c++ - 立方体的法线似乎指向内部

Question

我编写了以下函数来绘制立方体:

void drawCube() {

   Point vertices[8] = { Point(-1.0, -1.0, -1.0), Point(-1.0, -1.0, 1.0), Point(1.0, -1.0, 1.0), Point(1.0, -1.0, -1.0),
                         Point(-1.0, 1.0, -1.0), Point(-1.0, 1.0, 1.0), Point(1.0, 1.0, 1.0), Point(1.0, 1.0, -1.0) };

   int faces[6][4] = {{0, 1, 2, 3}, {0, 3, 7, 4}, {0, 1, 5, 4}, {1, 2, 6, 5}, {3, 2, 6, 7}, {4, 5, 6, 7}};

   glBegin(GL_QUADS);

   for (unsigned int face = 0; face < 6; face++) {

     Vector v = vertices[faces[face][1]] - vertices[faces[face][0]];
     Vector w = vertices[faces[face][2]] - vertices[faces[face][0]];

     Vector normal = v.cross(w).normalised();

     glNormal3f(normal.dx, normal.dy, normal.dz);

     for (unsigned int vertex = 0; vertex < 4; vertex++) {

       switch (vertex) {
         case 0: glTexCoord2f(0, 0); break;
         case 1: glTexCoord2f(1, 0); break;
         case 2: glTexCoord2f(1, 1); break;
         case 3: glTexCoord2f(0, 1); break;
       }

       glVertex3f(vertices[faces[face][vertex]].x, vertices[faces[face][vertex]].y, vertices[faces[face][vertex]].z);
     }
   }

   glEnd();
}

当用光照在立方体上进行渲染时，看起来当我旋转立方体时，正确的阴影过渡只发生在大约一半的面上。其余的只是保持非常暗的阴影，就好像我已经删除了正常的计算。

然后我决定移除几个面以查看立方体内部。那些在外面没有正确反射光线的面，在里面却能正确地反射光线。我如何确保每个面的法线从该面指向外，而不是指向立方体的中心？

Answer 1

要反转法线的方向，交换您用于叉积的顺序:

Vector normal = w.cross(v).normalised();