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**CSE325 Computers and Sculpture**

Prof. George Hart

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**Symmetry Intuitive notion – mirrors, rotations, …**

Mathematical concept — set of transformations Possible 2D and 3D symmetries Sculpture examples: M.C. Escher sculpture Carlo Sequin’s EscherBall program Constructions this week based on symmetry

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**Intuitive uses of “symmetry”**

left side = right side Human body or face n-fold rotation Flower petals Other ways?

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**Mathematical Definition**

Define geometric transformations: reflection, rotation, translation (“slide”), glide reflection (“slide and reflect”), identity, … A symmetry is a transformation The symmetries of an object are the set of transformations which leave object looking unchanged Think of symmetries as axes, mirror lines, …

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**Frieze Patterns Imagine as infinitely long.**

Each frieze has translations. A smallest translation “generates” all translations by repetition and “inverse”. Some have vertical mirror lines. Some have a horizontal mirror. Some have 2-fold rotations. Analysis shows there are exactly seven possibilities for the symmetry.

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**Wallpaper Groups Include 2 directions of translation**

Might have 2-fold, 3-fold, 6-fold rotations, mirrors, and glide-reflections 17 possibilities Several standard notations. The following slides show the “orbifold” notation of John Conway.

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Wallpaper Groups o 2222 xx ** *2222 22*

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Wallpaper Groups *442 x* 22x 2*22 442 4*2

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Wallpaper Groups 333 *333 3*3 Images by Xah Lee 632 *632

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3D Symmetry Three translation directions give the 230 “crystallographic space groups” of infinite lattices. If no translations, center is fixed, giving the 14 types of “polyhedral groups”: 7 families correspond to a rolled-up frieze Symmetry of pyramids and prisms Each of the seven can be 2-fold, 3-fold, 4-fold,… 7 correspond to regular polyhedra

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**Roll up a Frieze into a Cylinder**

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**Seven Polyhedra Groups**

Octahedral, with 0 or 9 mirrors Icosahedral, with 0 or 15 mirrors Tetrahedral, with 0, 3, or 6 mirrors Cube and octahedron have same symmetry Dodecahedron and icosahedron have same symmetry

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**Symmetries of cube = Symmetries of octahedron**

In “dual position” symmetry axes line up

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**Cube Rotational Symmetry**

Axes of rotation: Three 4-fold — through opposite face centers four 3-fold — through opposite vertices six 2-fold — through opposite edge midpoints Count the Symmetry transformations: 1, 2, or 3 times 90 degrees on each 4-fold axis 1 or 2 times 120 degrees on each 3-fold axis 180 degrees on each 2-fold axis Identity transformation = 24

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**Cube Rotations may or may not Come with Mirrors**

If any mirrors, then 9 mirror planes. If put “squiggles” on each face, then 0 mirrors

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**Icosahedral = Dodecahedral Symmetry**

Six 5-fold axes. Ten 3-fold axes. Fifteen 2-fold axes There are 15 mirror planes. Or squiggle each face for 0 mirrors.

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**Tetrahedron Rotations**

Four 3-fold axes (vertex to opposite face center). Three 2-fold axes.

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**Tetrahedral Mirrors Regular tetrahedron has 6 mirrors (1 per edge)**

“Squiggled” tetrahedron has 0 mirrors. “Pyrite symmetry” has tetrahedral rotations but 3 mirrors:

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**Symmetry in Sculpture People Sculpture (G. Hart)**

Sculpture by M.C. Escher Replicas of Escher by Carlo Sequin Original designs by Carlo Sequin

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People

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Candy Box M.C. Escher

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**Sphere with Fish M.C. Escher, 1940**

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**Carlo Sequin, after Escher**

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**Polyhedron with Flowers M.C. Escher, 1958**

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**Carlo Sequin, after Escher**

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**Sphere with Angels and Devils M.C. Escher, 1942**

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**Carlo Sequin, after Escher**

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M.C. Escher

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**Construction this Week**

Wormballs Pipe-cleaner constructions Based on one line in a 2D tessellation

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**The following slides are borrowed from**

Carlo Sequin

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**Escher Sphere Construction Kit**

Jane Yen Carlo Séquin UC Berkeley I3D 2001 [1] M.C. Escher, His Life and Complete Graphic Work

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**Introduction M.C. Escher graphic artist & print maker**

myriad of famous planar tilings why so few 3D designs? [2] M.C. Escher: Visions of Symmetry

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**Spherical Tilings Spherical Symmetry is difficult Hard to understand**

Hard to visualize Hard to make the final object [1]

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Our Goal Develop a system to easily design and manufacture “Escher spheres” - spherical balls composed of tiles provide visual feedback guarantee that the tiles join properly allow for bas-relief output for manufacturing of physical models

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**Interface Design How can we make the system intuitive and easy to use?**

What is the best way to communicate how spherical symmetry works? [1]

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**Spherical Symmetry The Platonic Solids R3 R5 tetrahedron octahedron**

cube dodecahedron icosahedron R3 R2 R5 R3

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**How the Program Works Choose a symmetry based on a Platonic solid**

Choose an initial tiling pattern to edit starting place Example: Tetrahedron R3 R2 R3 R2 R3 R2 Tile 1 Tile 2

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**Initial Tiling Pattern**

+ easier to understand consequences of moving points + guarantees proper tiling ~ requires user to select the “right” initial tile - can only make monohedral tiles [2] Tile 1 Tile 2 Tile 2

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**Modifying the Tile Insert and move boundary points**

system automatically updates the tile based on symmetry Add interior detail points

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**Adding Bas-Relief Stereographically projected and triangulated**

Radial offsets can be given to points individually or in groups separate mode from editing boundary points

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**Creating a Solid The surface is extruded radially**

inward or outward extrusion, spherical or detailed base Output in a format for free-form fabrication individual tiles or entire ball

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Video

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**Fabrication Issues Many kinds of manufacturing technology**

we use two types based on a layer-by-layer approach Fused Deposition Modeling (FDM) Z-Corp 3D Color Printer - parts made of plastic starch powder glued together each part is a solid color parts can have multiple colors assembly

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FDM Fabrication moving head Inside the FDM machine support material

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Z-Corp Fabrication de-powdering infiltration

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Results FDM

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Results FDM | Z-Corp

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Results FDM | Z-Corp

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Results Z-Corp

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**Conclusions Intuitive Conceptual Model Editing in Context**

symmetry groups have little meaning to user need to give the user an easy to understand starting place Editing in Context need to see all the tiles together need to edit the tile on the sphere editing in the plane is not good enough (distortions) Part Fabrication need limitations so that designs can be manufactured radial manipulation Future Work predefined color symmetry injection molded parts (puzzles) tessellating over arbitrary shapes (any genus)

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