select front facing faces in blender

select_front_facing.py

def select_front_facing(context):
    """
    from: http://freespace.virgin.net/hugo.elias/routines/r_dot.htm

    When deciding if a polygon is facing the camera, you need 
    only calculate the dot product of the normal vector of     
    that polygon, with a vector from the camera to one of the 
    polygon's vertices. 
    
    - If the dot product is less than zero, the polygon is facing the camera. 
    - If the value is greater than zero, it is facing away from the camera.
    """
    
    region = context.region  
    rv3d = context.space_data.region_3d  
    obj = context.active_object
    vertlist = obj.data.vertices

    # [ ] be in object mode
    
    # neat eye location code with the help of paleajed
    eye = Vector(rv3d.view_matrix[2][:3])
    eye.length = rv3d.view_distance
    eye_location = rv3d.view_location + eye  

    face_list = []
    for idx, polygon in enumerate(obj.data.polygons):
        
        vert_index = polygon.vertices[0]
        pnormal = obj.matrix_world * polygon.normal
        world_coordinate = obj.matrix_world * vertlist[vert_index].co
        
        result_vector = eye_location-world_coordinate
        dot_value = pnormal.dot(result_vector.normalized())            

        if dot_value < 0.0:
            polygon.select = False
        else:
            polygon.select = True
            face_list.append(idx)
     
    return face_list