discrete differentialbrickisland.net/.../2017/09/cmu_ddg_fall2017_02_simpicialcomple… · •for...
TRANSCRIPT
![Page 1: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/1.jpg)
DISCRETE DIFFERENTIAL GEOMETRY:
AN APPLIED INTRODUCTIONKeenan Crane • CMU 15-458/858B • Fall 2017
![Page 2: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/2.jpg)
Keenan Crane • CMU 15-458/858B • Fall 2017
DISCRETE DIFFERENTIAL GEOMETRY:
AN APPLIED INTRODUCTION
LECTURE 2: THE SIMPLICIAL COMPLEX
![Page 3: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/3.jpg)
Administrivia• First reading assignment was due 10am today! (Please use Andrew ID)
• First homework assignment (A1) — due 9/27
• Basic tool we’ll be using all term: (discrete) exterior calculus
• Written part out today, coding part out next week
• Special recitation on how to use code framework: 9/8 @ 4–5pm, GHC 4215
![Page 4: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/4.jpg)
Reading: Overview of DDG
“…I’m intimidated by the math…” “…I’m intimidated by the coding…”
DDG is by its very nature interdisciplinary—everyone will feel a bit uncomfortable!
We are aware of this fact! Everyone will be ok. :-) Lots of details; focus on the ideas.
![Page 5: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/5.jpg)
Assignment 1—Written Out Later Today!•First assignment
•Written part out today
•Coding part out next week•Topic: exterior algebra & exterior calculus
•Basic tools used throughout semester
•Can’t skip this one!•Won’t cover in class until next week
•But, great intro available in our course notes•Good idea to get started now! (Read notes first.)
•All administrative details (handin, etc.) in assignment.
![Page 6: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/6.jpg)
Today: What is a Mesh?•Many possibilities…
•Simplicial complex
•Abstract vs. geometric simplicial complex
•Oriented, manifold simplicial complex
•Application: topological data analysis
•Cell complex
•Poincaré dual, discrete exterior calculus
•Data structures:
• adjacency list, incidence matrix, halfedge
![Page 7: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/7.jpg)
Connection to Differential Geometry?
topological space* abstract simplicial complex
*We’ll talk about this later in the course!
![Page 8: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/8.jpg)
Convex Set
![Page 9: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/9.jpg)
Convex Set—Examples
(A) (B) (C)
(D) (E) (F)
Which of the following sets are convex?
![Page 10: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/10.jpg)
Convex Set
convex not convex
![Page 11: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/11.jpg)
Convex Set—Examples
(A) (B) (C)
(D) (E) (F)
![Page 12: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/12.jpg)
S conv(S)
Convex Hull
![Page 13: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/13.jpg)
Convex Hull—Example
A: A cube.
(1, 1, 1)
(-1, -1, -1)
![Page 14: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/14.jpg)
Simplex
![Page 15: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/15.jpg)
Simplex—Basic IdeaRoughly speaking, a k-simplex is a point, a line segment, a triangle, a tetrahedron…
…much harder to draw for large k!
k = 0 k = 1 k = 2 k = 3 k = 4 k = 5
![Page 16: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/16.jpg)
Linear Independence
linearly independentlinearly dependent
![Page 17: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/17.jpg)
Linear Independence
linearly independentlinearly dependent
v1
v2
v1v2
![Page 18: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/18.jpg)
Affine Independence
(Colloquially: might say points are in “general position”.)
(A) (B)
![Page 19: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/19.jpg)
Affine Independence
(Colloquially: might say points are in “general position”.)
(A) (B)
affinely independent affinely dependent
![Page 20: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/20.jpg)
Simplex—Geometric Definition
![Page 21: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/21.jpg)
Simplex—Geometric Definition
Q: How many affinely-independent points can we have in n dimensions?
![Page 22: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/22.jpg)
Barycentric Coordinates—1-Simplex•We can describe a simplex more explicitly using barycentric coordinates.
•For instance, a 1-simplex is comprised of all weighted combinations of the two points where the weights sum to 1:
a
b
![Page 23: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/23.jpg)
Barycentric Coordinates—k-Simplex•More generally, any point in a k-simplex can be expressed as a weighted
combination of the vertices, where the weights sum to 1.
•The weights ti are called the barycentric coordinates.
(Also called a “convex combination.”)
![Page 24: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/24.jpg)
Simplex—Example
(1,0,0)
(0,1,0)
(0,0,1)
(Also known as the “probability simplex.”)
![Page 25: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/25.jpg)
Simplicial Complex
![Page 26: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/26.jpg)
Simplicial Complex—Rough Idea•Roughly speaking, a simplicial complex is “a bunch of simplices*”
•…but with some specific properties that make them easy to work with.
•Also have to resolve some basic questions—e.g., how can simplices intersect?
Plural of simplex; not “simplexes.” Pronounced like vertices and vortices.
![Page 27: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/27.jpg)
Face of a Simplex
*Doesn’t have to be a proper subset, i.e., a simplex is its own face.
Q: Anything missing from this picture?
p0
p1
p2
p0
p0 p0
p0
p1
p1
p1
p1
p2
p2p2
p2
A: Yes—formally, the empty set ø.
![Page 28: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/28.jpg)
Simplicial Complex—Geometric Definition
simplicial complex not a geometric simplicial complex…
![Page 29: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/29.jpg)
Simplicial Complex—Example
Mr. GoPuff
![Page 30: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/30.jpg)
Simplicial Complex—Example
Mr. GoPuff
0 1
2
3 4
5
67
8
9 10
Q: What are all the simplices in Mr. Go Puff?
A: {7,10,8}{6,7,9} {2,3} {3,4} {4,5} {0} {1}
{6,7} {7,9} {9,6} {7,8} {8,10} {10,7} {2} {3} {4} {5}
{6} {7} {8} {9} {10}
{ø}
Didn’t really use the geometry here…
![Page 31: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/31.jpg)
Abstract Simplicial Complex
• Only care about how things are connected, not how they are arranged geometrically.• Later in class, will serve as our discretization of a topological space
geometric simplicial complex abstract simplicial complex*
*…visualized by mapping it into R3.
![Page 32: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/32.jpg)
Abstract Simplicial Complex—Graphs• Any (undirected) graph G = (V,E) is an abstract simplicial (1-)complex
• 0-simplices are vertices
• 1-simplices are edges
![Page 33: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/33.jpg)
Abstract Simplicial Complex—Example
Q: Is this set an abstract simplicial complex? If so, what does it look like?A: Yes—it’s a pair of 2-simplices (triangles) sharing a single edge:
Vertices no longer have to be points in space; can represent anything at all.
![Page 34: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/34.jpg)
Application: Topological Data Analysis
(persistence diagram)
Forget (mostly) about geometry—try to understand data in terms of connectivity.
E.g., persistent homology:
https://youtu.be/h0bnG1Wavag
![Page 35: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/35.jpg)
Material Characterization via Persistence
Nakamura et al, “Persistent Homology and Many-Body Atomic Structure for Medium-Range Order in the Glass”
Regular Random Glass
![Page 36: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/36.jpg)
Persistent Homology—More ApplicationsM. Carrière, S. Oudot, M. Ovsjanikov, “Stable Topological Signatures for Points on 3D Shapes”
…and much much more (identifying patients with breast cancer, classifying players in basketball, new ways to compress images, etc.)
C. Carstens, K. Horadam,“Persistent Homology of Collaboration Networks ”
H. Lee, M. Chung, H. Kang, B. Kim, D. Lee“Discriminative Persistent Homology of Brain Networks”
![Page 37: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/37.jpg)
Anatomy of a Simplicial Complex• Closure: smallest simplicial complex containing a given set of simplices
• Star: union of simplices containing a given subset of simplices
• Link: closure of the star minus the star of the closureclosure star
link
![Page 38: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/38.jpg)
Vertices, Edges, and Faces•Just a little note about notation:
•For simplicial 1-complexes (graphs) we often write G = (V,E)
•Likewise, for simplicial 2-complexes (triangle meshes) we write K = (V,E,F)
– Vertices
– Edges
– Faces*
•K is for “Komplex!”
*Not to be confused with the generic face of a simplex…
G = (V,E) K = (V,E,F)
![Page 39: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/39.jpg)
Oriented Simplicial Complex
![Page 40: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/40.jpg)
Orientation—Visualized
![Page 41: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/41.jpg)
Orientation of a 1-Simplex•Basic idea: does a 1-simplex {a,b} go from a to b or from b to a?
•Instead of set {a,b}, now have ordered tuple (a,b) or (b,a)
•Why do we care? Eventually will have to do with integration…
b
aa
b
![Page 42: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/42.jpg)
Orientation of a 2-Simplex•For a 2-simplex, orientation given by “winding order” of vertices:
a
b
c
a
b
c
Q: How can we encode these oriented 2-simplices?A: Oriented tuples, up to circular shift.
(a,b,c)(b,c,a)(c,a,b)
(a,c,b)(c,b,a)(b,a,c)
![Page 43: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/43.jpg)
Oriented k-SimplexHow do we define orientation in general?
? ? ? ?
Similar idea to orientation for 2-simplex:
Hence, always* two orientations: even or odd permutations of vertices.Call even permutations of (0,…,k) “positive”; otherwise “negative.”
![Page 44: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/44.jpg)
Oriented 0-Simplex?What’s the orientation of a single vertex?
a
Only one permutation of vertices, so only one orientation! (Positive):
![Page 45: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/45.jpg)
Oriented 3-SimplexHard to draw pictures as k gets large!
But still easy to apply definition:
ab
c
d
even / positive odd / negative
![Page 46: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/46.jpg)
Oriented Simplicial Complex
Example.
![Page 47: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/47.jpg)
Relative Orientation
Example: Consider two triangles that intersect along an edge:
same relative orientation different relative orientation
Definition. Two distinct oriented simplices have the same relative orientation if the two (maximal) faces in their intersection have opposite orientation.
![Page 48: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/48.jpg)
Simplicial Manifold
![Page 49: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/49.jpg)
Manifold—First Glimpsemanifold nonmanifold
![Page 50: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/50.jpg)
Manifold—First Glimpsemanifold nonmanifold
Key idea: “looks like Rn up close”
![Page 51: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/51.jpg)
Simplicial Manifold—VisualizedWhich of these simplicial complexes look “manifold?”
(E.g., where might it be hard to put a little xy-coordinate system?)
![Page 52: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/52.jpg)
Simplicial Manifold—Definition
link (k=2) link (k=3)link (k=1)
*I.e., is homeomorphic to.
Aside: How hard is it to check if a given simplicial complex is manifold?•(k=1) trivial—is it a loop?•(k=2) trivial—is each link a loop?•(k=3) is each link a 2-sphere? Just check if V-E+F = 2 (Euler’s formula)•(k=4) is each link a 3-sphere? …Well, it’s known to be in NP! [S. Schleimer 2004]
![Page 53: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/53.jpg)
Manifold Triangle Mesh
Why? One reason: data structures…
Key example: For a triangle mesh (k=2):•every edge is contained in exactly two triangles
•…or just one along the boundary•every vertex is contained in a single “loop” of triangles
•…or a single “fan” along the boundary
nonmanifold edge nonmanifold vertex
![Page 54: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/54.jpg)
Topological Data Structures
![Page 55: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/55.jpg)
Topological Data Structures—Adjacency List• Store only top-dimensional simplices
• Implicitly includes all facets
• Pros: simple, small storage cost
• Cons: hard to access neighbors
0
1
2
30 2 10 3 23 0 13 1 2
Q: How might you list all edges touching a given vertex? What’s the cost?
Example.
![Page 56: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/56.jpg)
Topological Data Structures—Incidence Matrix
0 0
21 3
0
1
2
3
45
1
2
3
Example.
Q: Now what’s the cost of finding edges incident on a given vertex?
![Page 57: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/57.jpg)
Data Structures—Signed Incidence Matrix
Example.
A signed incidence matrix is an incidence matrix where the sign of each nonzero entry is determined by the relative orientation of the two simplices corresponding to that row/column.
(Closely related to discrete exterior calculus.)
![Page 58: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/58.jpg)
Topological Data Structures—Half Edge Mesh
Halfedge
twin
edge
next
vertex
face
struct Halfedge{ Halfedge* twin; Halfedge* next; Vertex* vertex; Edge* edge; Face* face;};
struct Vertex{ Halfedge* halfedge;};
halfedge
vertex
struct Edge{ Halfedge* halfedge;};ha
lfedge
edge
struct Face{ Halfedge* halfedge;};
halfedge
Face
Basic idea: each edge gets split into two half edges.• Half edges act as “glue” between mesh elements.• All other elements know only about a single half edge.
(You’ll use this one in your assignments!)
![Page 59: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/59.jpg)
Half Edge—Algebraic Definition
v0
e0
e1
e2e3
e4
h0
h1
h6
h7
f0f1
h8
h9
h2
h3h4
h5
v1
v3 v2“next”
“twin”
![Page 60: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/60.jpg)
Half Edge—ExampleSmallest examples (two half edges):
(images courtesy U. Pinkall)
![Page 61: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/61.jpg)
Data Structures—Quad Edge
(L. Guibas & J. Stolfi, “Primitives for the Manipulation of General Subdivisions and the Computation of Voronoi Diagrams”)
![Page 62: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/62.jpg)
Dual Complex
![Page 63: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/63.jpg)
Dual Mesh—Visualized
![Page 64: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/64.jpg)
simplicial complex
Poincaré Duality
cell complex(Poincaré Duality)
![Page 65: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/65.jpg)
Primal vs. Dual
primal
dual
0-cell1-cell2-cell
0-simplex 1-simplex 2-simplex
(Will say more when we talk about discrete exterior calculus!)
![Page 66: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/66.jpg)
Poincaré Duality in Nature
![Page 67: DISCRETE DIFFERENTIALbrickisland.net/.../2017/09/CMU_DDG_Fall2017_02_SimpicialComple… · •For instance, a 1-simplex is comprised of all weighted combinations of the two points](https://reader034.vdocuments.site/reader034/viewer/2022042418/5f3453a9c1a7bc6ca832ad61/html5/thumbnails/67.jpg)
Thanks!
Keenan Crane • CMU 15-458/858B • Fall 2017
DISCRETE DIFFERENTIAL GEOMETRY:
AN APPLIED INTRODUCTION