector3[] vertices = theMesh.vertices;
Vector3[] normals = theMesh.normals;
Vector2[] texcoords = theMesh.uv;
int[] triangles = theMesh.triangles;
int triangleCount = triangles.Length / 3;
Vector4[] tangents = new Vector4[vertexCount];
Vector3[] tan1 = new Vector3[vertexCount];
Vector3[] tan2 = new Vector3[vertexCount];
int tri = 0;
for (int i = 0; i < (triangleCount); i++)
{
int i1 = triangles[tri];
int i2 = triangles[tri + 1];
int i3 = triangles[tri + 2];

Vector3 v1 = vertices[i1];
Vector3 v2 = vertices[i2];
Vector3 v3 = vertices[i3];

Vector2 w1 = texcoords[i1];
Vector2 w2 = texcoords[i2];
Vector2 w3 = texcoords[i3];

float x1 = v2.x - v1.x;
float x2 = v3.x - v1.x;
float y1 = v2.y - v1.y;
float y2 = v3.y - v1.y;
float z1 = v2.z - v1.z;
float z2 = v3.z - v1.z;

float s1 = w2.x - w1.x;
float s2 = w3.x - w1.x;
float t1 = w2.y - w1.y;
float t2 = w3.y - w1.y;

float r = 1.0f / (s1 * t2 - s2 * t1);
Vector3 sdir = new Vector3((t2 * x1 - t1 * x2) * r, (t2 * y1 - t1 * y2) * r, (t2 * z1 - t1 * z2) * r);
Vector3 tdir = new Vector3((s1 * x2 - s2 * x1) * r, (s1 * y2 - s2 * y1) * r, (s1 * z2 - s2 * z1) * r);

tan1[i1] += sdir;
tan1[i2] += sdir;
tan1[i3] += sdir;

tan2[i1] += tdir;
tan2[i2] += tdir;
tan2[i3] += tdir;

tri += 3;
}

for (int i = 0; i < (vertexCount); i++)
{
Vector3 n = normals[i];
Vector3 t = tan1[i];

// Gram-Schmidt orthogonalize
Vector3.OrthoNormalize(ref n, ref t);

tangents[i].x = t.x;
tangents[i].y = t.y;
tangents[i].z = t.z;

// Calculate handedness
tangents[i].w = (Vector3.Dot(Vector3.Cross(n, t), tan2[i]) < 0.0) ? -1.0f : 1.0f;
}
theMesh.tangents = tangents;
}

目前还没有好好分析这段代码，但是效果很好。