you must do things you think you cannot do. eleanor...
TRANSCRIPT
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You must do things you think you cannot do.
Eleanor Roosevelt
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School of the Art Institute of Chicago
Geometry of
Art and Nature
Frank Timmes
flash.uchicago.edu/~fxt/class_pages/class_geom.shtml
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Syllabus
1 Sept 03 Basics and Celtic Knots
2 Sept 10 Golden Ratio
3 Sept 17 Fibonacci and Phyllotaxis
4 Sept 24 Regular and Semiregular tilings
5 Oct 01 Irregular tilings
6 Oct 08 Rosette and Frieze groups
7 Oct 15 Wallpaper groups
8 Oct 22 Platonic solids
9 Oct 29 Archimedian solids
10 Nov 05 Non-Euclidean geometries
11 Nov 12 Bubbles
12 Dec 03 Fractals
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• www2.spsu.edu/math/tile/
• www.clarku.edu/~djoyce/wallpaper/seventeen.html
• hverrill.net/Wallpaper/wallindex.html
• nothung.math.uh.edu/~patterns/
Sites of the Week
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Class #7
• Plane Symmetry
• There are only 17 wallpaper patterns
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Wallpaper
• A wallpaper pattern (also called a plane crystallographic group) is a pattern with translation symmetry in two directions.
p4m
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• They are essentially an arrangement of frieze patterns stacked upon one another to fill the plane.
pmm
2000, Alberto Erazo
Wallpaper
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• Any particular pattern is made from a combination of the four planar isometries: translation, rotation, reflection, and glide reflection.
p6m
2000, Alberto Erazo
Wallpaper
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• There are exactly 17 different wallpaper patterns.
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• I don’t expect you to memorize these 17, but I do expect you to learn some of the principles that would allow you to classify them, or create designs with them.
Malahide wallpaper
Wallpaper
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Notation I
• The 17 wallpaper patterns are “named” by their crystallographic notation.
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• Each name is four characters long: _ _ _ _
p111
p211
p1m1
p1g1
c1m1
p2mm
p2mg
p2gg
c2mm
p411
p4mm
p4gm
p311
p3m1
p31m
Notation I
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p111
p211
p1m1
p1g1
c1m1
p2mm
p2mg
p2gg
c2mm
p411
p4mm
p4gm
p311
p3m1
p31m
• The first blank is filled by p or c, for the type of cell.
• Lattices of parallelograms, rectangles, squares, and hexagons have p (primitive), while a rhombic lattice gets c (centered).
Notation I
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• The second blank is filled with the highest n-fold rotation. This could be 1 for no rotation, 2 for 2-fold (180º) rotation, 3 for a 3-fold (120º) rotation, 4 for a 4-fold (90º) rotation, or 6 for a 60º rotation.
p111
p211
p1m1
p1g1
c1m1
p2mm
p2mg
p2gg
c2mm
p411
p4mm
p4gm
p311
p3m1
p31mcmm
2000, Fay Pirooz
Notation I
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• The third blank is filled with m if there is a vertical mirror, g if there is a glide reflection, and 1 otherwise.
p111
p211
p1m1
p1g1
c1m1
p2mm
p2mg
p2gg
c2mm
p411
p4mm
p4gm
p311
p3m1
p31mpgg
2000, Philip Tolar
Notation I
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• The fourth blank is filled with m if there is a mirror at an angle consistent with the second blank, g if it is a glide reflection at an angle consistent with the second blank, and 1 otherwise.
p111
p211
p1m1
p1g1
c1m1
p2mm
p2mg
p2gg
c2mm
p411
p4mm
p4gm
p311
p3m1
p31mp6m
2000, Amy Labus
Notation I
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p111
p211
p1m1
p1g1
c1m1
p2mm
p2mg
p2gg
c2mm
p411
p4mm
p4gm
p311
p3m1
p31m
• In all cases, 1 designates no to the question asked for each blank.
p4
2000, Diane La Franca
Notation I
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P1
• This is the simplest symmetry group.
• It consists only of translations. The two translation axes may be inclined at any angle to each other.
• Its base tile and lattice are any parallelogram.
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P1
• For many actual wallpaper patterns, translations are the only isometries that leave the pattern invariant.
Egyptian
Stained Glass in Cathedral
of Bourges, Middle ages
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P1
• Instead, the pattern is raised or lowered on adjacent wallpaper strips.
• Usually, horizontal translations are not invariant on commercial wallpaper.
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P2
• This group differs only from P1 in that it contains 180° rotations, that is, rotations of order 2, also called "half-turns.”
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• The two translation axes may be inclined at any angle to each other. Its lattice is any parallelogram.
Egyptian
Egyptian
P2
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• In this example, as in all patterns with this symmetry, there are four essentially different centers of half-turns.
• It may be a little difficult to see the half-turns; they’re subtle.
P2
2000, Luke Grochocki
P2
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Long form Short form Lattice
p111 p1 Parallelogram
p211 p2 Parallelogram
p1m1 pm Rectangle
p1g1 pg Rectangle
c1m1 cm Rhombus
p2mm pmm Rectangle
p2mg pmg Rectangle
p2gg pgg Rectangle
c2mm cmm Rhombus
p411 p4 Square
p4mm p4m Square
p4gm p4g Square
p311 p3 Hexagon
p3m1 p3m1 Hexagon
p31m p31m Hexagon
• Most of the time you’ll see the short form of the wallpaper name rather than the long form.
• The short form of the name is derived by dropping any irrelevant or redundant characters from the long form.
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PM
• This is our first group with reflections.
• The axes of reflection are parallel to one axis of translation and perpendicular to the other axis of translation.
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PM
• The base tile is one-half the lattice unit.
Hindu Chinese
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PM
• Reflections are usually easy to see; humans almost seem to be designed to recognize them, although it helps if the axis of reflection is vertical.
pm
2000, Alberto Erazo
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PG
• This is the first group containing glide reflections.
• The axes of the glide reflection are parallel to one axis of translation and perpendicular to the other axis of translation.
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• Half-turns were hard to see, but glide reflections can be even harder to see.
PG
From a damask chair cover
in Kent, England; Elizabethan
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PG
pg
2000, Ray Ogar
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CM
• This group contains reflections and glide-reflections with parallel axes.
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• The translations may be inclined at any angle to each other, but the axes of the reflections bisect the angle formed by the translations, so the lattice for the translation group is a rhombus.
CM
From a drapery in a tomb
at Westminster; Elizabethan
From St. Denis, France,
12th century, Byzantine
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• A base tile for the symmetry group is half the rhombus.
CM
cm
2000, Mike Field
and John Nguyen
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PMM
• This symmetry group contains reflections whose axes are perpendicular.
• There are no glide-reflections, except those which have the same axis as an axis of reflection.
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PMM
• The only rotations are half-turns whose fixed points lie at intersections of axes of reflection.
Contemporary fabric
Egyptian
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PMM
• The lattice is rectangular and the base tile is a quarter-rectangle.
pmm
2000, Mike Field
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Long form Short form Lattice
p111 p1 Parallelogram
p211 p2 Parallelogram
p1m1 pm Rectangle
p1g1 pg Rectangle
c1m1 cm Rhombus
p2mm pmm Rectangle
p2mg pmg Rectangle
p2gg pgg Rectangle
c2mm cmm Rhombus
p411 p4 Square
p4mm p4m Square
p4gm p4g Square
p311 p3 Hexagon
p3m1 p3m1 Hexagon
p31m p31m Hexagon
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PMG
• This group has both reflections (with parallel axes) and half-turns.
• The fixed points of the half-turns do not lie on the axes of reflection.
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PMG
• The lattice is rectangular, and the base tile is a quarter-rectangle.
pmg
2000, Diane La Franca
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PGG
• This group contains no reflections, but it has glide-reflections and half-turns.
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PGG
• There are perpendicular axes for the glide-reflections, and the fixed points of the half-turns do not lie on these axes.
Arabic
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PGG
• The lattice is rectangular, and the base tile is a quarter-rectangle.
Street Paving,
Chattanooga, Tennessee
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CMM
• This group has perpendicular reflection axes and half-turns.
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• Some centers of the half-turns lie at intersections of the axes of reflection, but others lie at intersections of the axes of the glide reflections.
CMM
cmm
2000, Mike Field
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CMM
• The lattice is rhombic, and the base tile is a quarter of the rhombus.
Indian
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P4
• This is the first group with a 90° rotation, that is, a rotation of order 4. It also has rotations of order 2, that is, half-turns.
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P4
• The centers of the half-turns are midway between the centers of the order-4 rotations.
Moorish
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P4
• The lattice is square, and a quarter of a square is the base tile.
Street Paving,
Chattanooga, Tennessee
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P4M
• This group differs from p4 in that it also has reflections.
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P4M
• The axes of reflection are inclined to each other by 45° so that four axes of reflection pass through the centers of the order-4 rotations.
Persian Persian
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P4M
• The lattice is square, and a triangular eighth of the square is the base tile.
Patio tiles at
Pensacola Beach, Florida
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P4G
• Like p4m, this group also contains reflections and rotations of orders 2 and 4.
• But the axes of reflection are perpendicular (none at 45°), and none of the centers of the rotations of order 4 lie on the reflection axes.
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P4G
• The lattice is a square, and a triangular eighth of the square is the base tile.
Chinese
p4g
2000, Fay Pirooz
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• This is the first group that contains a 120° rotation, that is, a rotation of order 3, and the first one whose lattice is hexagonal.
P3
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P3
• The base tile is a rhombus, one third of the hexagon lattice.
Arab p3
1981, John Locke
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P3M1
• This group contains reflections whose axes are inclined at 60° to one another and rotations of order 3.
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P3M1
• Some of the centers of rotation lie on the reflection axes, and some do not.
Chinese Persian
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P3M1
• The lattice is hexagonal, and the base tile is an equilateral triangle, one sixth of the hexagon.
p3m1
2000, Alberta Erazo
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P31M
• This group is similar to p3m1 in that it contains reflections and order-3 rotations.
• The axes of the reflections are again inclined at 60° to one another, but for this group all of the centers of rotation lie on the reflection axes.
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P31M
Chinese
• The lattice is hexagonal, and the base tile is a 30º - 30º - 120º triangle.
Chinese
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P6
• This group contains 60° rotations, and rotations of orders 2 and 3, but neither reflections nor glide reflections.
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P6
MoorishPersian
• The lattice is hexagonal, with the base tile being an equilateral triangle.
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P6
p6
2000, Mike Field
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P6M
• This most complicated group has rotations of order 2, 3, and 6 as well as reflections.
• The axes of reflection meet at all the centers of rotation.
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P6M
• At the centers of the order-6 rotations, six reflection axes meet and are inclined at 30° to one another.
Byzantine Chinese
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P6M
• The lattice is hexagonal and the base tile is a 30º - 60º - 90º triangle, one twelfth of a hexagon.
p6m
1998, Shelly Newberg
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In today’s in-class construction, you’ll play with the lattice patterns that fascinated a young M.C. Escher.