Select Front Facing Polygons

This snippet deals with selecting front facing polygons, thanks to paleajed for the hint on the eye code, and vrav for the suggestion to multiply obj_matrix with the face normal (turns out my demo object suzanne gets rotated around the x axis after insertion into the scene)

	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

view raw select_front_facing.py hosted with ❤ by GitHub

Paper on Visibility in Computer Graphics by Jirˇ´ı Bittner and Peter Wonka . This paper looks promising and might pave the way to get a decent faux3d svg render of 3d geometry.