import bpy
import bmesh
import mathutils
from bpy_extras import view3d_utils

def get_3d_viewport_area():
    for area in bpy.context.screen.areas:
        if area.type == 'VIEW_3D':
            for region in area.regions:
                if region.type == 'WINDOW':
                    return area, region
    return None, None

def get_view_direction(region_3d):
    return -region_3d.view_rotation @ mathutils.Vector((0.0, 0.0, -1.0))

def face_visible_on_screen(region, rv3d, face, obj):
    """Returns True if any vertex of the face is inside the viewport."""
    for vert in face.verts:
        world_co = obj.matrix_world @ vert.co
        screen_pos = view3d_utils.location_3d_to_region_2d(region, rv3d, world_co)
        if screen_pos:
            x, y = screen_pos
            if 0 <= x <= region.width and 0 <= y <= region.height:
                return True
    return False

def select_offscreen_or_backfacing_inside():
    area, region = get_3d_viewport_area()
    if not area or not region:
        print("3D Viewport not found.")
        return

    space = next((s for s in area.spaces if s.type == 'VIEW_3D'), None)
    if not space:
        print("VIEW_3D space not found.")
        return

    rv3d = space.region_3d
    view_dir = get_view_direction(rv3d)

    obj = bpy.context.object
    if obj is None or obj.type != 'MESH':
        print("No mesh object selected.")
        return

    bpy.ops.object.mode_set(mode='EDIT')
    bm = bmesh.from_edit_mesh(obj.data)
    bm.faces.ensure_lookup_table()

    for f in bm.faces:
        f.select = False

    for face in bm.faces:
        normal_world = obj.matrix_world.to_3x3() @ face.normal
        dot = normal_world.normalized().dot(view_dir)

        is_backfacing = dot > 0.2
        is_onscreen = face_visible_on_screen(region, rv3d, face, obj)
        is_offscreen = not is_onscreen

        # ✅ Final logic
        if is_onscreen and not is_backfacing:
            face.select = True

    bmesh.update_edit_mesh(obj.data)
    print("Selected: offscreen OR backfacing onscreen faces.")

select_offscreen_or_backfacing_inside()