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### Object Intersection

CSE 681

Object Representation

- Implicit forms
- F(x,y,z) = 0

testing

- Explicit forms
- Analytic form x = F(y,z)

generating

- Parametric form (x,y,z) = P(t)

CSE 681

Ray-Object Intersection

- Implicit forms
- F(x,y,z) = 0

Ray: P(t) = (x,y,z) = source + t*direction = s + t*c

Solve for t: F(P(t)) = 0

CSE 681

Ray-Sphere Intersection

Implicit form for sphere at origin of radius 1

Ray:

Solve: …

- Use quadratic equation…

CSE 681

Ray-Sphere Intersection

At2 + Bt + C = 0

A = |c|2

B = 2s·c

C = |s|2 - 1

B2-4AC < 0 => no intersection

= 0 => just grazes

> 0 => two hits

CSE 681

Z = z2

X = x2

X = x1

Z = z1

Y = y1

Axis-Aligned Cuboid (rectangular solid, rectangular parallelpiped)Ray equation

Planar equations

P(t) = s + tc

Solve for intersections with planes

tx1 = (x1 - sx)/cx

tx2 = (x2 - sx)/cx

ty1 = (y1 - sx)/cx

…

CSE 681

Rectangle

For each intersection, note whether entering or leaving square side of plane

Ray 2

Ray is inside after last entering and before first leaving

Ray 1

CSE 681

Normal Vector

Given ordered sequence of points defining a polygon how do you find a normal vector for the plane?

Note: 2 normal vectors to a plane, colinear and one is the negation of the other

Ordered: e.g., clockwise when viewed from the front of the face

Right hand v. left hand space

CSE 681

Normal Vector

mx = S(yi - ynext(i)) (zi - znext(i))

V1

V2

my = S(zi - znext(i)) (xi - xnext(i))

V3

mz = S(xi - xnext(i)) (yi - ynext(i))

V5

V2

V4

V1

V1

V3

V2

V3

V5

V4

V5

CSE 681

V4

Ray-Plane

Ax + by + cz + d = 0

n ·P = -d

P = s + t*dir

n· (s+t*dir) = -d

n·s + t*(n·dir) = -d

t = -(d + n·s)/(n·dir)

CSE 681

Ray-Polyhedron

Polyhedron - volume bounded by flat faces

Each face is defined by a ring of edges

Each edge is shared by 2 and only 2 faces

The polyhedron can be convex or concave

Faces can be convex or concave

CSE 681

Intersection going from outside to inside

Convex Polyhedronvolume bounded by finite number of infinite planes

Computing intersections is similar to cube but using ray-plane intersection and arbitrary number of planes

Intersection going from inside to outside

CSE 681

Intersection going from outside to inside

Convex PolyhedronIntersection going from inside to outside

Record maximum entering intersection - enterMax

Record minimum leaving intersection - leaveMin

If (enterMax < leaveMin) polyhedron is intersected

CSE 681

Concave Polyhedron

Find closest face (if any) intersected by ray

Need ray-face (ray-polygon) intersection test

CSE 681

Ray-Polyhedron

1. Intersect ray with plane

2. Determine if intersection point is inside of 2D polygon

A) Convex polygon

B) Concave polygon

CSE 681

VxE

Ray-Convex PolygonE

Test to see if point is on

‘inside’ side of each edge

n

V

Dot product of

normal

Cross product of

ordered edge

vector from edge source to point of intersection

CSE 681

Ray-concave polygon

Project plane and point of intersection to 2D plane

2D point-inside-a-polygon test

Project to plane of 2 smallest coordinates of normal vector

Form semi-infinite ray and count ray-edge intersections

CSE 681

World space

Transformed objectse.g., Ellipse is transformed sphere

Intersect ray with transformed object

Use inverse of object transformation to transform ray

Intersect transformed ray with untransformed object

CSE 681

Transformed objects

World space ray

Object space ray

Intersect ray with object in object space

Transform intersection point and normal back to world space

CSE 681

Ray-Cylinder

CSE 681

Ray-Ellipsoid

Geometric construction: all points p such that |p-a| + |p-b| = r

a

b

d2

d1

|P(t)-a| + | P(t)- b| = r

CSE 681

Ray-Quadric

http://en.wikipedia.org/wiki/Quadric

Ray-Whatever

Algebraic solution v. Numeric solution

CSE 681

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