Affine Transformations

1. Affine Transformations Jim Van Verth (jim@essentialmath.com) Lars M. Bishop (lars@essentialmath.com)

2. Affine Transformations • Extension of linear transforms • Lines preserved • Distances and angles can change • Transform in an affine space • Almost all apply to both vectors and points – exception is translation Essential Math for Games

3. Affine Transformations • The most common ones are translation, scale and rotation • Translation moves an object along a direction vector • Scale shrinks/grows the object size • Rotation rotates the object around an axis vector Essential Math for Games

4. Affine Transformations • Can’t use 3x3 matrix for R3 R3 affine xforms • Perform linear transforms, not affine transforms • Need another representation Essential Math for Games

5. What’s Wrong With A 3x3? • Can’t construct a 3x3 matrix that will translate the point (0,0,0) • The zeroes in the coordinate vector cancel out all the coefficients! Essential Math for Games

6. Homogeneous Coordinates • One way is to use homogeneous coordinates • Initially developed in the 40s for use in geometry • Later found their way into computer graphics • Allow us to perform affine transforms via matrices Essential Math for Games

7. Homogeneous Coordinates • Convert point in Rn to one of equivalent points in homogeneous space RPn • Point in RPn has n+1 coordinates • So point in R3 (x y z) has corresponding points in RP3 (x’ y’ z’ w) Essential Math for Games

8. Homogeneous Coordinates • How to convert an R3 coordinate to its homogeneous representation • Pick a number for the fourth element, w, and scale the real coordinate by it (x y z) => (xw yw zw w) • Standard value for w is 1, so: (x y z) => (x y z 1) Essential Math for Games

9. Homogeneous Coordinates • Map back to R3 by dividing out the w (x' y' z' w) => (x'/w y'/w z'/w) • Usually assumew = 1, so just drop thew (x' y' z' 1) => (x' y' z') • But not always, so be careful! Essential Math for Games

10. Homogeneous Matrices • N-dimensional affine transformation is n+1 dimensional linear transformation • A 3D homogeneous transformation matrix is a 4x4 matrix • Treat 3D homogeneous point as a 4D vector Essential Math for Games

11. Translation Revisited • Remember translating the point (0,0,0)? • No problem; right column is translation vector Essential Math for Games

12. Translation Txyz(a,b,c) Essential Math for Games

13. Transforming Point • Can represent affine transform as block matrix multiplication • Transform point P = (x, 1), get • So y is translation part, what does A represent? Essential Math for Games

14. Transforming Vectors • Represent vector with w = 0 (x, y, z) => (x, y, z,0) • Can also write as v => (v,0) • Think of as point at infinity Essential Math for Games

15. Transforming Vectors • Apply affine transform to vector, get • Applies linear transform A to vector • So linear transforms are subset of affine transforms • Note no translation: makes sense, vectors have no position! Essential Math for Games

16. Other Affine Transformations • Remaining affine transformations are also linear transformations • Can be applied to vectors as well • Notice that the top-left 3x3 block in the translation example was the identity? • Just place the linear transform in the upper-left 3x3 block, as follows… Essential Math for Games

17. Scaling Sxyz(a,b,c) Essential Math for Games

18. Rotating About the x-axis Rx() Essential Math for Games

19. Rotating About the y-axis Ry() Essential Math for Games

20. Rotation About the z-axis Rz() Essential Math for Games

21. Rotation Matrices • Have some useful properties • Are orthogonal, i.e., R-1 = RT • Column vectors are transformed axis of old coordinate system • We’ll see how we can use this later Essential Math for Games

22. Concatenation of Transformations • Transforms concatenated into a single matrix by multiplying them together, in the order they are to occur • Can then use single matrix to perform transforms on many points Essential Math for Games

23. Concatenation • Can perform a series of transformations • v’ = M1v • v’’ = M2v’ • w = M3v’’ • Substitute to get • w = M3M2v’ • w = M3M2M1v Essential Math for Games

24. Concatenation • Can combine series into one matrix that performs all of them • M= M3 M2 M1 • w = M  v • Example: Euler rotations Essential Math for Games

25. Euler Angles • Common representation for 3D orientation (common  best) • Three Euler angles: • Yaw = shaking your head, “no” • Pitch = nodding your head, “yes” • Roll = cocking your head sideways Essential Math for Games

26. The Euler Transform • Each angle corresponds to a rotation about an axis (dependent on coordinate system) z x y Essential Math for Games

27. The Euler Transform • Looking down the x-axis: • Yaw = Rz • Pitch = Ry • Roll = Rx z x y Essential Math for Games

28. The Euler Transform • For this coordinate system, the Euler Transform E E(y,p,r) = Rx(r) Ry(p) Rz(y) • Again, Eis orthogonal (it’s a pure rotation matrix) Essential Math for Games

29. Concatentation of Transforms • Can concatenate any sequence of scale, rotation, translate • Matrix products are non-commutative, so the order matters • Translating the point (0, 0, 0) and then rotating will not yield the same point as rotating (0, 0, 0) then translating (rotation of (0, 0, 0) does nothing) Essential Math for Games

30. Concatentation of Transforms • Similarly, scaling and rotation are non-commutative • Example: • rotate (1 0 0) by 90° around z, scale by (sx, sy, sz), get (0 sy 0) • scale (1 0 0) by (sx, sy, sz), rotate (1 0 0) by 90° around z, get (0 sx 0) Essential Math for Games

31. Transformation Order • To perform scale, then rotation, then translation (generally the order desired), use Essential Math for Games

32. Matrix Breakdown • Rotation, scale in upper left 3x3 • Translation rightmost column • Does rotation/scale first, then translate Essential Math for Games

33. Better Format • Problem: want to • Translate object in space and change rotation and scale arbitrarily • Handle rotation/scale separately • Can’t do easily with 4x4 matrix format • Involves SVD, Polar decomposition • Messy, but we can do better using… Essential Math for Games

34. Rigid Body Transforms • Any sequence of translation and rotation transformations • Not scale or shear • Object shape is not affected (preserves angles and lengths) • Usually include uniform scale despite this Essential Math for Games

35. Better Format • Scale – one uniform scale factor s • Rotation – 3x3 matrix R • Translation – single vector t Essential Math for Games

36. Better Format • Want to concatenate transforms T1, T2 in this form, or • Do this by Essential Math for Games

37. Better Format • Advantages • Clear what each part does • Easier to change individual elements • Concat: 39 mult 27 add vs. 48 mult 32 add • Disadvantages • Eventually have to convert to 4x4 matrix anyway for renderer/video card Essential Math for Games

38. Better Format • Matrix conversion Essential Math for Games

39. Inverting Rigid Body Xforms • We can easily invert our rigid body transforms symbolically: Essential Math for Games

40. Inverting Rigid Body Xforms (2) • In fact, the result itself may be written as a rigid body transform: Essential Math for Games

41. References • Rogers, F. David and J. Alan Adams, Mathematical Elements for Computer Graphics, 2nd Ed, McGraw-Hill, 1990. • Watt, Alan, 3D Computer Graphics, Addison-Wesley, Wokingham, England, 1993. Essential Math for Games