AST-MESH-001: Mesh Optimization

What This Rule Detects

This rule flags Static Mesh assets with optimization concerns:

• High polygon count without LODs: Dense meshes that don’t reduce at distance
• High polygon count without Nanite: Meshes that could benefit from Nanite
• Low polygon count with Nanite enabled: Overhead not justified for simple meshes
• Missing LOD coverage: High-poly meshes with insufficient LOD chain

The default threshold is 50,000 triangles before flagging high-poly concerns.

Why This Matters

Triangle Count Isn’t the Only Cost

Triangle count matters, but it’s not the only geometry cost. Understanding the full picture:

Epic’s Production Example (HLOD)

In a Fortnite example, Epic demonstrated using HLOD/proxy geometry to reduce a distant region from 424 meshes and 556,445 triangles down to 1 mesh with 1,215 triangles—an enormous reduction that demonstrates why distance-based simplification exists.

This isn’t about arbitrary limits—it’s about matching geometry detail to actual visibility.

Nanite: What It Is and Isn’t

Nanite is a different geometry pipeline, not a magic “make everything faster” checkbox.

What Nanite does well:

• Automatic LOD based on screen coverage
• Only processes triangles that contribute to pixels
• Handles massive triangle counts (millions) gracefully
• Excellent for dense, detailed environment geometry

What Nanite doesn’t support:

• Skeletal meshes (not supported at all)
• Translucent/masked materials (will use fallback)
• Two-sided foliage (partially supported in UE 5.7 with Nanite Foliage experimental)
• Min/Max Draw Distance (not supported—Nanite handles culling differently)

UE 5.7 note: Nanite Foliage is now experimental, designed for dense vegetation rendering. Check release notes for current status.

Memory: High-poly meshes with no LODs load the full mesh even when displaying at low detail. LODs allow streaming only what’s needed.

Real Example: SM_HeroStatue with 500k triangles, no LODs, Nanite disabled. Placed 20 times in a courtyard scene. At distance, still processing 10 million triangles for what could be 20k with LOD2.

When This Is Acceptable

• Hero props at fixed camera distance: Character select screens, cinematic closeups
• Nanite-enabled high-detail meshes: Nanite handles LOD automatically
• Intentionally simple meshes: Props designed to be low-poly
• Procedural geometry: Runtime-generated meshes that can’t have authored LODs

The Problem

Optimization Scenarios

SM_Environment_Rock (75,000 triangles)

❌ No LODs, No Nanite:
   → Full 75k triangles at any distance
   → 20 instances = 1.5M triangles always

✅ With LODs:
   LOD0: 75,000 triangles (0-10m)
   LOD1: 25,000 triangles (10-30m)
   LOD2: 5,000 triangles (30-100m)
   LOD3: 1,000 triangles (100m+)
   → Typical scene: ~300k triangles instead of 1.5M

✅ With Nanite:
   → Automatic triangle reduction based on screen coverage
   → Only processes triangles that contribute to pixels
   → Ideal for dense, detailed geometry

The Checks

The Fix

Option 1: Generate LODs (Most Common)

Use Unreal’s automatic LOD generation:

1. Open Static Mesh Editor
2. Window → LOD Settings
3. Set Number of LODs to 4-5
4. Click Apply Changes

Recommended LOD reduction settings:

Option 2: Enable Nanite

For dense, detailed geometry (50k+ triangles):

1. Open Static Mesh Editor
2. Check Enable Nanite Support
3. Save asset

Nanite is ideal for:

• Environment hero meshes
• Scanned/photogrammetry assets
• Dense architectural details
• Any mesh >100k triangles

Nanite is NOT ideal for:

• Simple props (<10k triangles)
• Skeletal meshes (not supported)
• Translucent materials
• Two-sided foliage with masked materials

Option 3: Simplify Source Mesh

If the mesh is over-detailed for its use case:

In DCC (Blender/Maya/Max):

1. Apply decimation modifier
2. Target 50% of original triangles
3. Preserve UV seams and hard edges
4. Re-export to Unreal

In Unreal (Reduction Settings):

1. LOD Settings → LOD0 Reduction Settings
2. Set Percent Triangles to target value
3. Apply Changes

Option 4: Disable Nanite on Simple Meshes

For flagged low-poly Nanite meshes:

1. Open Static Mesh Editor
2. Uncheck Enable Nanite Support
3. Add manual LODs if >10k triangles
4. Save asset

Why disable on simple meshes:

• Nanite has per-mesh processing overhead
• Simple meshes render faster with traditional pipeline
• Memory overhead for Nanite data structures

Option 5: Use HLOD for Dense Scenes

For scenes with many high-poly meshes:

1. Create HLOD Proxy meshes for distant viewing
2. Configure HLOD layers in World Settings
3. Let HLOD system swap simplified geometry at distance

Useful for cityscapes, forests, and other dense environments.

LOD Screen Size Tuning

Screen size determines when LODs swap. Adjust based on mesh importance:

// In Static Mesh Editor or via Blueprint
LOD0.ScreenSize = 1.0    // Always use when close
LOD1.ScreenSize = 0.4    // ~40% screen height
LOD2.ScreenSize = 0.15   // ~15% screen height
LOD3.ScreenSize = 0.05   // ~5% screen height

Aggressive (performance-focused):

LOD transitions at larger screen sizes = earlier LOD swap

Conservative (quality-focused):

LOD transitions at smaller screen sizes = longer LOD0 usage

Triangle Budget Guidelines

Checking Mesh Stats

In the Static Mesh Editor:

1. Statistics panel shows triangle counts
2. LOD dropdown shows per-LOD stats
3. Nanite section shows cluster/triangle info

In viewport:

• stat RHI shows total triangles rendered
• stat Nanite shows Nanite-specific stats
• LOD coloration mode visualizes LOD distribution

Configuration

Threshold: Triangle count before flagging high-poly (default: 50,000)

To adjust in Project Settings:

Blueprint Health Analyzer → Rule Thresholds → AST-MESH-001 → 100000.0

Higher thresholds for Nanite-heavy projects. Lower thresholds (25,000) for mobile or VR where triangle budget is tight.

Related Rules

• MAT-INST-001 - Complex materials compound mesh cost
• AST-TEX-001 - Texture memory adds to mesh memory
• RND-SET-001 - Nanite/Lumen settings affect mesh rendering path