The Modern CAD/CAM Workflow:

Scan, Design, Edit, Analyze, and Fabricate

Without Triangles Duane Storti

Mechanical Engineering

University of Washington

Seattle, WA

“Big picture” context: Technology democratization

• Ubiquity of 3D imaging (CT, MR, …)

democratizing volumetric scanning

• CUDA democratizing parallel computing

• 3D printing democratizing manufacturing

Near a threshold where:

“If you can think/imagine it, you can build it.”

- Chris Anderson (Wired), Walt Disney, Lego?

But can you model “it” with current computer-aided

design (CAD) software?

Motivational problems

• Designing “Bigfoot”

• Current models/algorithms scale badly

• Boundary representations (b-reps)/triangulations

• Sculptured surfaces lots of patches/triangles

• Boolean ops scale with product of triangle counts

• Modeling bones with pins crashed CAD systems

• Solution: Hack the 3D printer build set-up

• Designing/printing objects with graded properties

• Brain phantom, graded octet truss

• Crucial advantage of 3D printing (additive mfg.)

• B-reps ill-suited for describing interior composition

Brain Phantom: Graded radiological activity (Solution: Test page hack)

Graded Octet Truss via Vat Photo-Polymerization

(Solution: Hack the build stack)

Need better software for interacting with image stack models (and less hackery!)

• Interactive tools for

• Viewing, Editing, Boolean operations

• Modeling of graded material properties

• Analysis

• Preparation for 3D printing

• GPU-based parallel computing with CUDA

enables live interaction with new approach:

• Voxel SDF-reps (Models are image stacks!)

• Irony alert!

GPU Technology built to render triangles beautifully,

saves the day when there are no triangles…

Voxel SDF-rep Image Stack Modeler

• CUDA/Python Implementation by Chris Uchytil

using:

• Pyopengl

• Pycuda: graphics interop

• Numba: bulk of CUDA code including kernels

• Pyside: user interface

• Demonstrations recorded in real-time

on workstation with GTX 1080

• Demo videos: Triangle free!

Import bones from CT (and label file). Perform basic modeling ops.

Import bones, adjust spacing, and construct union

Creating Voxel SDF-reps: torus, cylinder, pin

Pinned bones: uniform material

Pinned bones: unions with properties

Modeling Ops:

Swept Solids

C implementation

By Di Zhang

Modeling Ops:

Skeletal Editing

How? And why without triangles?

What do you value?

As a rendering, this is ??? _

How? Why without triangles?

What do you value?

As a rendering, this is beautiful

How? Why without triangles?

What do you value?

As a rendering, this is beautiful

As a solid model, it is ??? _

How? Why without triangles?

What do you value?

As a rendering, this is beautiful

As a solid model, it is broken!

What is inside/outside?

Limitations of Current CAD Systems

Primarily boundary representation (B-rep)

• Robustness issues

• Limited support for variable material

• Difficult to import scanned objects

• Leads to “Bigfoot” crashes!

-

Classify points as in/out by function evaluation

Signed Distance Functions (SDFs) very desirable: (1) Simplified root finding (2) Skeletal editing

Can we create functions and/or SDFs for: (1) Real engineering parts? (2) Parts captured via scan?

Alternative approach:

Implicit or Function-based (f-rep) models

Coordinates F-rep Sphere SDF-rep Sphere

Cartesian x2 + y 2 + z 2 – R2 < 0 𝑥2 + 𝑦2 + 𝑧2 – R < 0

Spherical r 2 – R2 < 0 r – R < 0

F-rep for Engineering Part: by Mark Ensz

F-rep for Engineering Part: Hex Nut

Engineering Part: F-rep wood screw

SDF-reps (Signed Distance Functions)

• Few analytic SDF primitives

• Compute voxelized approximation of SDF

• Create “label” file

• Sample sign of F-rep on grid

• Segmentation of 3D imaging

• Convert label file to grid of SDF values

• 3D distance transform

• Upwind differencing (scheme used in level set methods)

• Interpolate as needed (wavelets)

Issues on the 3D printing end of the workflow

• Convert models to STL files

• De facto standard for 3D printing

• Bag of triangles Lots of problems/limitations

• Need to slice STL to determine layer descriptions

• Instead use voxel SDF-rep

• Model is an image stack

• Send images to printer as slice descriptions

• Variable material Auxiliary property stack

• Use grayscale or color images to encode materials

• New operations on inhomogeneous objects

Take-away points

Advances on the design side (CAD software) are

essential to realize the potential of 3D printing.

Implicit approaches are helpful and link directly to

Image stack/Voxel methods that provide:

Straightforward modeling of graded materials/properties

Unified format for 3D operations:

scan, design, analyze and fabricate

Real-time interactivity using GPU-based parallelism

References

[1] Yurtoglu, M., GPU-based Parallel Computation of Integral Properties of Volumetrically Digitized

Objects, PhD Dissertation, University of Washington, 2017.

[2] Peterson, G., Schwartz, J., Zhang, D., Weiss, B., Ganter,M., Storti, D. and Boydston, AJ.

Production of Materials with Spatially-Controlled Crosslink Density via Vat Photopolymerization, ACS

Applied Materials & Interfaces. 8, 29037−29043 (2016).

DOI:10.1021/acsami.6b09768

[3] Zhang, D., A GPU Accelerated Signed Distance Voxel Modeling System, PhD Dissertation,

University of Washington, 2016.

[4] Storti, D, and Yurtoglu, M., CUDA for Engineers: An Introduction to High-Performance Parallel

Computing, Addison-Wesley Professional, NY, 2015.

[5] Storti D, Ganter MA, Ledoux WR, Ching RP, Hu Y, Haynor D. Wavelet SDF-Reps: Solid Modeling

With Volumetric Scans. ASME. International Design Engineering Technical Conferences and

Computers and Information in Engineering Conference, Volume 6: 33rd Design Automation

Conference, Parts A and B (2007):501-513. doi:10.1115/DETC2007-34703.

[6] Storti, D., Ganter, M., Ledoux, W., Ching, R., Hu, Y., and Haynor, D. Artifact vs. Anatomy: Dealing

With Conflict of Geometric Modeling Descriptions. No. 2007-01-2450. SAE Technical Paper, (2007).

[7] Mark T. Ensz, Duane W. Storti, and Mark A. Ganter, Implicit Methods for Geometry Creation, Int. J.

Comput. Geom. Appl. 08, 509 (1998).

DOI: http://dx.doi.org/10.1142/S0218195998000266

Acknowledgments/Thanks

• Students (Current and Former UW AM lab members) • Chris Uchytil (code and videos), Ben Weiss

• Mete Yurtoglu (Google), Di Zhang (Bodylabs), Mark Ensz (Sandia)

• Siu Kwan Lam (Continuum Analytics) – Numba support

• UW Colleagues:

• Mark Ganter (AM Lab Co-director)

• Nick Boechler Lab (ME), AJ Boydston Lab (Chemistry)

• Bil Ledoux (VA) – motivation/sample data/support

• Mike Miller (U. Indiana Med. School, Siemens,…)

• UW College of Engineering (Strategic Research Initiative Program)

• Ricoh

• NVIDIA

• Thank you for your attention!