folding & unfolding in computational geometry: introduction joseph orourke smith college (many...
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Folding & Unfolding Folding & Unfolding in Computational in Computational
Geometry:Geometry:IntroductionIntroduction
Joseph O’RourkeJoseph O’RourkeSmith CollegeSmith College
(Many slides made by Erik (Many slides made by Erik Demaine)Demaine)
Folding and Unfolding in Folding and Unfolding in Computational GeometryComputational Geometry
1D: Linkages
2D: Paper
3D: Polyhedra
Preserve edge lengths Edges cannot cross
Preserve distances Cannot cross itself
Cut the surface while keeping it connected
CharacteristicsCharacteristics
TangibleApplicableElementaryDeepFrontier Accessible
OutlineOutline
Topics:1D: Linkages2D: Paper3D: Polyhedra
Lectures ScheduleLectures Schedule
Sunday 7:30-8:30 0 Introduction and Overview
Monday 9:00-9:50 1 Part Ia: Linkages and Universality
Monday 10:00-10:50 2 Part Ib: Pantographs and Pop-ups
Monday 1:30-2:30 Discussion
Monday 2:40-3:30 3 Part Ic: Locked Chains
Monday 3:40-4:30 4 Part IIa: Flat Origami
Tuesday 9:00-9:50 5 Part IIb: One-Cut Theorem
Tuesday 10:00-10:50 6 Part IIIa: Folding Polygons to Polyhedra
Tuesday 1:30-2:30 Discussion
Tuesday 2:40-3:30 7 Part IIIb: Unfolding Polyhedra to Nets
Tuesday 3:40-4:30 Guest Lecture: Jane Sangwine-Yeager
Wednesday 9:00-9:50 8 Part Id: Protein Folding: Fixed-angle Chains
Wednesday 10:00-10:50
9 Part Ie: Unit-Length Chains: Locked?
Thursday 9:00-9:50 10 Part IIc: Skeletons, Roofs, Medial Axis
Thursday 10:00-10:50 11 Part IId: Medial Axis Models
Friday 9:00-9:50 12 Part IIIc: Cauchy’s Rigidity Theorem
Friday 10:00-10:50 13 Part IIId: Bellows, Volume, Reconstruction
Outline: TonightOutline: Tonight
Topics:1D: Linkages2D: Paper3D: Polyhedra
Within each:DefinitionsOne “application”One open problem
OutlineOutline1 1 ― 1D: Linkages― 1D: Linkages
Definitions Configurations Locked chain in 3D Fixed-angle chains
Application: Protein foldingOpen Problem: unit-length locked
chains?
Linkages / FrameworksLinkages / Frameworks
Link / bar / edge = line segmentJoint / vertex = connection between
endpoints of bars
Closed chain / cycle / polygon
Open chain / arc
Tree General
ConfigurationsConfigurations
Configuration = positions of the vertices that preserves the bar lengths
Non-self-intersecting configurations Self-intersecting
Non-self-intersecting = No bars cross
Locked QuestionLocked Question
Can a linkage be moved between any twonon-self-intersecting configurations?
?
Can any non-self-intersecting configuration be unfolded, i.e., moved to “canonical” configuration?
Equivalent by reversing and concatenating motions
Canonical ConfigurationsCanonical Configurations
Chains: Straight configuration
Polygons: Convex configurations
Trees: Flat configurations
Locked 3D Chains Locked 3D Chains [Cantarella & [Cantarella & Johnston 1998; Johnston 1998; Biedl, Demaine, Demaine, Lazard, Lubiw, Biedl, Demaine, Demaine, Lazard, Lubiw, O’Rourke, Overmars, Robbins, Streinu, Toussaint, O’Rourke, Overmars, Robbins, Streinu, Toussaint, Whitesides 1999]Whitesides 1999]
Cannot straighten some chains, even with universal joints.
Locked 2D TreesLocked 2D Trees[Biedl, Demaine, Demaine, Lazard, Lubiw, O’Rourke, [Biedl, Demaine, Demaine, Lazard, Lubiw, O’Rourke, Robbins, Streinu, Toussaint, Whitesides 1998]Robbins, Streinu, Toussaint, Whitesides 1998]
Theorem: Not all trees can be flattened No petal can be opened unless all others are
closed significantly No petal can be closed more than a little
unless it has already opened
Can Chains Lock?Can Chains Lock?
Can every chain, with universal joints, be straightened?
Chains Straightened?
2D Yes
3DNo:
some locked
4D & beyond
Yes“Polygonal Chains Cannot Lock in 4D.”Roxana Cocan and J. O'RourkeComput. Geom. Theory Appl., 20 (2001) 105-129.
OpenOpen11: Can Equilateral Chains : Can Equilateral Chains Lock?Lock?
Does there exist an open polygonal chain embedded in 3D, with all links of equal length, that is locked?
ProteinProteinFoldingFolding
Protein FoldingProtein Folding
Fixed-angle chainFixed-angle chain
FlattenableFlattenable
A configuration of a chain if flattenable if it can be reconfigured, without self-intersection, so that it lies flat in a plane.
Otherwise the configuration is unflattenable, or locked.
Unflattenable fixed-angle Unflattenable fixed-angle chainchain
Open ProblemsOpen Problems11 : : Locked Equilateral Locked Equilateral Chains?Chains?
(1)Is there a configuration of a chain with universal joints, all of whose links have the same length, that is locked?
(2)Is there a configuration of a 90o fixed-angle chain, all of whose links have the same length, that is locked?
Perhaps: No?
Perhaps: Yes for 1+?
OutlineOutline2 2 ― 2D: Paper― 2D: Paper
Definitions Foldings Crease patterns
Application: Map FoldingOpen Problem: Complexity of Map
Folding
FoldingsFoldings
Piece of paper = 2D surface Square, or polygon, or polyhedral surface
Folded state = isometric “embedding” Isometric = preserve intrinsic distances
(measured alongpaper surface)
“Embedding” = no self-intersections exceptthat multiple surfacescan “touch” withinfinitesimal separation Flat origami crane
Nonflat folding
Structure of FoldingsStructure of Foldings
Creases in folded state =discontinuities in the derivative
Crease pattern = planar graph drawn with straight edges (creases) on the paper, corresponding tounfolded creases
Mountain-valleyassignment = specifycrease directions as or
Nonflat folding
Flat origami crane
Map FoldingMap Folding
Motivating problem: Given a map (grid of unit squares),
each crease marked mountain or valley Can it be folded into a packet
(whose silhouette is a unit square)via a sequence of simple folds?
Simple fold = fold along a line
1 6 72 5 83 4 9
Map FoldingMap Folding
Motivating problem: Given a map (grid of unit squares),
each crease marked mountain or valley Can it be folded into a packet
(whose silhouette is a unit square)via a sequence of simple folds?
Simple fold = fold along a line
1 6 72 5 83 4 9
Easy?Easy?
Hard?Hard?
Map FoldingMap Folding
Motivating problem: Given a map (grid of unit squares),
each crease marked mountain or valley Can it be folded into a packet
(whose silhouette is a unit square)via a sequence of simple folds?
Simple fold = fold along a line
2 5 83 4 9
1 6 7
Map FoldingMap Folding
Motivating problem: Given a map (grid of unit squares),
each crease marked mountain or valley Can it be folded into a packet
(whose silhouette is a unit square)via a sequence of simple folds?
Simple fold = fold along a line
2 5 8
1 76
Map FoldingMap Folding
Motivating problem: Given a map (grid of unit squares),
each crease marked mountain or valley Can it be folded into a packet
(whose silhouette is a unit square)via a sequence of simple folds?
Simple fold = fold along a line
1 76
Map FoldingMap Folding
Motivating problem: Given a map (grid of unit squares),
each crease marked mountain or valley Can it be folded into a packet
(whose silhouette is a unit square)via a sequence of simple folds?
Simple fold = fold along a line
76
Map FoldingMap Folding
Motivating problem: Given a map (grid of unit squares),
each crease marked mountain or valley Can it be folded into a packet
(whose silhouette is a unit square)via a sequence of simple folds?
Simple fold = fold along a line
69
More generally: Given an arbitrary crease pattern, is it flat-foldable by simple folds?
OpenOpen22: Map Folding : Map Folding Complexity?Complexity?
Given a rectangular map, with designated mountain/valley folds in a regular grid pattern, how difficult is it to decide if there is a folded state of the map realizing those crease patterns?
OutlineOutline3 3 ― 3D: Polyhedra― 3D: Polyhedra
Edge-Unfolding Definitions
Cut tree: spanning treeNet
Applications: Manufacturing Open Problem: Does every polyhedron
have a net?
Unfolding PolyhedraUnfolding Polyhedra
Cut along the surface of a polyhedron
Unfold into a simple planar polygon without overlap
Edge UnfoldingsEdge Unfoldings
Two types of unfoldings: Edge unfoldings: Cut only along edges General unfoldings: Cut through faces too
Cut Edges form Spanning Cut Edges form Spanning TreeTree
Lemma: The cut edges of an edge unfolding of a convex polyhedron to a simple polygon form a spanning tree of the 1-skeleton of the polyhedron.
o spanning: to flatten every vertexo forest: cycle would isolate a surface pieceo tree: connected by boundary of polygon
Commercial SoftwareCommercial Software
Lundström Design, http://www.algonet.se/~ludesign/index.html
OpenOpen33: Edge-Unfolding Convex : Edge-Unfolding Convex PolyhedraPolyhedra
Does every convex polyhedron have an edge-unfolding to a net (a simple, nonoverlapping polygon)?
[Shephard, 1975]
Archimedian SolidsArchimedian Solids
Nets for Archimedian SolidsNets for Archimedian Solids
Cube with one corner Cube with one corner truncatedtruncated
SclickenriederSclickenrieder11::steepest-edge-unfoldsteepest-edge-unfold
“Nets of Polyhedra”TU Berlin, 1997
SclickenriederSclickenrieder33::rightmost-ascending-edge-rightmost-ascending-edge-unfoldunfold
OpenOpen33: Edge-Unfolding Convex : Edge-Unfolding Convex PolyhedraPolyhedra
Does every convex polyhedron have an edge-unfolding to a net (a simple, nonoverlapping polygon)?
[Shephard, 1975]