Coordinate Transformation



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Coordinate Transformation Coordinate Transformations In this chater, we exlore maings where a maing is a function that "mas" one set to another, usually in a way that reserves at least some of the underlyign geometry of the sets. For examle, a -dimensional coordinate transformation is a maing of the form T (u; v) hx (u; v) ; y (u; v)i The functions x (u; v) and y (u; v) are called the comonents of the transformation. Moreover, the transformation T mas a set S in the uv-lane to a set T (S) in the xy-lane: If S is a region, then we use the comonents x f (u; v) and y g (u; v) to nd the image of S under T (u; v) : EXAMPLE 1 Find T (S) when T (u; v) uv; u v and S is the unit square in the uv-lane (i.e., S [0; 1] [0; 1]). Solution: To do so, let s determine the boundary of T (S) in the xy-lane. We use x uv and y u v to nd the image of the lines bounding the unit square: Side of Square Result of T (u; v) Image in xy-lane v 0; u in [0; 1] x 0; y u ; u in [0; 1] y-axis for 0 y 1 u 1; v in [0; 1] x v; y 1 v ; v in [0; 1] y 1 x ; x in [0; 1] v 1; u in [0; 1] x u; y u 1; u in [0; 1] y x 1; x in [0; 1] u 0; u in [0; 1] x 0; y v ; v in [0; 1] y-axis for 1 y 0 1

As a result, T (S) is the region in the xy-lane bounded by x 0; y x 1; and y 1 x : Linear transformations are coordinate transformations of the form T (u; v) hau + bv; cu + dvi where a; b; c; and d are constants. Linear transformations are so named because they ma lines through the origin in the uv-lane to lines through the origin in the xy-lane. If each oint (u; v) in the uv-lane is associated with a column matrix, [u; v] t ; then the linear transformation T (u; v) hau + bv; cu + dvi can be written in matrix form as u a b u T v c d v The matrix of coe cients a; b; c; d is called the matrix of the transformation. EXAMPLE Find the image of the unit square under the linear transformation u 1 u T v 1 1 v Solution: Since linear transformations ma straight lines to straight lines, we need only nd the images of the 4 vertices of the unit square. To begin with, the oint (0; 0) is maed to (0; 0) : Associating the oint (1; 0) to the column vector [1; 0] t yields 1 1 1 T 0 1 1 0 1

Thus, the oint (1; 0) is maed to the oint (3; associating (0; 1) with [0; 1] t leads to 0 1 0 1 T 1 1 1 1 1 ) : Likewise, and associating (1; 1) with [1; 1] t leads to 1 1 1 3 T 1 1 1 1 That is, (0; 1) is maed to (1; 1) and (1; 1) is maed to (3; ) : Thus, the unit square in the uv-lane is maed to the arallelogram in the xy-lane with vertices (0; 0) ; (; 1) ; (1; 1) ; and (3; ) : Check your Reading: Is the entire u-axis maed to 0 by T (u; v) hv cos (u) ; v sin (u)i? Coordinate Systems Coordinate transformations are often used to de ne often used to de ne new coordinate systems on the lane. The u-curves of the transformation are the images of vertical lines of the form u constant and the v-curves are images of horizontal lines of the form v constant. Together, these curves are called the coordinate curves of the transformation. 3

EXAMPLE 3 Find the coordinate curves of T (u; v) uv; u v Solution: The u-curves are of the form u k where k is constant. Thus, x kv; y k v so that v xk and thus, y x k + k which is a family of straight lines with sloe 1k and intercet k : The v-curves are of the form v c; where c is a constant. Thus, x uc and y u c: Since u xc; the v-curves are of the form y x c which is a family of arabolas oening uwards with vertices on the y-axis. c A coordinate transformation T (u; v) is said to be 1-1 on a region S in the uvlane if each oint in T (S) corresonds to only one oint in S: The air (u; v) in S is then de ned to be the coordinates of the oint T (u; v) in T (S) : For examle, in the next section we will exlore the olar coordinate transformation T (r; ) hr cos () ; r sin ()i or equivalently, x r cos () and y r sin () : 4

EXAMPLE 4 What are the coordinate curves of the olar coordinate transformation T (r; ) hr cos () ; r sin ()i Solution: The r-curves are of the form r R for R constant. If r R; then x R cos () ; y R sin () As a result, x + y R cos () + R sin () R ; which is the same as x + y R : Thus, the r-curves are circles of radius R centered at the origin. The -curves, in which c for c constant, are given by x r cos (c) ; y r sin (c) As a result, we have y r sin (c) tan (c) x r cos (c) which is the same as y kx with k tan (c) : Thus, the -curves are lines through the origin of the xy-lane. Since corresonds to angles, the olar coordinate transformation is not 1-1 in general. However, if we restrict to [0; ) and require that r > 0, then the olar coordinate transformation is 1-1 onto the xy-lane omitting the origin. 5

Check your Reading: What oint corresonds to r 0 in examle 4? Rotations About the Origin Rotations about the origin through an angle are linear transformations of the form T (u; v) hu cos () v sin () ; u sin () + v cos ()i (1) The matrix of the rotation through an angle is given by cos () sin () R () sin () cos () given that ositive angles are those measured counterclockwise (see the exercises). EXAMPLE 5 Rotate the triangle with vertices (0; 0) ; (; 0) ; and (0; ) through an angle 3 about the origin. Solution: To begin with, the matrix of the rotation is cos R () 3 sin 3 1 3 sin 3 cos 3 3 1 so that the resulting linear transformation is given by u 1 3 T u v 3 1 v The oint (0; 0) is maed to (0; 0). The oint (; 0) is associated with [; 0] t ; so that T 0 1 3 3 1 0 3 1 That is, (; 0) is maed to 1; 3 : Similarly, it can be shown that (0; ) is maed to 3; 1 : 6

Often rotations are used to ut gures into standard form, and often this requires rotating a line y mx onto the x-axis. If we notice that m tan () ; then it follows that 1 cos () m +1 sin () m m +1 Thus, the rotation matrix for rotating the x-axis to the line y mx is cos () sin () 1 1 m sin () cos () m + 1 m 1 () Conversely, rotation through an angle will rotate y mx to the x-axis (and corresonds to using m in lace of m in () ). blueexample 6 blackrotate the triangle with vertices at (0; 0) ; (1; ) ; and ( 4; ) so that one edge lies along the x-axis. Solution: The line through (0; 0) and (1; ) is y x; which imlies that the rotation matrix is cos ( ) sin ( ) 1 1 sin ( ) cos ( ) + 1 1 Thus, the oint (1; ) is maed to 1 T 1 1 1 5 1 5 0 7

while the oint ( 1; 4) is maed to 4 T 1 1 5 1 4 0 5 Notice that this reveals that that the triangle is a right triangle. Check your Reading: Why do all linear transformations ma (0; 0) to (0; 0)? Rotation of Conics into Standard Form If A; B; and C are constants, then the level curves of Q (x; y) Ax + Bxy + Cy (3) are either lines, circles, ellises, or hyerbolas. If B 6 0; then a curve (??) is the image under rotation of a conic in standard osition in the uv-lane. Seci cally, (??) is the image of a conic in standard osition in the uv-lane of a rotation x cos () sin () u (4) y sin () cos () v 8

that mas the u-axis to a rincial axis of the conic, which is a line y mx containing the oints closest to or furthest from the origin. Thus, Lagrange multiliers can be used to determine the equation y mx of a rincial axis, after which relacing x and y by the rotation transformation imlied by () and (4) will rotate a conic (??) into standard form. blueexample 7 form: blackrotate the following conic into standard 5x 6xy + 5y 8 (5) Solution: Our goal is to nd the extrema of the square of the distance from a oint (x; y) to the origin, which is f (x; y) x +y ; subject to the constraint (5) The associated Lagrangian is L (x; y; ) x + y 5x 3xy + 5y 1 Since L x x (10x 3y) and L y y ( 3x + 10y) ; we must solve the equations x (10x 6y) ; y ( 6x + 10y) Since cannot be zero since (0; 0) cannot be a critical oint, we eliminate using the ratio of the two equations: x y (10x 6y) ( 6x + 10y) or x y 10x 6y 6x + 10y Cross-multilication yields 10xy 6x 10xy 6y so that y x. Thus, the rincial axes i.e., the lines containing the extrema are y x and y x: Using y x means m 1 and corresondingly, x y cos () sin () sin () u cos () v 1 1 1 1 1 u v That is, x (u v) and y (u + v), which uon substitution into (5) yields u v 5 u v u + v u + v 6 + 5 8 5 u uv + v 6 u v + 5 u + uv + v 8 5u + 5v 6u + 6v + 5u + 5v 16 Consequently, the ellise (5) is a rotation of the ellise u 4 + v 1 16 9 4u + 16v 16

as is shown below: Exercises: Find the image in the xy-lane of the given curve in the uv-lane under the given transformation. If the transformation is linear, identify it as such and write it in matrix form. 1. T (u; v) u; v ; v u. T (u; v) huv; u + vi ; v 3 3. T (u; v) hu v; u + vi ; v 0 4. T (u; v) hu + 3; v + i ; u + v 1 5. T (u; v) h4u; 3vi ; u + v 1 6. T (u; v) u + v; u v ; v u 7. T (u; v) u v ; uv ; v 1 u 8. T (u; v) u v ; uv ; v u 9. T (r; ) hr cos () ; r sin ()i ; r 1 10. T (r; ) hr cos () ; r sin ()i ; 4 Find several coordinate curves of the given transformation (e.g., u 1; 0; 1; and v 1; 0; 1; ). What is the image of the unit square under the given transformation? If the transformation is linear, identify it as such and write it in matrix form. 11. T (u; v) hu + 1; v + 5i 1. T (u; v) hu + 1; 3v i 13. T (u; v) h v; ui 14. T (u; v) hu + v + 1; v + i 15. T (u; v) u v ; uv 16. T (u; v) u + v; u v 17. T (u; v) hu + 3v; 3u + vi 18. T (u; v) u+v 19. Rotation about the origin 0. Rotation about the origin through an angle 4 through an angle 3 1. T (u; v) hi. T (u; v) hu cos (v) ; u sin (v)i 3. T (r; ) he r cos () ; e r sin ()i 4. T (r; t) hr cosh (t) ; r sinh (t)i ; u v 10

Find a conic in standard form that is the ullback under rotation of the given curve. 5. 5x + 6xy + 5y 8 6. 5x 6xy + 5y 9 7. xy 1 8. xy 4 9. 7x + 6 3 xy + 13y 16 30. 13x 6 3 xy + 7y 16 31. 5x 7xy + 73y 100 31. 73x + 7xy + 5y 100 33. The conic section x + xy + y x + y is not centered at the origin. 34. The conic section Can you rotate it into standard osition? 5x + 6xy + 5y 4x + 4y is not centered at the origin. Can you rotate it into standard osition? 35. Show that if x y cos () sin () sin () cos () u v then we must also have u v cos () sin () sin () cos () x y What is the signi cance of this result? 36. Use matrix multilication to show that a rotation through angle followed by a rotation through angle is the same as a single rotation through angle +. 37. The arabolic coordinate system on the xy-lane is the image of the coordinate transformation T (u; v) u v ; uv Determine the coordinate curves of the transformation, and sketch a few for seci c values of u and v: 38. The tangent coordinate system on the xy-lane is the image of the coordinate transformation T (u; v) u u + v ; v u + v Determine the coordinate curves of the transformation, and sketch a few for seci c values of u and v: 39. The ellitic coordinate system on the xy-lane is the image of the coordinate transformation T (u; v) hcosh (u) cos (v) ; sinh (u) sin (v)i 11

Determine the coordinate curves of the transformation, and sketch a few for seci c values of u and v: 40. The biolar coordinate system on the xy-lane is the image of the coordinate transformation sinh (v) T (u; v) cosh (v) cos (u) ; sin (u) cosh (v) cos (u) Determine the coordinate curves of the transformation, and sketch a few for seci c values of u and v: 41. Write to Learn: A coordinate transformation T (u; v) hf (u; v) ; g (u; )i is said to be area reserving if the area of the image of any region S in the uvlane is the same as the area of R: Write a short essay exlaining why a rotation through an angle is area reserving. 4. Write to Learn (Male): A coordinate transformation T (u; v) hf (u; v) ; g (u; )i is said to be conformal (or angle-reserving) if the angle between lines in the uv-lane is maed to the same angle between the image lines in the xy-lane. Write a short essay exlaining why a linear transformation with a matrix of a b A b a is a conformal transformation. 43. Write to Learn: What tye of coordinate system is imlied by the coordinate transformation T (u; v) hu; F (u) + vi? What are the coordinate curves? What is signi cant about tangent lines to these curves? Write a short essay which addresses these questions. 44. Write to Learn: Suose that we are working in an XY -coordinate system that is centered at (; q) and is at an angle to the x-axis in an xycoordinate system. 1

Write a short essay exlaining how one would convert coordinates with resect to the XY axes to coordinates in the xy-coordinate system. 13

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