vertex normal

Basis:
    Cross Product
        Define two input vectors, Vector1 (x1, y1, z1) and Vector2 (x2, y2, z2), and one output vector, OutVect(ox, oy, oz). The components of OutVect are calculated as follows:
        ox = (y1 * z2) - (y2 * z1)
        oy = (z1 * x2) - (z2 * x1)
        oz = (x1 * y2) - (x2 * y1)

    Dot Product
        Define two vectors. Vector1 (x1, y1, z1) and Vector2 (x2, y2, z2).
        DotProduct = (x1*x2 + y1*y2 + z1*z2)

surface normal:
    three vertices V1, V2 and V3, ordered in counterclockwise order, obtain the direction of the normal by computing
    (V2 - V1) x (V3 - V1), where x is the cross product of the two vectors

    U = (V2 - V1)
    V = (V3 - V1)
    surfaceNormal.x = U.y * V.z - U.z * V.y
    surfaceNormal.y = U.z * V.x - U.x * V.z
    surfaceNormal.z = U.x * V.y - U.y * V.x

vertex normal:
    Algorithm1:
    Use cross-products to calculate the face normals for the triangles surrounding a given vertex, add them together, and normalize.
        1. Read the faces in your model file
        2. Calculate the face normal
        3. Add it to the normal of each vertex making up that face
        4. After that's done, normalize all the vertex normals

    Algorithm2:
        Sum the face normals (component wise) for avery triangle containing the vertex
        Devide the summed normal (component wise) by the number of triangles containing the vertex to get the normal for the vertex
        Normalize all the vertex normals


reference:
    google "Face and Vertex Normal Vectors mdsn"
    https://msdn.microsoft.com/en-us/library/windows/desktop/bb324491%28v=vs.85%29.aspx

    google "vertex normal wiki"
    https://en.wikipedia.org/wiki/Vertex_normal

    google "A Comparison of Algorithms for Vertex Normal Computation"
    A Comparison of Algorithms for Vertex Normal Computation.pdf