CSE 167: Introduction to Computer Graphics Lecture #13: Curves. Jürgen P. Schulze, Ph.D. University of California, San Diego Fall Quarter 2017

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1 CSE 167: Introduction to Computer Graphics Lecture #13: Curves Jürgen P. Schulze, Ph.D. University of California, San Diego Fall Quarter 2017

2 Announcements Project 4 due Monday Nov 27 at 2pm Next Tuesday: midterm #2 Topics: everything since first midterm including lecture this Thursday 2

3 Lecture Overview Polynomial Curves Introduction Polynomial functions Bézier Curves Introduction Drawing Bézier curves Piecewise Bézier curves 3

4 Modeling Creating 3D objects How to construct complex surfaces? Goal Specify objects with control points Objects should be visually pleasing (smooth) Start with curves, then surfaces What can curves be used for? 4

5 Curves Surface of revolution 5

6 Curves Extruded/swept surfaces 6

7 Curves Animation Provide a track for objects Use as camera path 7

8 Video Bezier Curves 8

9 Curves Can be generalized to surface patches 9

10 Curve Representation Why not specify many points along a curve and connect with lines: Can t get smooth results when magnified more points needed Large storage and CPU requirements Instead: specify a curve with a small number of control points Known as a spline curve or spline. Control point 10

11 Spline: Definition Wikipedia: Term comes from flexible spline devices used by shipbuilders and draftsmen to draw smooth shapes. Spline consists of a long strip fixed in position at a number of points that relaxes to form a smooth curve passing through those points. 11

12 Lecture Overview Polynomial Curves Introduction Polynomial functions Bézier Curves Introduction Drawing Bézier curves Piecewise Bézier curves 12

13 Interpolating Control Points Interpolating means that curve goes through all control points A.k.a. Anchor Points Seems most intuitive But hard to control exact behavior 13

14 Approximating Control Points Curve is influenced by control points Various types Most common: polynomial functions Bézier spline (our focus) B-spline (generalization of Bézier spline) NURBS (Non Uniform Rational Basis Spline): used in CAD tools 14

15 Mathematical Definition A vector valued function of one variable x(t) Given t, compute a 3D point x=(x,y,z) Could be interpreted as three functions: x(t), y(t), z(t) Parameter t moves a point along the curve x(t) z y x(0.0) x(0.5) x(1.0) x 15

16 Tangent Vector Derivative Vector x : Points in direction of movement Length corresponds to speed x(t) z y x (0.0) x (0.5) x (1.0) 16 x

17 Lecture Overview Polynomial Curves Introduction Polynomial functions Bézier Curves Introduction Drawing Bézier curves Piecewise Bézier curves 17

18 Polynomial Functions Linear: (1 st order) Quadratic: (2 nd order) Cubic: (3 rd order) 18

19 Polynomial Curves in 3D Linear Evaluated as: 19

20 Polynomial Curves in 3D Quadratic: (2 nd order) Cubic: (3 rd order) We usually define the curve for 0 t 1 20

21 Control Points Polynomial coefficients a, b, c, d can be interpreted as control points Remember: a, b, c, d have x,y,z components each But: they do not intuitively describe the shape of the curve Goal: intuitive control points 21

22 Weighted Average Based on linear interpolation (LERP) Weighted average between two values Value could be a number, vector, color, Interpolate between points p 0 and p 1 with parameter t Defines a curve that is straight (first-order spline) p 0. 0<t<1 t=0. p 1 t=1 x(t) = Lerp( t, p 0, p 1 )= ( 1 t)p 0 + t p 1 22

23 Linear Polynomial x(t) = (p 1 p 0 ) t + p 0 vector point Curve is based at point p 0 Add the vector, scaled by t p 0..5(p 1 -p 0 ). p 1 -p 0. 23

24 Matrix Form Geometry matrix Geometric basis Polynomial basis In components 24

25 Summary 1. Grouped by points p: weighted average 2. Grouped by t: linear polynomial 3. Matrix form: 25

26 Tangent Weighted average Polynomial Matrix form 26

27 Lecture Overview Polynomial Curves Introduction Polynomial functions Bézier Curves Introduction Drawing Bézier curves Piecewise Bézier curves 27

28 Bézier Curves Are a higher order extension of linear interpolation p 1 p 1 p 1 p 2 p 3 p 0 p 0 p 0 p 2 Linear Quadratic Cubic 28

29 Bézier Curves Give intuitive control over curve with control points Endpoints are interpolated, intermediate points are approximated Demo: 29

30 Cubic Bézier Curve Most commonly used case Defined by four control points: Two interpolated endpoints (points are on the curve) Two points control the tangents at the endpoints Points x on curve defined as function of parameter t p 1 p 0 x(t) p 2 30 p 3

31 Algorithmic Construction Algorithmic construction De Casteljau algorithm, developed at Citroen in 1959, named after its inventor Paul de Casteljau (pronounced Cast-all- Joe ) Developed independently from Bézier s work: Bézier created the formulation using blending functions, Casteljau devised the recursive interpolation algorithm 31

32 De Casteljau Algorithm A recursive series of linear interpolations Works for any order Bezier function, not only cubic Not very efficient to evaluate Other forms more commonly used But: Gives intuition about the geometry Useful for subdivision 32

33 De Casteljau Algorithm Given: Four control points A value of t (here t 0.25) p 1 p 0 p 2 p 3 33

34 De Casteljau Algorithm p 1 q 1 q 0 (t) = Lerp( t,p 0,p ) 1 p 0 q 0 q 1 (t) = Lerp( t,p 1,p ) 2 p 2 q 2 (t) = Lerp( t,p 2,p ) 3 q 2 p 3 34

35 De Casteljau Algorithm q 0 r 0 q 1 r 0 (t) = Lerp( t,q 0 (t),q 1 (t)) r 1 (t) = Lerp( t,q 1 (t),q 2 (t)) r 1 q 2 35

36 De Casteljau Algorithm r 0 x r 1 x(t) = Lerp( t,r 0 (t),r 1 (t)) 36

37 De Casteljau Algorithm p 1 p 0 x p 2 Demo p 3 37

38 Recursive Linear Interpolation x = Lerp t,r 0,r 1 ( ) r = Lerp ( t,q,q ) ( ) r 1 = Lerp t,q 1,q 2 q 0 = Lerp( t,p 0,p ) 1 q 1 = Lerp( t,p 1,p ) 2 q 2 = Lerp( t,p 2,p ) 3 p 0 p 1 p 2 p 3 p 1 q 0 r 0 p 2 x q 1 r 1 p 3 q 2 p 4 38

39 Expand the LERPs q 0 (t) = Lerp( t,p 0,p 1 )= ( 1 t)p 0 + tp 1 q 1 (t) = Lerp( t,p 1,p 2 )= ( 1 t)p 1 + tp 2 q 2 (t) = Lerp( t,p 2,p 3 )= ( 1 t)p 2 + tp 3 r 0 (t) = Lerp( t,q 0 (t),q 1 (t))= 1 t r 1 (t) = Lerp( t,q 1 (t),q 2 (t))= 1 t ( ) ( )p 0 + tp 1 )+ t ( 1 t)p 1 + tp 2 ( )p 1 + tp 2 )+ t ( 1 t)p 2 + tp 3 ( ) 1 t ( ) 1 t x(t) = Lerp t,r 0 (t),r 1 (t) = ( 1 t) ( 1 t) ( 1 t)p 0 + tp 1 )+ t ( 1 t)p 1 + tp 2 +t ( 1 t) ( 1 t)p 1 + tp 2 )+ t ( 1 t)p 2 + tp 3 39 ( ) ( ) ( ) ( ) ( ) ( )

40 Weighted Average of Control Points Regroup for p: x(t) = 1 t ( )p 0 + tp 1 )+ t ( 1 t)p 1 + tp 2 ( )p 1 + tp 2 )+ t( 1 t)p 2 + tp 3 ) ( ) ( 1 t) 1 t +t 1 t ( ) ( ) ( ) 1 t ( ) x(t) = ( 1 t) 3 p 0 + 3( 1 t) 2 tp 1 + 3( 1 t)t 2 p 2 + t 3 p 3 B 0 (t ) B 1 (t ) x(t) = t 3 + 3t 2 3t + 1 p 0 + 3t 3 6t 2 + 3t ( ) ( ) + 3t 3 + 3t 2 p 2 + t 3 p 3 B 2 (t ) ( ) B 3 (t ) ( ) p 1 40

41 Cubic Bernstein Polynomials x(t) = B 0 ( t)p 0 + B 1 ( t)p 1 + B 2 ( t)p 2 + B 3 ( t)p 3 The cubic Bernstein polynomials : B 0 ( t)= t 3 + 3t 2 3t + 1 B 1 ( t)= 3t 3 6t 2 + 3t B 2 ( t)= 3t 3 + 3t 2 ( t)= t 3 B 3 B i (t) = 1 Weights B i (t) add up to 1 for any value of t 41

42 General Bernstein Polynomials B 1 0 B 1 1 ( t)= t + 1 B 2 0 ( t)= t 2 2t + 1 B 3 0 ( t)= t 3 + 3t 2 3t + 1 ( t)= t B 2 1 ( t)= 2t 2 + 2t B 3 1 ( t)= 3t 3 6t 2 + 3t B 2 2 ( t)= t 2 B 3 2 ( t)= 3t 3 + 3t 2 B 3 3 ( t)= t 3 42 B i n ( t)= n i ( 1 t )n i t i = n! i! ( n i)! B n i ( t) = 1 n! = factorial of n ( ) i n (n+1)! = n! x (n+1)

43 Any order Bézier Curves nth-order Bernstein polynomials form nth-order Bézier curves B i n ( t)= n i ( 1 t)n i t n i=0 x( t)= B n i ( t)p i ( ) i 43

44 Demo: Bezier curves of multiple orders 44

45 Useful Bézier Curve Properties Convex Hull property Affine Invariance 45

46 Convex Hull Property A Bézier curve is always inside the convex hull Makes curve predictable Allows culling, intersection testing, adaptive tessellation p 1 p 3 p 0 p 2 46

47 Affine Invariance Transforming Bézier curves Two ways to transform: First transform control points, then compute spline points First compute spline points, then transform them Results are identical Invariant under affine transformations 47

48 Cubic Polynomial Form Start with Bernstein form: x(t) = ( t 3 + 3t 2 3t + 1)p 0 + ( 3t 3 6t 2 + 3t)p 1 + ( 3t 3 + 3t 2 )p 2 + ( t 3 )p 3 Regroup into coefficients of t : x(t) = ( p 0 + 3p 1 3p 2 + p 3 )t 3 + ( 3p 0 6p 1 + 3p 2 )t 2 + ( 3p 0 + 3p 1 )t + ( p 0 )1 x(t) = at 3 + bt 2 + ct + d ( ) ( ) ( ) ( ) a = p 0 + 3p 1 3p 2 + p 3 b = 3p 0 6p 1 + 3p 2 c = 3p 0 + 3p 1 d = p 0 Good for fast evaluation Precompute constant coefficients (a,b,c,d) Not much geometric intuition 48

49 Cubic Matrix Form x(t) = a t 3 b c t 2 d t 1 a = p 0 + 3p 1 3p 2 + p 3 b = ( 3p 0 6p 1 + 3p 2 ) c = ( 3p 0 + 3p 1 ) d = p 0 ( ) ( ) t x(t) = [ p 0 p 1 p 2 p 3 ] t t G Bez B Bez T xx tt = GG BBBBBB BB BBBBBB TT = CC TT 49

50 Matrix Form Other types of cubic splines use different basis matrices Efficient evaluation Pre-compute C Use existing 4x4 matrix hardware support 50

51 Lecture Overview Polynomial Curves Introduction Polynomial functions Bézier Curves Introduction Drawing Bézier curves Piecewise Bézier curves 51

52 Drawing Bézier Curves Draw line segments or individual pixels Approximate the curve as a series of line segments (tessellation) Uniform sampling Adaptive sampling Recursive subdivision 52

53 Uniform Sampling Approximate curve with N straight segments N chosen in advance Evaluate x i = x t i ( ) where t i = i N for i = 0, 1,, N Connect points with lines Too few points? Poor approximation: curve is faceted Too many points? x i = a i3 N + b i2 3 N + c i 2 N + d Slow to draw too many line segments x(t) x 0 x 1 x 2 x 3 x4 53

54 Adaptive Sampling Use only as many line segments as you need Fewer segments where curve is mostly flat More segments where curve bends Segments never smaller than a pixel x(t) 54

55 Recursive Subdivision Any cubic curve segment can be expressed as a Bézier curve Any piece of a cubic curve is itself a cubic curve Therefore: Any Bézier curve can be broken down into smaller Bézier curves 55

56 De Casteljau Subdivision p 1 p 0 q r 0 0 r x 1 p 2 De Casteljau construction points are the control points of two Bézier sub-segments q 2 p 3 56

57 Adaptive Subdivision Algorithm Use De Casteljau construction to split Bézier segment in two For each part If flat enough : draw line segment Else: continue recursion Curve is flat enough if hull is flat enough Test how far the approximating control points are from a straight segment If less than one pixel, the hull is flat enough 57

58 Lecture Overview Polynomial Curves Introduction Polynomial functions Bézier Curves Introduction Drawing Bézier curves Longer curves 58

59 More Control Points Cubic Bézier curve limited to 4 control points Cubic curve can only have one inflection (point where curve changes direction of bending) Need more control points for more complex curves k-1 order Bézier curve with k control points Hard to control and hard to work with 59 Intermediate points don t have obvious effect on shape Changing any control point changes the whole curve Want local support: each control point only influences nearby portion of curve

60 Piecewise Curves Sequence of line segments Piecewise linear curve Sequence of cubic curve segments Piecewise cubic curve (here piecewise Bézier) 60

61 Global Parameterization Given N curve segments x 0 (t), x 1 (t),, x N-1 (t) Each is parameterized for t from 0 to 1 Define a piecewise curve Global parameter u from 0 to N x(u) = x 0 (u), 0 u 1 x 1 (u 1), 1 u 2 x N 1 (u ( N 1)), N 1 u N x(u) = x i (u i), where i = u (and x(n) = x N 1 (1)) Alternate solution: u defined from 0 to 1 x(u) = x i (Nu i), where i = Nu 61

62 Piecewise Bézier curve Given 3N + 1 points p 0,p 1,,p 3N Define N Bézier segments: x 0 (t) = B 0 (t)p 0 + B 1 (t)p 1 + B 2 (t)p 2 + B 3 (t)p 3 x 1 (t) = B 0 (t)p 3 + B 1 (t)p 4 + B 2 (t)p 5 + B 3 (t)p 6 x N 1 (t) = B 0 (t)p 3N 3 + B 1 (t)p 3N 2 + B 2 (t)p 3N 1 + B 3 (t)p 3N p 7 p 8 p 0 p p 2 1 x 0 (t) p 3 x 1 (t) p 6 x 2 (t) p 9 x 3 (t) p 10 p 11 p 12 p 4 p 5 62

63 Piecewise Bézier Curve Parameter in 0<=u<=3N x(u) = x 0 ( 1 u), 0 u 3 3 x 1 ( 1 u 1), 3 u 6 3 x N 1 ( 1 3 u (N 1)), 3N 3 u 3N 1 x(u) = x i ( u i 3 ), where i = 1 u 3 x(8.75) x 0 (t) x 1 (t) x 2 (t) x 3 (t) u=0 x(3.5) u=12 63

64 Parametric Continuity C 0 continuity: Curve segments are connected C 1 continuity: C 0 & 1st-order derivatives agree Curves have same tangents Relevant for smooth shading C 2 continuity: C 1 & 2nd-order derivatives agree Curves have same tangents and curvature Relevant for high quality reflections

65 Piecewise Bézier Curve 3N+1 points define N Bézier segments x(3i)=p 3i C 0 continuous by construction C 1 continuous at p 3i when p 3i - p 3i-1 = p 3i+1 - p 3i C 2 is harder to achieve and rarely necessary p 4 p 1 p 2 P 3 p 6 p 1 p 2 P 3 p 6 p 4 p 5 p 0 p 5 p 0 C 1 discontinuous C 1 continuous 65

66 Piecewise Bézier Curves Used often in 2D drawing programs Inconveniences Must have 4 or 7 or 10 or 13 or (1 plus a multiple of 3) control points Some points interpolate, others approximate Need to impose constraints on control points to obtain C 1 continuity Solutions User interface using Bézier handles to ascertain C 1 continuity Generalization to B-splines or NURBS 66

67 Bézier Handles Segment end points (interpolating) presented as curve control points Midpoints (approximating points) presented as handles Can have option to enforce C 1 continuity Adobe Illustrator 67

68 Demo: Bezier handles ml 68

69 Rational Curves Weight causes point to pull more (or less) Can model circles with proper points and weights, Below: rational quadratic Bézier curve (three control points) pull less 69

70 B-Splines B as in Basis-Splines Basis is blending function Difference to Bézier blending function: B-spline blending function can be zero outside a particular range (limits scope over which a control point has influence) B-Spline is defined by control points and range in which each control point is active. 70

71 NURBS Non Uniform Rational B-Splines Generalization of Bézier curves Non uniform: Combine B-Splines (limited scope of control points) and Rational Curves (weighted control points) Can exactly model conic sections (circles, ellipses) OpenGL support: see glunurbscurve Demos: html 71

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