How to find back-faces and delete them with python?

Question

Is there a python code to find and delete back-faces? As a way to do "back-face culling".

Answer 1

If you know a little bit of python you can modify and expand upon this snippet:

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.

this could be further optimized, but I think that obfuscates the concepts. Run this code from 3dview (a 3d view context) either through a camera or user/perspective view - with the object you want to work on selected and in Object mode.

from mathutils import Vector 

def get_locals(context):
    region = context.region  
    rv3d = context.space_data.region_3d  
    obj = context.active_object
    vertlist = obj.data.vertices
    return region, rv3d, obj, vertlist

and

def select_front_facing(context):

    region, rv3d, obj, vertlist = get_locals(context)

    # 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  

    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

Here is a short working demo, you can run this from text editor: https://gist.github.com/zeffii/5766177

I use it in my basic SVG renderer, to fake a wireframe material which occludes back-facing, it isn't perfect for all possible use-cases but it works to 'select back-facing'. You can run the SVG renderer directly from the Text Editor, and it will set every front facing Polygon to the select == True state.

Answer 2

Have you looked at this Blender Artists thread? (Post #9) They changed the object's material to a node material. In the nodes, they mapped the back faces of the object to alpha zero using a geometry node.

Summary: you can make the back faces transparent to simulate back-face culling. I'm not sure how much faster it will be, but it will be nicer than deleting portions of your geometry.

Answer 3

And then, for the adventurous amongst you, I made a patch for Blender source code that allows getting occlusion information for BMesh elements.

Syntax:

state = bmelem.occlusion_get(rv3d)

rv3d being a RegionView3D

Get the patch here.