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Vectors, Points, Lines and Planes Jim Van Verth Lars M. Bishop

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Essential Math for Games Geometric Representation Problem Represent geometric data in computer? Solution Vectors and points Can represent most abstract objects as combinations of these E.g. lines, planes, polygons

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Essential Math for Games What Is a Vector? Geometric object with two properties direction length (if length is 1, is unit vector) Graphically represented by

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Essential Math for Games Scale Graphically change length of vector v by

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Essential Math for Games Addition Graphically put tail of second vector at head of first draw new vector from tail of first to head of second b a a + b

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Essential Math for Games Algebraic Vectors Algebraically, vectors are more than this Any entity that meets certain rules (lies in vector space) can be called vector Ex: Matrices, quaternions, fixed length polynomials Mostly mean geometric vectors, however

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Essential Math for Games Vector Space Set of vectors related by +,· Meet rules v + w = w + v (commutative +) (v + w) + u = v + (w + u) (associative +) v + 0 = v(identity +) v + (-v) = 0(inverse +) ( ) v = ( v)(associative ·) ( + )v = v + v(distributive ·) (v + w) = v + w (distributive ·)

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Essential Math for Games Number Spaces Cardinal – Positive numbers, no fractions Integer – Pos., neg., zero, no fractions Rational – Fractions (ratios of integers) Irrational – Non-repeating decimals (, e ) Real – Rationals+irrationals Complex – Real + multiple of -1 a+bi

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Essential Math for Games Real Vector Spaces Usually only work in these R n is an n-dimensional system of real numbers Represented as ordered list of real numbers (a 1,…,a n ) R 3 is the 3D world, R 2 is the 2D world

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Essential Math for Games Linear Combination Combine set of n vectors using addition and scalar multiplication v = 1 v v 2 + … + n v n Collection of all possible linear combinations for given v 1 … v n is called a span Linear combination of 2 perpendicular vectors span a plane

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Essential Math for Games Linear Dependence A system of vectors v 1, …,v n is called linearly dependant if for at least one v i v i = 1 v 1 +…+ i-1 v i-1 + i+1 v i+1 +…+ n v n Otherwise, linearly independent Two linearly dependant vectors are said to be collinear I.e. w =. v I.e. they point the same direction

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Essential Math for Games Linear Dependence Example Center vector can be constructed from outer vectors Vector Prerequisites

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Essential Math for Games Vector Basis Ordered set of n lin. ind. vectors = { v 1, v 2, …, v n } Span n-dimensional space Represent any vector as linear combo v = 1 v v 2 + … + n v n Or just components v = ( 1, 2, …, n )

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Essential Math for Games Vector Representation 3D vector v represented by (x, y, z) Use standard basis { i, j, k } Unit length, perpendicular (orthonormal) v = xi + yj + zk Number of units in each axis direction v1v1 v3v3 v2v2

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Essential Math for Games Vector Operations Addition: +,- Scale: · Length: || v || Normalize:

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Essential Math for Games Addition Add a to b b a a + b

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Essential Math for Games Scalar Multiplication change length of vector v by

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Essential Math for Games Length ||v|| gives length (or Euclidean norm) of v if ||v|| is 1, v is called unit vector usually compare length squared Normalize v scaled by 1/|| v || gives unit vector

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Essential Math for Games Vector Operations Games tend to use most of the common vector operations Addition, Subtraction Scalar multiplication Two others are extremely common: Dot product Cross product

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Essential Math for Games Dot product Also called inner product, scalar product a b

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Essential Math for Games Dot Product: Uses a a equals ||a|| 2 can test for collinear vectors if a and b collinear & unit length, | a b | ~ 1 Problems w/floating point, though can test angle/visibility a b > 0 if angle < 90° a b = 0 if angle = 90° (orthogonal) a b 90°

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Essential Math for Games Dot Product: Example Suppose have view vector v and vector t to object in scene ( t = o - e ) If v t < 0, object behind us, dont draw v t o e

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Essential Math for Games Dot Product: Uses Projection of a onto b is a b

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Essential Math for Games Dot Product: Uses Example: break a into components collinear and perpendicular to b a b

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Essential Math for Games Cross Product Cross product: definition returns vector perpendicular to a and b right hand rule length = area of parallelogram a b c

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Essential Math for Games Cross Product: Uses gives a vector perpendicular to the other two! || a b || = ||a|| ||b|| sin( ) can test collinearity ||a b|| = 0 if a and b are collinear Better than dot – dont have to be normalized

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Essential Math for Games Other Operations Several other vector operations used in games may be new to you: Scalar Triple Product Vector Triple Product These are often used directly or indirectly in game code, as well see

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Essential Math for Games Scalar Triple Product Dot product/cross product combo Volume of parallelpiped Test rotation direction Check sign w v u

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Essential Math for Games Triple Scalar Product: Example Current velocity v, desired direction d on xy plane Take If > 0, turn left, if < 0, turn right v d v d d v

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Essential Math for Games Vector Triple Product Two cross products Useful for building orthonormal basis Compute and normalize:

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Essential Math for Games Points Points are positions in space anchored to origin of coordinate system Vectors just direction and length free- floating in space Cant do all vector operations on points But generally use one class in library

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Essential Math for Games Two points related by a vector (Q - P) = v P + v = Q Point-Vector Relations v Q P

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Essential Math for Games Vector, point related by origin (P - O) = v O + v = P Vector space, origin, relation between them make an affine space Affine Space v P O e3e3 e2e2 e1e1

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Essential Math for Games Cartesian Frame Basis vectors {i, j, k}, origin (0,0,0) 3D point P represented by (p x, p y, p z ) Number of units in each axis direction relative to origin o pxpx pzpz pypy

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Essential Math for Games Affine Combination Like linear combination, but with points P = a 1 P 1 + a 2 P 2 + … + a n P n a 1,…,a n barycentric coord., add to 1 Same as point + linear combination P = P 1 + a 2 (P 2 -P 1 ) + … + a n (P n -P 1 ) If vectors ( P 2 -P 1 ), …, (P n -P 1 ) are linearly independent, {P 1, …, P n } called a simplex (think of as affine basis)

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Essential Math for Games Convex Combination Affine combination with a 1,…,a n between 0 and 1 Spans smallest convex shape surrounding points – convex hull Example: triangle

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Essential Math for Games Points, Vectors in Games Points used for models, position vertices of a triangle Vectors used for velocity, acceleration indicate difference between points, vectors

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Essential Math for Games Parameterized Lines Can represent line with point and vector P + tv Can also represent an interpolation from P to Q P + t(Q-P) Also written as (1-t)P + tQ P v Q

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Essential Math for Games Planes 2 non-collinear vectors span a plane Cross product is normal n to plane n

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Essential Math for Games Planes Defined by normal n = (A, B, C) point on plane P0 Plane equation Ax+By+Cz+D = 0 D=-(A·P0 x + B·P0 y + C·P0 z )

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Essential Math for Games Planes Can use plane equation to test locality of point If n is normalized, gives distance to plane n Ax+By+Cz+D > 0 Ax+By+Cz+D = 0 Ax+By+Cz+D < 0

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Essential Math for Games References Anton, Howard and Chris Rorres, Elementary Linear Algebra, 7 th Ed, Wiley & Sons, Axler, Sheldon, Linear Algebra Done Right, Springer Verlag, Blinn, Jim, Notation, Notation, Notation, Morgan Kaufmann, 2002.

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