compute the face normal and the viewer''s direction , do the dot product and see if it is >90 actually check only for the numerator of the formula to be >0 ( this is a backfaceculling ) when you are done, check the vector''s direction if one is ''against'' another , you are facing that surface, do it for every cube''s surface , but you may also want to take a look at quaternions to check the direction is a little bit tricky you could divide the space in octants and retreive the spherical coordinates giving only the vector direction, ..good work..

Unfortunately not because i have never had the need to do soemthing like that but computing a normal and doing a dot product is quite straighforward, the tricky part is calculating the vector directions, try to search for polar coordinates or spherical

try thi code

void GetAngles( fVect V,float *fPan, float *fPitch )
{
float dx, dy, dz;
dx = (float)V[0];
dy = (float)V[1];
dz = (float)V[2];
if(dz > 0.0001f)
{
*fPan = (float) -atan(dx/dz);
dz /= cos((float)(*fPan));
*fPitch = (float) atan(dy/dz);
} else if(dz < -EPSILON)
{
*fPan = (float) (__PI-atan(dx/dz));
dz /= cos((float)(*fPan)-__PI);
*fPitch = (float) -atan(dy/dz);
} else
{
*fPan = 0.0f;
*fPitch = (float)-__PI/2.0f;
}
}