Avoiding Mesh Problems: Guidelines On N-Gons In Subdivision Modeling

What are N-Gons and Why to Avoid Them

N-gons are polygons in a 3D mesh that have more than 4 vertices and edges. Common polygon types in modeling are triangles (3 vertices), quads (4 vertices), and sometimes five-sided polygons. Anything higher than four vertices is generally considered an N-gon.

N-gons can cause issues when using subdivision surface modifiers. As the mesh divides, shading interpolations across an N-gon become irregular, leading to rendering artifacts. Edge flow also becomes chaotic, creating distortions in texture maps.

For these reasons, N-gons are typically avoided in subdivision workflow. Keeping geometry to clean quads and triangles ensures smooth predictive shading and texturing after multiple subdivisonlevels .

Definition of N-Gons

The “N” in N-gon stands for “n-number of sides” where N > 4. While triangles and quads have specific geometric definitions, N-gons is a more general term referring to any polygon with an excess of vertices.

Some common examples of N-gons include 5-sided pentagons, 6-sided hexagons, 7-sided heptagons, and so on. These arise in meshes when additional edges are created within an existing quad face. Essentially whenever a face contains an internal “hole” with edges that break up the quad into multiple sub-sections, an N-gon exists.

Problems Caused by N-Gons

N-gons pose several issues in subdivision surfaces relating to shading smoothness and texture distortions.

Shading Artifacts

As an N-gon mesh subdivides at render time, the interpolation of vertex normals across the N-gon face can become irregular. This leads to angular distortions and faceting in what should be a smooth shaded surface.

These shading irregularities tend to worsen with more subdivisions as the N-gon deforms further. While not always noticeable in raw polygon renders, these artifacts become very apparent when using smooth shading algorithms common in modern rendering.

Texturing Distortions

Due to unpredictable vertex flow, texture maps can become severely distorted when wrapping onto subdivided N-gon faces. As new split vertices are interpolated across an N-gon on subdivision, there is uneven distribution. This causes texture stretching, compression artifacts, or discontinuities.

The more irregular an N-gon’s shape, the more distortion occurs. Simple pentagons or hexagons may show minimal artifacts. But heptagons and octagons exhibit faster texture degradation, readily visible at mid-level subdivisions.

When N-Gons May Be Acceptable

While N-gons should generally always be avoided, there are a few special cases where they may be acceptable:

Invisible or non-critical areas of a mesh.
When mid-level subdivisions are not needed.
If the mesh has additional supporting geometry.
Only as intermediate placeholder geometry.

As examples, N-gons may be fine on hidden interior shapes inside objects, or on proxy collision geometry unseen by camera. Just ensure there are no rendering or texturing requirements on these N-gon-containing surfaces.

Low polygon models not needing smoother subdivisions can also potentially have minor N-gons. The problems only manifest at higher subdivision levels, so some N-gons in basic proxy meshes may not pose issues.

Having surrounding clean topology can help isolate and contain certain N-gon areas as well. This prevents irregular vertex flow from cascading outward across the whole mesh.

Lastly as workflow aids, N-gons are often used in early blocking stages with the expectation they will be fixed in more finalized modeling passes. The goal is resolving all N-gons in the finished model though.

Converting N-Gons to Quads and Tris

To eliminate N-gons, the basic solution is to split them into quad and triangle faces. This involves adding new edges to divide the complex N-gon shape into multiple simpler polygons.

Identifying Problematic Geometry

The first step is to visually inspect the mesh and identify areas with N-gons. Mesh analysis tools also help highlight them.

Signs to look for:

Polygons with more than 4 edges.
Overlapping uneven geometry.
Twisted irregular interior vertices.
High frequency texture distortions.
Shading triangles or other artifacts.

Select suspects faces and check the polygon count in the installed modeling package. Quads show 4 vertices, triangles display 3 vertices. Anything higher indicates N-gons.

Splitting Strategy

When splitting N-gons, aim to achieve all quad topology for best subdivision surface flow. But quads with occasional triangles are also acceptable where it simplifies cutting.

Try to maintain even spacing on new edges and incrementally work toward clean quads. Prevent triangles from sharing vertices or overlapping where possible. This avoids pole and vertex normalization issues.

When dealing with complex irregular N-gons, first simplify down to larger understandable shapes. For example convert an 8-sided polygon to simpler fused quad and triangle combinations. Then make further adjustments from that improved starting point.

Edge Loop Cutting

The primary tool for splitting is adding new edges to cut across the N-gon faces. Insert pairs of edge loops to slice the N-gon into quad regions. Use single edge cuts to divide off specific triangles where needed.

Properly placed cuts will reduce twisting geometry and establish cleaner vertex flow paths. Carefully adjust new edges to minimize irregular vertices. Deleting old problematic edges may help guide edge loops as well.

Supporting loops a few edges out from cuts can further improve flow. Attempt to create clear loops running perpendicular to other loops rather than overlapping T-junctions.

Retopology

For cases with dense N-gons or chaotic base topology, it may be necessary to recreate that section with an entirely new quad mesh. This manual process focuses on generating predictable quad geometry by hand.

Retopology allows the greatest flexibility in defining clean mesh flow around a surface. The challenge is accurately matching the original model shape with all quad faces.

Modeling Guidelines to Avoid N-Gons

Above are tips on fixing existing N-gons, but the best practice is to prevent them from ever forming during modeling. Here are approaches to create meshes with only quad dominant topology from the start.

Planning Edge Flow

Thoughtfully consider how major edge paths will flow over the modeled forms. Visualize clean lines traveling around key features and branching to span across flatter areas. These become the primary loops defining quad regions.

Sketch simple polygon layouts on concept sculpts to map flows. Identify major loops creating enclosed shapes over larger surface zones first. Then connect with smaller edge rings and spans branching between them.

Focus first on primary visible areas most affected by subdivisions. N-gons in less visible regions can be left as quick fills to resolve later.

Using Support Loops and Edge Rings

Supplementary edge paths added between main loops provide better structure for quads. These guide additional cuts later when needing to densify a mesh area or split irregular faces.

Support loops intersect perpendicular with major loops, branching directly between them. They divide broader shape zones into quadrants for easier subsections.

Edge rings continue along the curvature of surface contours in alignment with the major loops. They help isolate specific long edges needing further subdivision.

Leaving these supplemental paths from the start leaves clear cutting guides already in place for quick quad conversions.

Checking for N-Gons

Frequently inspect the topology after laying down new edges to catch stray N-gons before they accumulate. N-gons can easily slip in between other quads during rapid mesh development.

Temporarily enable visible polygon colors in app viewports to clearly spot higher order polys. Toggle angle based face shading as well for hints on twisting faces.

Maintaining clean quad flow constantly eliminates long revise times down the road. Catching runaway N-gons early restricts them from spreading deeper in the mesh.

Retopologizing Bad Geometry

When receiving imported assets from other sources, there will often be dense N-gons from unstructured modeling. Attempt to rework these areas into clean quadrilateral polygon layouts.

Trace surface contours to generate new optimized edge flow paths along the mesh surface. Delete away old messy topology leaving only the validated quad geometry.

Retopology may be extensive on some assets, but it provides the best subdivision results in the end. Aim to establish good modeling habits on any mesh lacking structure.

Subdivision Surface Modifier Settings

When using subdivision surface modifiers, be aware of settings that can help highlight lingering N-gon issues needing correction.

Viewport Display Settings

Configure viewport properties to emphasize mesh problems needing cleanup:

Activate Face Orientation overlay color to reveal twisted polys.
Check Triangles settings – highlights potential small artifact faces from quad cuts.
Enable Creasing visualization to check smoothing group borders.

Combined these identify irregular shading and abrupt angle changes on mesh surfaces from remaining poor topology.

Render Settings

When test rendering a mesh at higher subdivisions, enable render diagnostics to check for artifacts:

Amplify lighting normalization factors which exposes shading errors.
Exaggerate texture map sampling rates to check UV distortions faster.
Use render preview test patterns to isolate texturing or smoothing defects.

Debugging rendering this way calls out flaws that may not be obvious when model interacting in the viewport alone.

Example Workflow for Clean Quad Modeling

Reviewing the complete process on an example model helps showcase clean quad modeling principles in action.

Establishing Primary Edge Flow

After blocking overall forms, identify key surface contours and possible deformation zones to guide initial edge paths.

Place primary loops following the length of the model, such as around arms, legs, spine.
Add branching secondaries across wider areas like the torso, hips, shoulders.
Align edge rings along circular extrusion directions – knees, elbows, knuckles.

Focus on major features first before dense polys. Nail down overall flow line behavior, then increase edges.

Filling In Facets Between Loops

Connect together edge paths to create enclosed facet grids:

Triangulate narrower grooves where quads would taper and stretch.
Build clean quad rows between primary loops.
Use quads for broad zones and triangles for tight vertices.
Delete and redraw edges creating irregular polygons.

Taking time to refine topology, fix errant N-gons, and balance tris/quads prepares clean base mesh for subdividing.

Check Mesh Before Subdivision

Prior to applying subdivision surface modifiers, validate mesh topology:

Eyeball model for uneven loops or polarity shifts.
Toggle on face overlays to check for hidden irregularities.
Select suspicious elements and inspect polygon counts.
Remove any lingering complex polygons.

Addressing remaining poor topology eliminates obvious defects when smoothing starts accentuating issues.

Smoothing Preview and Render Debug

When previewing smoothed mesh, utilize available rendering and topology diagnostics:

Examine base mesh wireframe overlay to check edge flow.
Enable exaggerated texture maps to highlight UV distortions.
Amplify render diagnostics factor to multiply any defects.
Quick proxy test for rapid smoothing iteration, then recheck problem zones.

Actively analyzing mesh and rendering behavior at each step captures problems early before final submission.

Resources for Further Learning

For expanded practices on optimal subdivision modeling workflows, reference these additional resources:

“Subdivision Surface Modeling” digital courses – industry experts outline methodology.
“Essential Polygonal Modeling Techniques” playlists and tutorials – tips on quad mesh construction.
r/ComputerGraphicsModeling subreddit – user discussion on optimizing topology.
Topology Guides Volumes I and II by William Vaughan – detailed visualization of flow principles.

With applied study around sequential workflows and procedural best practices, skills rapidly improve modeling clean subdivision ready meshes.