# Intro to Modeling Modeling in 3D

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1 Intro to Modeling Modeling in 3D Polygon sets can approximate more complex shapes as discretized surfaces 2 1

2 2 3 Curve surfaces in 3D Sphere, ellipsoids, etc Curved Surfaces Modeling in 3D ) ( r z y x 4 Solid Models in 3D Sphere, ellipsoids, etc Modeling in 3D ) ( r z y x

3 Modeling in 3D Solid Models in 3D Boolean solids 5 Polygons Multi-sided planar element composed of edges and vertices. Vertices (singular vertex) are represented by points Edges connect vertices as line segments (x2,y2) E1 (x1,y1) E3 E2 (x3,y3) 6 3

4 Polygons Simple polygons - no edges cross Non-simple - some edges cross Simple Non-simple 7 Polygons Convex Polygon - a polygon that has no included angles larger than 180 degrees Concave Polygon - has at least one included angle greater than 180 degrees Convex Concave 8 4

5 Polygons Test for convexity- any line segment drawn between two points inside a polygon must remain inside the polygon if the poly is convex Convex Concave 9 Polygons A concave polygon can be broken up into two or more convex polygons OpenGL assumes all polygons are simple and convex 10 5

6 Rasterizing Polygons Assume that all polygons lie on the image plane (2D), then rasterization is to fill in pixels lying within a closed polygon and to do so efficiently To do this, algorithm needs to determine if a pixel is inside or outside a polygon How do we do this?? 11 Rasterizing Polygons Scan Line Approaches: - Inside test: Point P is inside a polygon iff a scanline intersects polygon edges an odd numberof times moving from P in either direction (inside) 12 6

7 Rasterizing Polygons Inside test special cases: - Horizontal edges can be ignored -Use Min-max test when scanline passes thru vertex Min-max: count twice if slope changes sign count once otherwise 13 Rasterizing Polygons Inside test: To fill polygons, one scanline at a time determine which pixels are inside and set "on" 14 7

8 Rasterizing Polygons Faster is to fill polygons, one scanline at a time determine which pixels intersect and sort by edge Scanline fill algorithm For each scanline: Find intersections Sort intersections Fill in pixels between pairs of intersections But, intersection calculations are expensive 15 Rasterizing Polygons Faster still, use edge coherence - Many edges that intersect scanline s intersect scanline s+1 Compute scanline intersection: y = mx + b and y s = s, then s = mx s + b or x s = (s-b)/m Compute difference in next scanline for intersection y s+1 = s+1, s+1 = mx s+1 + b or x s+1 = (s +1 - b)/m and, x s+1 = x s + 1/m <= USE THIS TO UPDATE Edge tables allow us to define in efficient, easy manner 16 8

9 Modeling in 3D 17 Defining 3D Polygons 3D Polygons live in 3D 18 9

10 Defining 3D Polygons 3D Polygons are truncated planes in 3-space E1 (x1,y1,z1) (x2,y2,z2) E2 E3 (x3,y3,z3) Express given the vertex and edge coordinates Start with the generic form of a plane Ax + By + Cz + D = 0 19 Defining 3D Polygons Vertices and edges are ordered, CW or CCW, usually counter clockwise Therefore, edges are directed line segments and may be treated as vectors (x2,y2,z2) E2 E1 E3 (x3,y3,z3) (x1,y1,z1) 20 10

11 Defining 3D Polygons Every plane has a vector n normal (perpendicular, orthogonal) to it From two vectors that line on the plane, we can use the cross product to find the normal, n = u v v P u 21 Vector Operations Apply the right hand rule Cross product results in a vector 22 11

12 Vector Operations Dot product can be used to define angles: For example, assume w & v are unit length v and = cos -1 ( w v ) projection w v w Also generalizes for non-normal vectors (see text) 23 Defining 3D Polygons In our plane equation: Ax + By + Cz + D = 0, [A,B,C] T = the normal of the plane From the edges find two vectors Use the cross product to find the normal, giving us A, B, C Then, to find D, plug in any vertex into the equation and solve for D 24 12

13 Defining 3D Polygons Once we know our plane equation: Ax + By + Cz + D = 0, we still need to manage the truncation which leads to the polygon itself Functionally, we will need to do this to know if a point lies in a polygon or not, for example 25 Defining 3D Polygons To do this, we can project the 3d polygon into 2d and see if the point is in the 2d z using the inside test (xi,yi,zi) y x (xi,yi ) 26 13

14 Defining 3D Polygons Project 3d to 2d based on largest of A,B,C z (xi,yi,zi) y This example: Z (or C) is principal component of N, normal so project on to xy-plane x (xi,yi ) 27 Z-buffer For 3D with multiple polygons, must deal with visibility image plane Project vertices into image plane and with projected vertex include depth in additional depth- or Z-buffer 28 14

15 Z-buffer Scan convert each projected polygon: For each polygon in scene project verticies for each pixel inside poly calculate z if z < closest draw into frame buffer update z buffer 29 Curves and Surfaces 15

16 Curves Curves are one dimensional entities where the function is nonlinear Curves 31 Representations for curves Parametric representation x = x(u), y = y(u) or simply p(u) where p = x y - More robust and general than other forms - Gives better control over curves and surfaces Notation: p(u) = c 0 + c 1 u + c 2 u 2 where each c is a vector, c i = c ix c iy 32 16

17 Representations for curves Example, quadric parametric curve: p(u) = c 0 + c 1 u + c 2 u 2 Like curve: x = 3u 2 y = 2u + 3 for u = [-1,1] c 0 = 0 c 1 = 0 c 2 = Note, more coefficients than a quadratic 33 Representations for curves Parametric curve, p(u) can as easily represent a curve in 3D (x,y,z) Simply: x = f x (u) y = fy(u) z = f z (u) Quadric coefs become: c ox c oy c oz c 1x c 1y c 1z c 0 = c 1 = c 2 = c 2x c 2y c 2z 34 17

18 Parametric Cubic (pc) curves Extension of quadrics to cubics Represented as: p(u) = c 0 + c 1 u + c 2 u 2 + c 3 u 3 Also called Hermite curves after the 17th century mathematician 35 Parametric Cubic (pc) curves Algebraic form: p(u) = c 0 + c 1 u + c 2 u 2 + c 3 u 3 (Note,unless otherwise specified u goes from 0,1) Not very intuitive, 12 values of c Instead want a better specify the curve More intuitive: control by start point p(0) and ending point p(1) and their derivatives 36 18

19 Parametric Cubic (pc) curves From: p(u) = c 0 + c 1 u + c 2 u 2 + c 3 u 3 Build a geometric form in order to specify a curves by their end points and tangents p(1) p (0) p (1) p(0) 37 Parametric Cubic curves This leads to the Geometric Form: p(u) = F 1 (u)p(0) + F 2 (u)p(1) + F 3 (u)p (0) + F 4 (u)p (1) where: F 1 (u) = 2u 3 3u 2 +1 F 2 (u) = -2u 3 + 3u 2 F 3 (u) = u 3 2u 2 + u F 4 (u) = u 3 u

20 F curves: Hermite curves basis 39 Joining Multiple Segments use p = [p 0 p 1 p 2 p 3 ] T use p = [p 3 p 4 p 5 p 6 ] T Get continuity at join points but not continuity of derivatives C0, C1, C2 continuity 40 20

21 Bezier Curves Family of curves developed in the 1970's by Bezier, a engineer for Renault, car manufacturer Bezier's curves are only guaranteed to pass through the end points, but other control points controlled the derivative at the end points Specifically, the tangent was controlled by the next control point in, the 2nd derivative by the second control point in, and the nth derivative by the nth and so on Bezier Curves 42 21

22 Bezier Curves The general form of the Bezier curve is: n p(u) = p i f i (u) 0 < u < 1 i=0 Vertices p control the curve and blending functions, f i (u), that satisfy the "derivative" condition Bernstein polynomials were a family of functions that were chosen by Bezier to satisfy his needs, these are not the only functions that could be used though 43 Convex Hull Property The properties of the Bernstein polynomials ensure that all Bezier curves lie in the convex hull of their control points Hence, even though we do not reach all the input data, we cannot be too far away Bezier curve p 1 p 2 convex hull p 0 p

23 Bernstein Polynomials The blending functions are a special case of the Bernstein polynomials d! k d k bkd ( u) u (1 u) k!( d k)! These polynomials give the blending polynomials for any degree Bezier form - All zeros at 0 and 1 - For any degree they all sum to 1 - They are all between 0 and 1 inside (0,1) 45 Bezier Curves p(u) = (1-u) 2 p 0 + 2u(1-u)p 1 + u 2 p 2 p1 p2 p

24 n = 3 n = 5 n = 4 n = 6 47 Bezier Curves Bezier curves have intuitive control, are nicely formed, convex hull of all points 48 24

25 B Splines One problem with the curves we have looked at is that changing a single control point affects the whole curve (this is called global propogation,) Also, depends on # of control pts B-Splines offer an alternative, to only affect the local region if a single control point is modified (i.e. local propogation) 49 B Splines B-splines are also called Basis Splines They have a form similar to Bezier, B- splines are defined as: n p(u) = p i f i,k (u) i=0 0 < u < n k where n +1 is the number of control points and k controls the degree of the blending (or basis) functions 50 25

26 B Splines B-splines' blending functions are defined recursively: and f i,1 (u) = 1 if t i < u < t i +1 = 0 otherwise f i,m (u) = m goes from 2 to k (u - t i ) f i,m-1 (u) (u - t i + m ) f i+1,m-1 (u) _ t i+m-1 -t i t i+m -t i+1 t i 's are knot points relating u to control points, p i 51 B Splines Knot points,t i 's, follow along like this: t i = 0 if i < k t i = i - k + 1 if k < i < n t i = n- k + 2 if i > n 52 26

27 B Splines For 6 control pts & k = 1, n= 5 and 0 < u < 6 We get the degenerate case: p(u) = p 0 0 < u < 1 p(u) = p 1 1 < u < 2 p(u) = p 2 2 < u < 3 p(u) = p 3 3 < u < 4 p(u) = p 4 4 < u < 5 p(u) = p 5 5 < u < 6 53 B Splines 54 27

28 B Splines For 6 control pts & k = 2, n= 5 and 0 < u < 5 We get a linear average of neighbors: p(u) = (1 - u)p 0 + u p 1 0 < u < 1 p(u) = (2 - u)p 1 + (u - 1)p 2 1 < u < 2 p(u) = (3 - u)p 2 + (u - 2)p 3 2 < u < 3 p(u) = (4 - u)p 3 + (u - 3)p 4 3 < u < 4 p(u) = (5 - u)p 4 + (u - 4)p 5 4 < u < 5 55 B Splines 56 28

29 B Splines For 6 control pts & k = 3, n= 5 and 0 < u < 4 We get: for 0 < u < 1 p 1 (u) = (1 - u) 2 p 0 +.5u(4-3u) p u 2 p 2 for 1 < u < 2 p 2 (u) =.5(2 - u) 2 p 1 +.5(-2u 2 + 6u - 3) p 2 +.5(u - 1) 2 p 3 for 2 < u < 3 p 3 (u) =.5(3 - u) 2 p 2 +.5(-2u u - 11) p 3 +.5(u - 2) 2 p 4 for 3 < u < 4 p 4 (u) =.5(4 - u) 2 p 3 +.5(-3u u - 32) p 4 + (u - 3) 2 p 5 57 B Splines Note, influence grows with degree k = 2 k = 3 k =

30 B Splines Thus, local influence of control points on curve as: 59 Cubic B-spline p(u) = u T M S p = b(u) T p MS

31 Rendering Curves How do we draw the curve, given p(u)? void evaluatecurve(u,pixelx,pixely) p(1) tangent p (0) p (1) tangent p(0) 61 Rendering Curves Polyline approximation p(1) p(0) piecewise linear discrete approximation 62 31

32 Curves vs. Surfaces Curves are one dimensional entities where the function is nonlinear Surfaces are formed from twodimensional functions - Linear functions give planes and polygons y Curves z x Surfaces 63 Curves vs. Surfaces Parametric curve, p(u) can as easily represent a curve in 3D (x,y,z) x = f x (u) y = fy(u) z = f z (u) u = 1 u =

33 Parametric surface is very similar: p(u,v) Curves vs. Surfaces Simply: x = f(u, v) y = f(u, v) z = f(u, v) (Just, more difficult to comprehend in a 2D representation, we must go behind the image plane) 65 Parametric Surfaces Surfaces require 2 parameters x=x(u,v) y=y(u,v) z=z(u,v) p(u,v) = [x(u,v), y(u,v), z(u,v)] T z Want same properties as curves: - Smoothness - Differentiability - Ease of evaluation y p(0,v) p(u,0) p(u,1) x p(1,v) 66 33

34 34 67 Bezier Patches p v b u b v u p ij j i j i ) ( ) ( ), ( Patch lies in convex hull 68 B-Spline Patches v u p v b u b v u p T S S T ij j i j i M P M ) ( ) ( ), ( defined region

35 Splitting a Cubic Bezier p 0, p 1,p 2,p 3 determine a cubic Bezier polynomial and its convex hull Consider left half l(u) and right half r(u) 69 l(u) and r(u) Since l(u) and r(u) are Bezier curves, we should be able to find two sets of control points {l 0, l 1, l 2, l 3 } and {r 0, r 1, r 2, r 3 } that determine them 70 35

36 Efficient Form l 0 = p 0 r 3 = p 3 l 1 = ½(p 0 + p 1 ) r 1 = ½(p 2 + p 3 ) l 2 = ½(l 1 + ½( p 1 + p 2 )) r 1 = ½(r 2 + ½( p 1 + p 2 )) l 3 = r 0 = ½(l 2 + r 1 ) Requires only shifts and adds! 71 Surfaces Can apply the recursive method to surfaces if we recall that for a Bezier patch curves of constant u (or v) are Bezier curves in u (or v) First subdivide in u - Process creates new points - Some of the original points are discarded original and discarded original and kept new 72 36

37 Second Subdivision 16 final points for 1 of 4 patches created 73 Normals We can differentiate with respect to u and v to obtain the normal at any point p p( u, v) u x( u, v) / u y( u, v) / u z( u, v) / u p( u, v) v x( u, v) / v y( u, v) / v z( u, v) / v n p( u, v) p( u, v) u v 74 37

38 Utah Teapot Most famous data set in computer graphics Widely available as a list of 306 3D vertices and the indices that define 32 Bezier patches 75 38

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