Vector Algebra Transformations. Lecture 4

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1 Vector Algebra Transformations Lecture 4 Cornell CS4620 Fall 2008 Lecture Steve Marschner 1

2 Geometry A part of mathematics concerned with questions of size, shape, and relative positions of figures Coordinates are used to represent points and vectors a naming scheme The same point can be described by different coordinates Points vectors (60,-40) Coordinates [KAIST] 2

3 3

4 Geometry Linear Algebra Scalar Vector Linear independence Linear transformations Frames (=Coordinates) Points Vectors Affine transformations Translation Rotation 4

5 y z [a b c]t v x 5

6 6

7 t denotes transpose 7

8 Transforming geometry Move a subset of the space (in 2D case, plane) using a mapping from the plane to itself 2008 Steve Marschner 8

9 Linear transformations One way to define a transformation is by matrix multiplication: Such transformations are linear, which is to say: (and in fact all linear transformations can be written this way) 2008 Steve Marschner 9

10 What is a Matrix? A matrix is a set of elements, organized into rows and columns rows columns a b c d

11 Matrix operating on vectors Matrix is like a function that transforms the vectors on a plane Matrix operating on a general point => transforms x and y components System of linear equations: matrix is just the bunch of coefficients x = ax + by y = cx + dy a b x x' c d y y'

12 Bases & Orthonormal Bases Basis vectors (or axes): frame of reference Any point in a space is a linear combination of the basis vectors Usually, orthonormal matrices are used for defining coordinate frames Ortho Normal: orthogonal + normal [Orthogonal: dot product is zero Normal: magnitude is one ] x T y z T T x y 0 x z 0 y z 0

13 Geometry of 2D linear trans. 2x2 matrices have simple geometric interpretations uniform scale non-uniform scale rotation shear reflection 2008 Steve Marschner 13

14 Linear transformation gallery Uniform scale 2008 Steve Marschner 14

15 Linear transformation gallery Nonuniform scale 2008 Steve Marschner 15

16 Linear transformation gallery Rotation 2008 Steve Marschner 16

17 Matrices: Scaling, Rotation, Identity Scaling without rotation => diagonal matrix Rotation without stretching => orthonormal matrix O Identity ( do nothing ) matrix = unit scaling, no rotation r1 0 0 r2 [0,1]T scaling [0,r2]T [r1,0]t [1,0]T cos -sin sin cos [0,1]T rotation [1,0]T [-sin, cos ]T [cos, sin ]T

18 Linear transformation gallery Reflection can consider it a special case of nonuniform scale 2008 Steve Marschner 18

19 Linear transformation gallery Shear 2008 Steve Marschner 19

20 Composing transformations Want to move an object, then move it some more We need to represent S o T ( S compose T ) and would like to use the same representation as for S and T 2008 Steve Marschner 20

21 Composing transformations Composing linear transformations is straightforward Transforming first by MT then by MS is the same as transforming by MSMT only sometimes commutative e.g. 2D rotations & uniform scales e.g. non-uniform scales w/o rotation Note MSMT, or S o T, is T first, then S 2008 Steve Marschner 21

22 Translation P t P P ' ( x t x, y t y ) P t ty y P t P x tx

23 Composing translations Composing translations is easy Translation by ut then by us is translation by ut + us commutative! 2008 Steve Marschner 23

24 Is translation linear transformation? Translation is the simplest transformation: Inverse: 2008 Steve Marschner 24

25 Is translation linear transformation? No! because T(v)= I v + u!= I v Affine transformation T(v)=Mv + u 2008 Steve Marschner 25

26 Affine transformations straight lines preserved; parallel lines preserved ratios of lengths along lines preserved (midpoints preserved) Origin 2008 Steve Marschner 26

27 Combining linear with translation Need to use both in single framework Can represent arbitrary seq. as e. g. Transforming by MT and ut, then by MS and us, is the same as transforming by MSMT and MsuT+ us This will work but is a little awkward 2008 Steve Marschner 27

28 Homogeneous coordinates A trick for representing the foregoing more elegantly Extra component w for vectors, extra row/column for matrices for affine, can always keep w = 1 Represent linear transformations with dummy extra row and column 2008 Steve Marschner 28

29 Homogeneous coordinates Represent translation using the extra column 2008 Steve Marschner 29

30 Homogeneous coordinates Composition just works, by 3x3 matrix multiplication This is exactly the same result but cleaner and generalizes in useful ways as we ll see later 2008 Steve Marschner 30

31 Homogeneous coordinates in 3D (Affine transformation) ( v 1x v 2x v 3x o x c 1 v x c +o x v 1y v 2y v 3y o y c 2 v y c+o y V c +o = = 1 v 1z v 2z v 3z o z c 3 v z c+o z )( ) ( ) transformation coordinates ( ) 31

32 32

33 Coordinates : 33

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36 2D Affine transformation gallery Translation 2008 Steve Marschner 36

37 Affine transformation gallery Uniform scale 2008 Steve Marschner 37

38 Affine transformation gallery Nonuniform scale 2008 Steve Marschner 38

39 Affine transformation gallery Rotation 2008 Steve Marschner 39

40 Affine transformation gallery Reflection can consider it a special case of nonuniform scale 2008 Steve Marschner 40

41 Affine transformation gallery Shear 2008 Steve Marschner 41

42 Composite affine transformations In general not commutative: order matters! rotate, then translate translate, then rotate 2008 Steve Marschner 42

43 Composite affine transformations Another example scale, then rotate rotate, then scale 2008 Steve Marschner 43

44 Composing to change axes Want to rotate about a particular point could work out formulas directly Know how to rotate about the origin so translate that point to the origin 2008 Steve Marschner 44

45 Composing to change axes Want to rotate about a particular point could work out formulas directly Know how to rotate about the origin so translate that point to the origin 2008 Steve Marschner 45

46 Composing to change axes Want to rotate about a particular point could work out formulas directly Know how to rotate about the origin so translate that point to the origin 2008 Steve Marschner 46

47 Composing to change axes Want to scale along a particular axis and point Know how to scale along the y axis at the origin so translate to the origin and rotate to align axes 2008 Steve Marschner 47

48 Composing to change axes Want to scale along a particular axis and point Know how to scale along the y axis at the origin so translate to the origin and rotate to align axes 2008 Steve Marschner 48

49 Composing to change axes Want to scale along a particular axis and point Know how to scale along the y axis at the origin so translate to the origin and rotate to align axes 2008 Steve Marschner 49

50 Composing to change axes Want to scale along a particular axis and point Know how to scale along the y axis at the origin so translate to the origin and rotate to align axes 2008 Steve Marschner 50

51 Composing to change axes Want to scale along a particular axis and point Know how to scale along the y axis at the origin so translate to the origin and rotate to align axes 2008 Steve Marschner 51

52 Composing to change axes Want to scale along a particular axis and point Know how to scale along the y axis at the origin so translate to the origin and rotate to align axes 2008 Steve Marschner 52

53 More math background Coordinate systems Expressing vectors with respect to bases Linear transformations as changes of basis 2008 Steve Marschner 53

54 Affine change of coordinates Six degrees of freedom or 2008 Steve Marschner 54

55 Affine change of coordinates A new way to read off the matrix e.g. shear from earlier can look at picture, see effect on basis vectors, write down matrix Also an easy way to construct transforms e. g. scale by 2 across direction (1,2) 2008 Steve Marschner 55

56 Rigid motions (Proper Euclidean space) A transform made up of only translation and rotation is a rigid motion or a rigid body transformation The linear part is an orthonormal matrix Inverse of orthonormal matrix is transpose so inverse of rigid motion is easy: 2008 Steve Marschner 56

57 Transforming points and vectors Recall distinction points vs. vectors vectors are just offsets (differences between points) points have a location represented by vector offset from a fixed origin Points and vectors transform differently points respond to translation; vectors do not 2008 Steve Marschner 57

58 Transforming points and vectors Homogeneous coords. let us exclude translation just put 0 rather than 1 in the last place and note that subtracting two points cancels the extra coordinate, resulting in a vector! 2008 Steve Marschner 58

59 59

60 60

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62 Note that = 62

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68 68

69 Vectors: Cross Product The cross product of vectors A and B is a vector C which is perpendicular to A and B The magnitude of C is proportional to the sin of the angle between A and B The direction of C follows the right hand rule if we are working in a right-handed coordinate system A B B A A B A B sin( )

70 MAGNITUDE OF THE CROSS PRODUCT

71 DIRECTION OF THE CROSS PRODUCT The right hand rule determines the direction of the cross product

72 72

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