I write simple software renderer. In my pipeline i have stage of backface culling. But looks like it has some error (see picture).

I perform culling right after world transformation (is it correct?).

(i can't insert picture in post coz i don't have enough points, so i just upload it (cube model): http://imageshack.us/photo/my-images/705/bcerror.png/)

Vector3F view_dir(0.0f, 0.0f, 1.0f);

std::vector<Triangle> to_remove;

for (Triangle &t : m_triangles)
{
    Vector4F e1 = t.v2 - t.v1;
    Vector4F e2 = t.v3 - t.v1;
    Vector3F normal(
        e1.y * e2.z - e1.z * e2.y,
        e1.z * e2.x - e1.x * e2.z,
        e1.x * e2.y - e1.y * e2.x
        );
    normal.Normalize();

    float dot = Dot(view_dir, normal);
    if (dot <= 0)
        to_remove.push_back(t);
}

for (Triangle& t : to_remove)
    m_triangles.erase(std::remove(m_triangles.begin(), m_triangles.end(), t), m_triangles.end());

Camera sits in origin and points in screen (RH).

What is the reason?

For better explanation i upload picture with cube rotation screenshots: http://imageshack.us/photo/my-images/842/bcmove.png/

UPDATED: The error occurs only when triangle has non-zero offset from origin

UPDATED 2: If i process backface culling in clip space (after transforming all vertices with view and projection matrix), and just check z coordinate of triangle normal - it works perfect... Can i perform culing RIGHT BEFORE view/proj transforms? In this case looks like culling will not depends of projection and it's not right?..

UPDATED 3: I found answer and will post it in two hours - again coz of reputation lack.