Splines III – Bézier Curves

Splines III – Bézier Curves based on: Michael Gleicher: Curves, chapter 15 in Fundamentals of Computer Graphics, 3rd ed. (Shirley & Marschner) Slides by Marc van Kreveld

Interpolation vs. approximation • Interpolation means passing through given points, approximation means getting “close” to given points • Bézier curves and B-spline curves p2 p2 p1 p1 p0 p3 p0 p3 p1and p2are interpolated p1and p2are approximated

Bezier curves • Polynomial of any degree • A degree-d Bezier curve has d+1 control points • It passes through the first and last control point, and approximates the d – 1 other control points • Cubic (degree-3) Bezier curves are most common; several of these are connected into one curve

Bezier curves • Cubic Bezier curves are used for font definitions • They are also used in Adobe Illustrator and many other illustration/drawing programs

Bezier curves • Parameter u, first control point p0 at u=0 and last control point pd at u=1 • Derivative at p0 is the vector p0p1 , scaled by d • Derivative at pd is the vector pd-1pd , scaled byd • Second, third, …, derivatives at p0 depend on the first three, four, …, control points

Cubic Bezier curve example 3 p1 p2 p0 p2p3 p2p3 p0p1 p0p1 p3 3

Quintic Bezier curve example 5 p2 p3 p1 p4 p5 p4p5 p4p5 p0p1 p0p1 p0 5

Cubic Bezier curves • p0 = f(0) = a0 + 0 a1 + 02a2 + 03a3p3 = f(1) = a0 + 1 a1 + 12 a2 + 13a33(p1 – p0) = f’(0) = a1+ 20 a2 + 302a33(p3 – p2) = f’(1) = a1 + 21 a2 + 312a3 basis matrix

Cubic Bezier curves • f(u) = (1 – 3u + 3u2 – u3) p0 + ( 3u – 6u2 + 3u3) p1 + ( 3u2 – 3u3) p2 + ( u3) p3 • Bezier blending functionsb0,3 = (1 – u)3 b1,3 = 3 u (1 – u)2 b2,3 = 3 u2 (1 – u) b3,3 = u3

Bezier curves • In general (degree d):bk,d(u) = C(d,k) uk(1 – u)d-kwhere , for 0  k  d(binomial coefficients) • The bk,d(u) are called Bernstein basis polynomials

Bezier curves degrees 2 (left) up to 6 (right)

Properties of Bezier curves • The Bezier curve is bounded by the convex hull of the control points  intersection tests with a Bezier curve can be avoided if there is no intersection with the convex hull of the control points

Properties of Bezier curves • Any line intersects the Bezier curve at most as often as that line intersects the polygonal lie through the control points (variation diminishing property)

Properties of Bezier curves • A Bezier curve is symmetric: reversing the control points yields the same curve, parameterized in reverse p2 p3 p1 p4 p3 p2 p5 p0 p4 p1 p0 p5

Properties of Bezier curves • A Bezier curve is affine invariant: the Bezier curve of the control points after an affine transformation is the same as the affine transformation applied to the Bezier curve itself (affine transformations: translation, rotation, scaling, skewing/shearing) p2 p2 p3 p3 p1 p1 p4 p4 p5 p5 p0 p0

Properties of Bezier curves • There are simple algorithms for Bezier curves • evaluating • subdividing a Bezier curve into two Bezier curves  allows computing (approximating) intersections of Bezier curves the point at parameter value u on the Bezier curve

Bezier curves in PowerPoint • The curve you draw in PowerPoint is a Bezier curve; however you don’t specify the intermediate two control points explicitly • Select draw curve • Draw a line segment (p0 and p3) • Right-click; edit points • Click on first endpoint and move the appearing marker (p1) • Click on last endpoint and move the appearing marker (p2)

Splines from Bezier curves • To ensure continuity • C0 : last control point of first piece must be same as first control point of second piece • G1 : last two control points of first piece must align with the first two control points of the second piece • C1 : distances must be the same as well q2 p2 p1 p3 q3 q0 q1 p0

Intuition for Bezier curves • Keep on cutting corners to make a “smoother” curve • In the limit, the curve becomes smooth p1 p0 p2

Intuition for Bezier curves • Suppose we have three control points p0 ,p1, p2;a linear connection gives two edges • Take the middle p3 of p0p1, and the middle p4 of p1p2 and place p’1 in the middle of p3 and p4 • Recurse on p0, p3, p’1 and also on p’1 , p4, p2 p1 p3 p4 p’1 p’1 p0 p0 p0 p2 p2 p2 gives a quadratic Bezier curve

De Casteljau algorithm • Generalization of the subdivision scheme just presented; it works for any degree • Given points p0, p1, …, pd • Choose the value of u where you want to evaluate • Determine the u-interpolation for p0p1, for p1p2, … , and for pd-1pd , giving d – 1 points • If one point remains, we found f(u), otherwise repeat the previous step with these d – 1 points

De Casteljau algorithm u = 1/3 p2 p1 p3 p0

De Casteljau algorithm u = 1/3 p2 p1 p3 p0 one point remains, the point on the curve at u = 1/3

Splitting a Bezier curve • The De Casteljau algorithm can be used to split a Bezier curve into two Bezier curves that together are the original Bezier curve p2 p1 r1 r2 r0 q3 q2 p3 q1 r3 p0 q0

Splitting a Bezier curve p2 p1 p3 r1 p0 r2 r0 q3 q2 Question: Recalling that Bezier splines are C1 only if (in this case) the vector q2q3 is the same as r0r1 , does this mean that the spline is no longer C1 after splitting?!? q1 r3 q0

Splitting a Bezier curve p2 p1 p3 r1 p0 r2 r0 q3 q2 Answer: q0q1q2q3 parameterizes the part u [0, 1/3] and r0r1r2r3 parameterizes the part u [1/3, 1] The condition for C1 continuity, q2q3 = r0r1 , applies only for equal parameter-length parameterizations q1 r3 q0

Splitting a Bezier curve • Splitting a Bezier curve is useful to find line-Bezier or Bezier-Bezier intersections p2 p1 u = ½ p3 p0

Intersecting a Bezier curve • To test if some line L intersects a Bezier curve with control points p0, p1, …, pd , test whether L intersects the poly-line p0, p1, …, pd • If not, L does not intersect the Bezier curve either • Otherwise, split the Bezier curve (with u = ½ ) andrepeat on the two pieces p2 p1 p3 p0

Intersecting a Bezier curve • If the line L separates the two endpoints of a Bezier curve, then they intersect • Repeating the split happens often only if the line L is nearly tangent to the Bezier curve p2 p1 p3 p0

Intersecting a Bezier curve • When determining intersection of a line segment and a Bezier curve we must make some small changes p2 p1 p3 p0

Splitting a Bezier curve for rendering • Splitting a Bezier curve several times makes the new Bezier curve pieces be closer and closer to their control polygons • At some moment we can draw the sequence of control polygons of the pieces and these will approximate the Bezier curve well (technically this approximation is only C0)

Splitting a Bezier curve for rendering p2 p1 u = ½ p3 p0

Splitting a Bezier curve for rendering p0 p3 p1 p2 p2 p1 p3 p0

Splitting a Bezier curve for rendering p0 p3 p1 p2 p2 p1 u = 1/2 p3 u = 3/4 u = 1/4 p0

Splitting a Bezier curve for rendering p0 p3 u = 1/2 p3 u = 3/4 u = 1/4 p0

Splitting a Bezier curve for rendering p3 p0 p3 p0 p0 p3 p3 p0

De Casteljau on quadratic, cubic and quartic Bezier curves

3D Bezier surfaces The 16 blending functions for cubic Bezier surfaces

Summary • Bezier curves are elegant curves that pass through the start and end points and approximate the points in between • They exist of any order (degree) but cubic is most common and useful • Continuity between consecutive curves can be ensured • The De Casteljau algorithm is a simple way to evaluate or split a Bezier curve

Questions • Consider figure 15.11, bottom left (also on slide 11). It looks like a circular arc, but it is not. Determine whether the quadratic Bezier curve shown here goes around (left and above) the midpoint of the circular arc with the same two endpoints or not • Can we ensure higher degrees of continuity than C1 with cubic Bezier splines? Discuss your answer • Suppose we want to define a closed Bezier curve of degree d. What properties must the control points have to make a C1 continuous curve? What is the minimum degree of the Bezier curve that is needed for this? What if we want a closed Bezier curve with an inflection point (boundary of a non-convex region)?

Questions • Apply the De Casteljau algorithm on the points (0,0), (4,0), (6,2), and (4,6) with u = ½ by drawing the construction (note that this is a cubic Bezier curve) • Apply the De Casteljau algorithm on the points (0,0), (4,0), (6,2), (6,8), and (10,4) with u = ½ by drawing the construction (note that this is a quartic Bezier curve)

Splines III – Bézier Curves