Question

My issue is regarding OpenGL, and Normals, I understand the math behind them, and I am having some success.

The function I’ve attached below accepts an interleaved Vertex Array, and calculates the normals for every 4 vertices. These represent QUADS that having the same directions. By my understanding these 4 vertices should share the same Normal. So long as they face the same way.

The problem I am having is that my QUADS are rendering with a diagonal gradient, much like this: Light Effect – Except that the shadow is in the middle, with the light in the corners.

I draw my QUADS in a consistent fashion. TopLeft, TopRight, BottomRight, BottomLeft, and the vertices I use to calculate my normals are TopRight – TopLeft, and BottomRight – TopLeft.

Hopefully someone can see something I’ve made a blunder on, but I have been at this for hours to no prevail.

For the record I render a Cube, and a Teapot next to my objects to check my lighting is functioning, so I’m fairly sure there is no issue regarding Light position.

void CalculateNormals(point8 toCalc[], int toCalcLength)
{
    GLfloat N[3], U[3], V[3];//N will be our final calculated normal, U and V will be the subjects of cross-product
    float length;

for (int i = 0; i < toCalcLength; i+=4) //Starting with every first corner QUAD vertice
{
    U[0] = toCalc[i+1][5] - toCalc[i][5]; U[1] = toCalc[i+1][6] - toCalc[i][6]; U[2] = toCalc[i+1][7] - toCalc[i][7]; //Calculate Ux Uy Uz 
    V[0] = toCalc[i+3][5] - toCalc[i][5]; V[1] = toCalc[i+3][6] - toCalc[i][6]; V[2] = toCalc[i+3][7] - toCalc[i][7]; //Calculate Vx Vy Vz

    N[0] = (U[1]*V[2]) - (U[2] * V[1]);
    N[1] = (U[2]*V[0]) - (U[0] * V[2]);
    N[2] = (U[0]*V[1]) - (U[1] * V[0]);

    //Calculate length for normalising
    length = (float)sqrt((pow(N[0],2)) + (pow(N[1],2)) + (pow(N[2],2)));

    for (int a = 0; a < 3; a++)
    {
        N[a]/=length;
    }

    for (int j = 0; i < 4; i++)
    {
                    //Apply normals to QUAD vertices (3,4,5 index position of normals in interleaved array)
        toCalc[i+j][3] = N[0]; toCalc[i+j][4] = N[1]; toCalc[i+j][5] = N[2];
    }
}
}

Answer 1

It seems like you are taking the vertex position values for use in calculations from indices 5, 6, and 7, and then writing out the normals at indices 3, 4, and 5. Note how index 5 is used on both. I suppose one of them is not correct.