Shells

The Shells tool analyzes disconnected components (shells) within a surface object, computing statistics for each shell and providing visualization options to differentiate and identify individual components. This analysis is valuable for understanding multi-component surfaces and assessing shell characteristics.

Overview

A surface may contain multiple disconnected shells—separate, independent mesh regions that don't share vertices or edges. The Shells tool:

• Identifies all distinct shells within the active surface
• Computes geometric statistics for each shell
• Visualizes shells using color mapping
• Exports statistical data for further analysis

This information helps with:

• Component identification: Understanding how many separate parts exist
• Quality assessment: Identifying unusually small or large shells
• Filtering decisions: Informing which shells to keep or remove
• Surface characterization: Understanding the composition of complex models

Accessing the Tool

Navigate to the Surface ribbon tab and locate Shells in the Analyze section. Select a surface object before activating the tool.

Visualization Methods

Four visualization methods are available, each colorizing shells based on different criteria:

Shell IDs

Visualize a color map representing shell IDs

Each shell is assigned a unique color based on its identification number. This visualization:

• Provides clear differentiation between all shells
• Makes it easy to count and identify separate components
• Uses a categorical color scheme for maximum distinction

This is the default method and is most useful for general shell identification.

Shell Volumes

Visualize a color map representing shell volumes

Shells are colorized based on their enclosed volume (for closed shells):

• Larger volumes appear as warmer colors
• Smaller volumes appear as cooler colors
• Open shells may display zero or undefined volume

This visualization helps identify the primary (largest) components versus smaller fragments or debris.

Shell Surface Areas

Visualize a color map representing shell surface areas

Shells are colorized based on their total surface area:

• Larger surface areas appear as warmer colors
• Smaller surface areas appear as cooler colors

Surface area provides a measure of shell size that works for both open and closed surfaces.

Triangle Counts

Visualize a color map representing the number of triangles in each shell

Shells are colorized based on their triangle count:

• More triangles appear as warmer colors
• Fewer triangles appear as cooler colors

This visualization reveals mesh density distribution and can help identify over- or under-tessellated regions.

Statistics Table

The statistics table displays computed metrics for each shell:

Column	Description
Shell ID	Unique identifier for each shell
Triangles	Number of triangles in the shell
Surface area	Total surface area (mm²)
Volume	Enclosed volume for closed shells (mm³)

The table allows you to:

• Sort shells by any metric
• Identify the largest or smallest shells
• Find shells matching specific criteria
• Select shells for further operations

Actions

Update

Click Update to recalculate shell statistics and update the visualization. Use this after:

• Selecting a different surface
• Modifying the surface geometry
• Changing the visualization method

Export Statistics

Click Export Statistics... to save the shell analysis data to a file. The export includes:

• All computed statistics for each shell
• Summary information
• Data suitable for spreadsheet analysis

Practical Applications

Model Validation

Before processing or exporting a model:

1. Run shell analysis
2. Verify the expected number of shells
3. Check for unexpected small shells (potential artifacts)
4. Confirm major components have appropriate volumes

Pre-Processing Assessment

Before mesh operations:

1. Analyze shell composition
2. Identify shells that should be removed (using Filter Shells)
3. Note shell volumes for verification after operations
4. Document original shell statistics

Multi-Component Analysis

For models with intentionally separate parts:

1. Visualize shells to confirm separation
2. Review volumes to verify each component
3. Identify any merged or split components
4. Use statistics for component verification

Quality Inspection

For incoming meshes or scan data:

1. Analyze for unexpected shell count
2. Identify very small shells (noise, artifacts)
3. Check that main geometry is intact
4. Export statistics for quality records

Technical Considerations

Shell Identification

Shells are identified through mesh connectivity:

• Vertices connected by edges belong to the same shell
• No path exists between vertices of different shells
• Each triangle belongs to exactly one shell

Volume Calculation

Volume is computed for closed (watertight) shells:

• Uses the signed volume method based on triangle orientations
• Requires consistent normal directions for accurate results
• Open shells or shells with inconsistent normals may show incorrect volumes

Performance

Shell analysis involves:

• Connectivity analysis: O(n) where n is triangle count
• Volume calculation: O(n) per shell
• Overall: Efficient for typical mesh sizes

Very complex meshes with thousands of shells may take longer to analyze.

Relationship to Other Tools

The Shells analysis tool complements:

• Filter Shells: Use analysis results to decide filtering criteria
• Split: Separates shells into distinct surface objects
• Merge: Combines separate surfaces (creates single object with multiple shells)
• Diagnostics and Fixes: Identifies topological issues within shells

Interpretation Guidelines

Expected vs. Unexpected Shells

Scenario	Interpretation
Single shell, expected	Normal solid object
Multiple shells, expected	Multi-component model (assembly)
Many small shells	Possible artifacts, noise, or fragments
Unexpected shell count	May indicate topology issues

Shell Size Distribution

Pattern	Typical Cause
One large, many tiny	Main geometry + artifacts
Several similar sizes	Multi-part assembly
Widely varying sizes	Mixed content (geometry + debris)
All similar sizes	Regular pattern or designed components

Common Issues and Solutions

Issue	Likely Cause	Solution
More shells than expected	Unintended disconnections	Check model for gaps; use Merge tool
Fewer shells than expected	Unintended connections	Check for bridging geometry; use Split
Zero volume for some shells	Open or inconsistent shells	Check shell closure; fix normals
Negative volumes	Inverted normals	Use Diagnostics and Fixes to correct normals
Visualization not updating	Analysis not refreshed	Click Update after changes