N-Gons: Pros, Cons And Recommendations For Different Workflows
Definition of N-Gons
N-gons are polygons with more than 4 vertices and edges. In 3D modeling, n-gons refer to faces on a mesh that have more than 4 vertices. Some common examples of n-gons are pentagons with 5 vertices, hexagons with 6 vertices, and octagons with 8 vertices.
N-gons provide flexibility in 3D modeling by allowing more than 4 vertices to define a single face. This enables models to have fewer overall faces in areas that do not need dense geometry with quads or triangles. The reduced face count can help optimize performance when rendering or simulate complex shapes.
Benefits of N-Gons
Flexibility in Modeling Complex Shapes
N-gons allow modelers to use any number of edges to build a face that fits a particular surface. For example, it may be easier to fit a curvy detail of a model with a single n-gon versus many quads or triangles. This gives more flexibility to reduce edge flow complexity.
Areas like cylinders, spheres, cones, and other rounded forms can utilize n-gons to greater effect than strict quad topology. This flexibility can speed up initial concept modeling, prototyping, and fleshing out basic forms without getting bogged down in all-quad topology.
Fewer Faces Can Improve Performance
Since n-gons have the ability to condense face density, they can help optimize scenes and improve performance. Areas of gentle curvature are good candidates for n-gons since fewer large faces reduce overall face counts. Hard surface models also benefit from n-gons to reduce repetitive edges in large flat areas.
Rendering engines can process n-gon meshes faster due to reduced draws calls for individual faces. Similarly, simulation is sped up with fewer polygon collisions to calculate physics. So n-gons introduce optimization potential in technical applications.
When to Avoid N-Gons
N-Gons and Subdivision Surfaces
While n-gons provide some modeling advantages, they can cause issues with subdivision modifier workflows. Subdivision relies on evenly spaced quads to smoothly add detail through edge loops.
Example Code Showing Distorted Subdivision with N-Gons
#Cube with default subdivision
mesh = bpy.ops.mesh.primitive_cube_add()
#Apply subsurf modifier at level 2
mod = mesh.modifiers.new("Subsurf", type='SUBSURF')
mod.levels = 2
bpy.ops.object.modifier_apply(modifier="Subsurf")
#Insert n-gon face on cube
bpy.ops.mesh.f2()
bpy.ops.mesh.edge_face_add()
#Apply subsurf again at level 2
mod = mesh.modifiers.new("Subsurf", type='SUBSURF')
mod.levels = 2
bpy.ops.object.modifier_apply(modifier="Subsurf")
#N-gon face causes uneven edge loops and shading artifacts
The above Python script demonstrates how adding an n-gon to a cube results in uneven geometry after subdivision. This causes shading artifacts due to uneven spacing.
N-Gons in Animation
During animations involving deformations, n-gon faces can collapse, fold unpredictably, or otherwise deform in unnatural ways. Quad topology with clean edge flow avoids such artifacts.
Example Code Demonstrating Deformations with N-Gon Faces
#Create default cube
mesh = bpy.ops.mesh.primitive_cube_add()
#Insert n-gon face
bpy.ops.mesh.f2()
bpy.ops.mesh.edge_face_add()
#Add armature modifier
arm_mod = mesh.modifiers.new("Armature", type='ARMATURE')
#Create bone at cube origin
arm = bpy.data.armatures.new("Armature")
obj = bpy.data.objects.new("Armature", arm)
bpy.context.collection.objects.link(obj)
#Create bone
b = arm.edit_bones.new("Bone")
b.head = 0,0,0
b.tail = 1,0,0
#Bind armature
mod = mesh.modifiers.new("Armature","ARMATURE")
mod.object = obj
#Bend bone 45 degrees
obj.pose.bones["Bone"].rotation_mode = 'XYZ'
obj.pose.bones["Bone"].rotation_euler.z = 0.125 * pi
#N-gon face collapses unevenly
Animating an armature modifier on the n-gon cube shows uneven collapsing due to a lack of surrounding topology and edge flow. Quads deform more evenly.
Best Practices for N-Gon Use
When used judiciously, n-gons can simplify meshes without compromising quality or flexibility. Consider these best practices when adding n-gons.
Ideal N-Gon Configurations
Convex and Planar Faces
N-gons tend to behave better when they have convex rather than concave geometry. Convex shapes avoid uneven collapsing during deformations. Simple planar n-gons also work well for flat panel type surfaces.
import bpy
import bmesh
#Create n-gon cube primitive
bpy.ops.mesh.primitive_cube_add()
bpy.ops.mesh.edge_face_add()
#Bevel edges to create more convex ngons
me = bpy.context.object.data
bm = bmesh.from_edit_mesh(me)
for e in bm.edges:
e.select = True
bpy.ops.mesh.bevel(offset=0.15, affect='EDGES')
bmesh.update_edit_mesh(me, True)
Adding a bevel modifier to round edges helps produce better shaped convex ngons. Remember to consider future deformation when planning n-gons.
Supporting Geometry
Example Adding Loops and Edge Creases
It helps to surround n-gons with clean quad rings to better hold shapes. Extra edge loops provide detailing support and prevent distortions.
#Box modeled with n-gons at center of faces
bpy.ops.mesh.primitive_cube_add()
bpy.ops.mesh.subdivide(number_cuts=2)
bpy.ops.mesh.edge_face_add() #Add ngons
#Insert loop cuts
bpy.ops.mesh.loopcut_slide(MESH_OT_loopcut={"number_cuts":4, "smoothness":0})
#Crease edges around ngons
bpy.ops.mesh.select_all(action='DESELECT')
bpy.ops.mesh.select_face_by_sides(number=5, extend=False)
for e in bm.edges:
if e.select:
e.crease = 1
bmesh.update_edit_mesh(me)
Surrounding n-gons with extra loops and creased edges maintains shape better when deforming or subdividing.
Finishing Workflow
Converting Final Model Faces to Quads
For final models, n-gons often should get converted to quads to maximize quality. Retopology cleaning tools make this process easier.
#Import high-poly sculpt with n-gons bpy.ops.import_scene.obj(filepath="sculpt.obj") #Retopologize object with quad-based mesh bpy.ops.mesh.retopomt() #Transfer vertex colors from original bpy.ops.object.vertex_color_transfer() #Bake normals to low-poly quads bpy.ops.object.bake(type='NORMALS', margin=5)
Retopology tools automatically generate optimized quad meshes while matching details from original n-gon sculpts. Vertex color and normal transfers preserve rendering accuracy.
For final models, cleanup to all quads ensures maximum quality and reliability for subdivision, animation, and rendering. But utilize n-gons freely during initial creative phases.
