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**Part II: Paper a: Flat Origami**

Joseph O’Rourke Smith College (Many slides made by Erik Demaine)

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**Outline Single-vertex foldability Continuous Foldability**

Kawaskai’s Theorem Maekawa’s Theorem Justin’s Theorem Continuous Foldability Map Folding (revisited)

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**Foldings Piece of paper = 2D surface**

Square, or polygon, or polyhedral surface Folded state = isometric “embedding” Isometric = preserve intrinsic distances (measured along paper surface) “Embedding” = no self- intersections except that multiple surfaces can “touch” with infinitesimal separation Nonflat folding Flat origami crane

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Foldings Configuration space of piece of paper = uncountable-dim. space of all folded states Folding motion = path in this space = continuum of folded states Fortunately, configuration space of a polygonal paper is path-connected [Demaine, Devadoss, Mitchell, O’Rourke 2004] Focus on finding interesting folded state

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Structure of Foldings Creases in folded state = discontinuities in the derivative Crease pattern = planar graph drawn with straight edges (creases) on the paper, corresponding to unfolded creases Mountain-valley assignment = specify crease directions as or Nonflat folding Flat origami crane

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**Flat Foldings of Single Sheets of Paper**

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**Single-Vertex Origami**

Consider a disk surrounding a lone vertex in a crease pattern (local foldability) When can it be folded flat? Depends on Circular sequence of angles between creases: Θ1 + Θ2 + … + Θn = 360° Mountain-valley assignment

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**Single-Vertex Origami without Mountain-Valley Assignment**

Kawasaki’s Theorem: Without a mountain-valley assignment, a vertex is flat-foldable precisely if sum of alternate angles is 180° (Θ1 + Θ3 + … + Θn−1 = Θ2 + Θ4 + … + Θn) Tracing disk’s boundary along folded arc moves Θ1 − Θ2 + Θ3 − Θ4 + … + Θn−1 − Θn Should return to starting point equals 0

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**Single-Vertex Origami with Mountain-Valley Assignment**

Maekawa’s Theorem: For a vertex to be flat-foldable, need |# mountains − # valleys| = 2 Total turn angle = ±360° = 180° × # mountains − 180° × # valleys

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**Single-Vertex Origami with Mountain-Valley Assignment**

Kawasaki-Justin Theorem: If one angle is smaller than its two neighbors, the two surrounding creases must have opposite direction Otherwise, the two large angles would collide These theorems essentially characterize all flat foldings

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**Local Flat Foldability**

Locally flat-foldable crease pattern = each vertex is flat-foldable if cut out = flat-foldable except possibly for nonlocal self-intersection Testable in linear time [Bern & Hayes 1996] Check Kawasaki’s Theorem Solve a kind of matching problem to find a valid mountain-valley assignment, if one exists Barrier:

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**Global Flat Foldability**

Testing (global) flat foldability is strongly NP-hard [Bern & Hayes 1996] Wire represented by “crimp” direction: False True T T F T F T T T F T F T self-intersects Not-all-equal 3-SAT clause

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Continuous Rolling

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**Continuous Foldability**

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Models of Simple Folds A single line can admit several different simple folds, depending on # layers folded Extremes: one-layer or all-layers simple fold In general: some-layers simple fold Example in 1D:

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**Simple Foldability [Arkin, Bender, Demaine, Demaine, Mitchell, Sethia, Skiena 2001]**

= = =

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Open Problems Open: Orthogonal creases on non-axis-aligned rectangular piece of paper? Open (Edmonds): Complexity of deciding whether an m × n grid can be folded flat (has a flat folded state) with specified mountain-valley assignment Even 2xN maps open! Open: What about orthogonal polygons with orthogonal creases, etc.? NP-Complete?

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