1. Curvy Stuff 102 2-The BREP & You
Edward Eaton Product Design Services Aurora, Illinois
Curvy Stuff 102
2-The BREP & You

2. BREP=Boundary Representation
Models are made of oversized (even infinite) surfaces

3. BREP=Boundary Representation
Closed boundaries on surfaces create faces

4. BREP=Boundary Representation
Many ways to create boundaries
Cut
Boss
Fillet
Surface cut
Split line
They make boundaries… not holes!

5. BREP=Boundary Representation
All of our model faces are ‘chopped’ out of these larger surfaces

6. BREP=Boundary Representation
Two types of surfaces
Torus
Cone
Sphere
Plane (prismatic)
Cylinder
Analytical
NURBS
(spline based)
Algorithmic

7. Why should I care?
Knowing the underlying surface types explains seemingly odd behavior
The more you know, the more tricks you can develop and employ

8. BREP=Boundary Representation
Explore the different surface types on a simple model

9. Bosses change perimeter of boundary
Planar faces
Planar face
Extrusion recognizes the underlying, untrimmed surface!
Test - extrude oversized profile ‘up to surface’
Actually just a boundary on an infinite surface
Bosses change perimeter of boundary

10. Torus faces Torus face Actually just a boundary on a larger surface
Reverse direction…
Extrusion recognizes the underlying, untrimmed surface!
Actually just a boundary on a larger surface
Test - extrude oversized profile ‘up to surface’

11. BREP=Boundary Representation
Analytical surfaces
Assets:
Simple
Fast
Flexible
Mathematically Exact
Faces extend
Bodies (almost) always merge
Plane (prismatic)
Cylinder
Torus
Sphere
Cone

12. BREP=Boundary Representation
Analytical surfaces
Liabilities:
Boring
Constant curvature
Sometimes surprising
Plane (prismatic)
Cylinder
Torus
Sphere
Cone

13. BREP=Boundary Representation
Tip: There are a lot of advantages to defaulting to Analytical surfaces

14. Algorithmic face Algorithmic face
Test - extrude barely oversized profile ‘up to surface’
Does not extend past the loft sections

15. Algorithmic face Cut back
Extrusion recognizes the underlying, untrimmed surface!
Test - extrude oversized profile ‘up to surface’

16. Special case Algorithmic profile Extruded
Behaves analytically in direction of extrusion
Behaves algorithmically in direction of spline
Extruded

17. BREP=Boundary Representation
Algorithmic surfaces
Assets:
Flexible
Superior aesthetics
Curvature continuous
NURBS (spline based)

18. BREP=Boundary Representation
Algorithmic surfaces
Liabilities:
Slow
Round off errors
Require exacting input
Offsetting trouble
Bad when 3-sided
No extension past original borders
NURBS (spline based)

19. Why should I care?
Build algorithmic surfaces oversized to provide runoff for features that interact with it

20. BREP and Algorithmic Surfaces

21. 1. Fillet (and draft) capping
SW needs to guess
Intent is buried in BREP
Another way to build exact same model

22. 2. Mirroring Algorithmic
Model symmetrical items in half then mirror, right?
Analytical
Algorithmic
Visible ridge
Sides aren’t tangent
Faces don’t merge

23. 2. Mirroring, cont… Whole Why is this? Half
Cutting model in half just adds boundary to surface
Mirroring removes boundary
Add features…

24. TIP: copy and paste the sketch to avoid tedious construction
2. Mirroring, cont…
RULE: Algorithmic faces that cross the plane of symmetry need to be modeled whole. ?
Section needs to be a single closed spline
We need to
insure it is
symmetrical
TIP: copy and paste the sketch to avoid tedious construction

25. 3. BREP & Shelling SW knows to use 4-sided surfaces
Shells don’t like 3-sided
surfaces
NURB surface
is 3-sided
Surface is still 4-sided

26. BREP=Boundary Representation
SW gives unprecedented access to the BREP
Pattern Face
‘Delete and repair’ face
Replace Face

27. Delete and repair face Didn’t want these areas Add revolved boss
Delete face just sidestepped at least 3 cut features!
Didn’t want these areas
Add revolved boss
Just extra boundaries on the revolved surface
I want the boundary
to look like this

28. Delete and repair face - cut
Cut-Sweep goes too far

29. Replace Face Sample everybody shows Can extrude up to surface Can’t
Revolve to surface
Sweep to surface
Existing lofts,etc
Use to patch up little spots

30. Replace Face Model Surface-Fill to create a subtle bulge
Now – replace face!
In the past we would have had to use big workarounds…

31. Why should I care? The BREP doesn’t care how you make faces
Use Solid and Surface features interchangeably

32. Surfaces vs. Solids
Surface features make faces directly, at price

33. Surfaces vs. Solids
Solid features are Macros for creating surface bodies
There are 12 faces that don’t directly contribute to the actual model .

34. So, what's the big difference between surface and solid modeling?
The difference…
So, what's the big difference between surface and solid modeling?

35. The difference… Surface bodies may have microscopic gaps between edges
Knitting forces two edges to become one

36. The difference… Surfaces have direction
If all of the surface point outward, you have a void
If all of the surface point inward, you have a solid

37. Why should I care?
Dirty little ‘secret’ – Solid models are surface models!
a) They are knitted surfaces that completely enclose a volume
b) …and all of the surfaces point in the same direction

38. Solids’ Achilles heel
So why all the talk about surface modelers vs Solid modelers?
Solid features have to be complete volumes that boolean into the work piece

39. Solids’ Achilles heel
What is Zero-Thickness geometry?

40. Solids’ Achilles heel
This kind of blend is virtually impossible in solids with algorithmic faces

41. create only the patch we need
Surfaces vs Solids
“On the tough stuff, surfaces models have an edge”
create a hole
create only the patch we need
merge

42. Face Modeling The BREP doesn’t care how you make faces
Use Solid and Surface features interchangeably

43. Face Modeling
The faces for the blend are best modeled using surfaces features…
… while the base is more efficiently modeled as a solid

44. Face Modeling - a a1. Go ahead and model the base as a solid
a2. Tell SW where the solid faces should end and the surface faces begin

45. Face Modeling - b
b1. Create the surface tangent to the base

46. Face Modeling - c c1. Check that everything is ok
c2. Swap the solid faces with the surface faces

47. Face Modeling - c c3. Cut in the crescent
c3. Angle the front of the crescent
Done!

48. Now, onto Evaluating model quality
End of BREP
Now, onto Evaluating model quality