Project 10: Feature-Based Modeler (Extrude/Revolve)

Build a feature pipeline that extrudes and revolves sketches into solids.

Project Overview

Attribute Value
Difficulty Level 3: Advanced
Time Estimate 3-4 weeks
Main Language C++
Alternative Languages Rust, C, Python
Knowledge Area Feature-based modeling
Tools Mesh viewer, validation scripts
Main Book “Geometric Modeling” by Michael E. Mortenson

What you’ll build: A feature pipeline that converts 2D sketches into 3D solids using extrude and revolve operations.

Why it teaches computational geometry: Feature modeling is how real CAD users build parts. This project connects sketches, topology, and solid generation.

Core challenges you’ll face:

  • Converting 2D profiles into 3D topology
  • Generating side faces and caps consistently
  • Handling self-intersections and invalid profiles

Real World Outcome

You will input a 2D profile and generate a valid solid. The output will be a closed mesh or B-Rep that can be visualized.

Example Output:

$ ./feature_model --extrude profile.json --distance 20 --output part.obj
Profile vertices: 16
Generated solid faces: 64
Validation: watertight

Verification steps:

  • Visualize the extruded/revolved solid
  • Check that caps and side faces are consistent

The Core Question You’re Answering

“How do sketch-based CAD features become real 3D solids?”

This is the bridge between design intent and geometry.

Concepts You Must Understand First

Stop and research these before coding:

  1. Profile validity
    • What makes a sketch suitable for extrusion?
    • Book Reference: “Geometric Modeling” by Michael E. Mortenson, Ch. 8
  2. Sweep and revolve
    • How does rotation create surfaces of revolution?
    • Book Reference: “Curves and Surfaces for CAGD” by Gerald Farin, Ch. 12
  3. Topology generation
    • How do you create consistent faces and edges from a profile?
    • Book Reference: “Geometric Modeling” by Michael E. Mortenson, Ch. 10

Questions to Guide Your Design

  1. Extrude strategy
    • Will you treat profiles as planar polygons or as curves?
    • How will you handle holes in profiles?
  2. Revolve strategy
    • How will you handle axis alignment and angle ranges?
    • How will you avoid self-intersections?

Thinking Exercise

Revolve Geometry

Take a 2D line segment offset from the axis and revolve it 360 degrees. What shape is produced?

Questions while working:

  • How do you represent the resulting surface in a mesh?
  • What happens if the profile intersects the axis?

The Interview Questions They’ll Ask

Prepare to answer these:

  1. “What is feature-based modeling?”
  2. “How does an extrusion create side faces and caps?”
  3. “What are common validity checks for sketches?”
  4. “How do you prevent self-intersection in a revolve?”
  5. “How do you convert a profile into a watertight solid?”

Hints in Layers

Hint 1: Starting Point Start with simple convex profiles before handling holes.

Hint 2: Next Level Generate side faces by connecting corresponding edges along the sweep.

Hint 3: Technical Details Ensure consistent winding for caps to maintain outward normals.

Hint 4: Tools/Debugging Validate the output mesh for open edges and inverted faces.

Books That Will Help

Topic Book Chapter
Sweeps and extrusion “Geometric Modeling” by Michael E. Mortenson Ch. 8
Surfaces of revolution “Curves and Surfaces for CAGD” by Gerald Farin Ch. 12
Solid topology “Geometric Modeling” by Michael E. Mortenson Ch. 10

Implementation Hints

  • Build extrusion first, then add revolve.
  • Keep geometry and topology generation separate.
  • Validate profile orientation before building solids.

Learning Milestones

  1. First milestone: You can extrude a simple profile into a solid.
  2. Second milestone: You can revolve a profile into a closed shape.
  3. Final milestone: You can explain why feature modeling needs robust checks.