3D Rendering and Ray Castingmisha/Fall19/06.pdf3D Rendering and Ray Casting Michael Kazhdan (601.457/657) HB Ch. 13.7, 14.6 FvDFH 15.5, 15.10 Announcements • We have a new CA: Cindy

3D Rendering and Ray Casting

Michael Kazhdan

(601.457/657)

HB Ch. 13.7, 14.6

FvDFH 15.5, 15.10

Announcements

• We have a new CA: Cindy Yang

Office Hours: Wednesday 1:00-2:00 @ Malone 122

Rendering

• Generate an image from geometric primitives

Geometric Primitives (3D)

Rendering

• Generate an image from geometric primitives

Rendering

Geometric Primitives (3D)

Raster Image (2D)

3D Rendering Example

What issues must be addressed by a

3D rendering system?

Overview

• 3D scene representation

• 3D viewer representation

• What do we see?

• How does it look?

Overview



• What do we see?


How is the 3D scene

described in a computer?

3D Scene Representation

• Scene is usually approximated by 3D primitives Point

Line segment

Triangles

Polygon

Polyhedron

Curved surface

Solid object

etc.

3D Point

• Specifies a location

Origin

3D Point

• Specifies a location Represented by three coordinates

Infinitely small

struct Point3D{

float x , y , z;};

(𝑥, 𝑦, 𝑧)

Origin

3D Vector

• Specifies a direction and a magnitude

3D Vector

• Specifies a direction and a magnitude Represented by three coordinates

Magnitude Ԧ𝑣 = 𝑑𝑥2 + 𝑑𝑦2 + 𝑑𝑧2

Has no location

Ԧ𝑣 = (𝑑𝑥, 𝑑𝑦, 𝑑𝑧)

struct Vector3D{

float dx , dy , dz;};

• Specifies a direction and a magnitude Represented by three coordinates

Magnitude Ԧ𝑣 = 𝑑𝑥2 + 𝑑𝑦2 + 𝑑𝑧2

Has no location

• Dot product of two 3D vectors Ԧ𝑣1, Ԧ𝑣2 = 𝑑𝑥1 ⋅ 𝑑𝑥2 + 𝑑𝑦1 ⋅ 𝑑𝑦2 + 𝑑𝑧1 ⋅ 𝑑𝑧2 Ԧ𝑣1, Ԧ𝑣2 = Ԧ𝑣1 ⋅ Ԧ𝑣2 ⋅ cos 𝜃

• Cross product of two 3D vectors Ԧ𝑣1 × Ԧ𝑣2 = Vector normal to 𝑣1 and 𝑣2 Ԧ𝑣1 × Ԧ𝑣2 = Ԧ𝑣1 ⋅ Ԧ𝑣2 ⋅ sin 𝜃

3D Vector

Ԧ𝑣1 = (𝑑𝑥1, 𝑑𝑦1, 𝑑𝑧1)

Ԧ𝑣2 = (𝑑𝑥2, 𝑑𝑦2 , 𝑑𝑧2)

𝜃

Cross Product: Review

• Let Ԧ𝑣1 = Ԧ𝑣2 × Ԧ𝑣3: 𝑑𝑥1 = 𝑑𝑦2 ⋅ 𝑑𝑧3 − 𝑑𝑧2 ⋅ 𝑑𝑦3 𝑑𝑦1 = 𝑑𝑧2 ⋅ 𝑑𝑥3 − 𝑑𝑥2 ⋅ 𝑑𝑧3 𝑑𝑧1 = 𝑑𝑥2 ⋅ 𝑑𝑦3 − 𝑑𝑦2 ⋅ 𝑑𝑥3

• Ԧ𝑣 × 𝑤 = −𝑤 × Ԧ𝑣 (remember “right-hand” rule)

• We can show:

Ԧ𝑣 × 𝑤 = Ԧ𝑣 ⋅ 𝑤 ⋅ sin 𝜃 ⋅ 𝑛,where 𝑛 is the unit vector normal to Ԧ𝑣 and 𝑤

Ԧ𝑣 × Ԧ𝑣 = 0

3D Line Segment

• Linear path between two points

Origin

3D Line Segment

• Use a linear combination of two points Parametric representation:

» 𝑝 𝑡 = 𝑝1 + 𝑡 ⋅ (𝑝2 − 𝑝1), (0 ≤ 𝑡 ≤ 1)

struct Segment3D{

Point3D p1 , p2;};

𝑝1

𝑝2

Origin

3D Ray

• Line segment with one endpoint at infinity Parametric representation:

» 𝑝 𝑡 = 𝑝1 + 𝑡 ⋅ Ԧ𝑣, (0 ≤ 𝑡 < ∞)

𝑝1

Ԧ𝑣

Origin

struct Ray3D{

Point3D p1;Vector3D v;

};

3D Line

• Line segment with both endpoints at infinity Parametric representation:

» 𝑝 𝑡 = 𝑝1 + 𝑡 ⋅ Ԧ𝑣, (−∞ < 𝑡 < ∞)

𝑝1

struct Line3D{

Point3D p1;Vector3D v;

};

Ԧ𝑣

Origin

Origin

3D Plane

• A linear combination of three points

𝑝1

𝑝3𝑝2

Origin

3D Plane

• A linear combination of three points Implicit representation:

» Φ 𝑝 = 𝑎𝑥 + 𝑏𝑦 + 𝑐𝑧 − 𝑑 = 0

» Φ 𝑝 = 𝑝, 𝑛 − 𝑑 = 0

𝑛 is the plane normal

» (May be) unit-length vector

» Perpendicular to plane

𝑑 is the signed (weighted) distance

of the plane from the origin.

struct Plane3D{

Vector3D n;float d;

};

𝑛 = (𝑎, 𝑏, 𝑐)

𝑑

𝑝1

𝑝3𝑝2

3D Polygon

• Area “inside” a sequence of coplanar points Triangle

Quadrilateral

Convex

Star-shaped

Concave

Self-intersecting

Holes (use > 1 polygon struct)

struct Polygon3D{

Point3D *points;int npoints;

};

Points are in counter-clockwise order

3D Sphere

• All points at distance 𝑟 from center point 𝑐 = (𝑐𝑥 , 𝑐𝑦, 𝑐𝑧)

Implicit representation:

» Φ 𝑝 = 𝑝 − 𝑐 2 − 𝑟2 = 0

Parametric representation:

» 𝑥 𝜙, 𝜃 = 𝑟 ⋅ cos 𝜙 ⋅ sin 𝜃 + 𝑐𝑥» 𝑦 𝜙, 𝜃 = 𝑟 ⋅ cos 𝜙 ⋅ sin 𝜃 + 𝑐𝑦» 𝑧 𝜃, 𝜙 = 𝑟 ⋅ sin𝜙 + 𝑐𝑧

struct Sphere3D{

Point3D center;float radius;

};

𝑟

Origin

𝑐

Other 3D primitives

• Cone

• Cylinder

• Ellipsoid

• Box

• Etc.

3D Geometric Primitives

• More detail on 3D modeling later in course Point

Line segment

Triangle

Polygon

Polyhedron

Curved surface

Solid object

etc.

Overview



• What do we see?


How is the viewing device

described in a computer?

Camera Models

• The most common model is pin-hole camera All captured light rays arrive along paths toward focal point

without lens distortion (everything is in focus)

Other models consider ...

Depth of field

Motion blur

Lens distortion

View plane

Eye position

(focal point)

Camera Parameters

• What are the parameters of a camera?

Camera Parameters

right

back

Up direction

Eye Position

ViewPlane

“Look at”Point

• Position Eye position: Point3D eye

• Orientation View direction: Vector3D view Up direction: Vector3D up

• Aperture Field of view angle: float xFov , yFov Resolution of film plane: int width , height Distance of film plane

(Orientation of film plane)

Other Models: Depth of Field

Close Focused Distance Focused

P. Haeberli

Other Models: Motion Blur

Brostow & Essa

• Mimics effect of open camera shutter

• Gives perceptual effect of high-speed motion

• Generally involves temporal super-sampling

Traditional Pinhole Camera

• The film sits behind the pinhole of the camera.

• Rays come in from the outside, pass through the

pinhole, and hit the film plane.

Film Plane Pinhole

Traditional Pinhole Camera

• The film sits behind the pinhole of the camera.


pinhole, and hit the film plane.

Film Plane Pinhole

Photograph is upside down

Virtual Camera

• The film sits in front of the pinhole of the camera.


film plane, and hit the pinhole.

Film PlanePinhole

Virtual Camera

• The film sits in front of the pinhole of the camera.


film plane, and hit the pinhole.

Film PlanePinhole

Photograph is right side up

Overview



• Ray Casting Where are we looking?

What do we see?

How does it look?

Ray Casting

• For each sample … Where: Construct ray from eye through view plane

What: Find first surface intersected by ray through pixel

How: Compute color sample based on surface radiance

Ray Casting

• Simple implementation:

Image RayCast( Camera camera , Scene scene , int width , int height){

Image image = new Image( width , height );for( int i=0 ; i<width ; i++ ) for( int j=0 ; j<height ; j++ ){

Ray ray = ConstructRayThroughPixel( camera , i , j );Intersection hit = FindIntersection( ray , scene );image[i][j] = GetColor( hit );

}return image;

}

Ray Casting

Where?




}return image;

}

Constructing a Ray Through a Pixel

right

back

Up direction

𝒑𝟎

ViewPlane

𝒗

𝒑 𝒊 [𝒋]


right

back

Up direction ViewPlane

The ray originates at 𝑝0 (the

position of the camera).

So the equation for the ray is:

Ray 𝑡 = 𝑝0 + 𝑡 ⋅ Ԧ𝑣

𝒑𝟎

𝒗

𝒑 𝒊 [𝒋]


right

back


If the ray passes through the point

𝑝 𝑖 [𝑗], then the solution for Ԧ𝑣 is:

Ԧ𝑣 =𝑝 𝑖 [𝑗]−𝑝0

𝑝 𝑖 [𝑗]−𝑝0

𝒑𝟎

𝒗

𝒑 𝒊 [𝒋]


right

back


If 𝑝 𝑖 [𝑗] represents the (𝑖, 𝑗)-th pixel of

the image, what is its position?

𝒑𝟎

𝒗

𝒑 𝒊 [𝒋]

Constructing Ray Through a Pixel

• 2D Example: Side view of camera at 𝑝0 Where is the 𝑖-th pixel, 𝑝[𝑖]? (𝑖 ∈ [0, height))

𝑑

towards𝑝0

up

𝜃 = field of view angle (given)

𝑑 = distance to view plane (arbitrary = you pick)

𝜃



𝑝2

𝑝1

2⋅𝑑⋅tan

𝜃/2



𝑝1 = 𝑝0 + 𝑑 ⋅ towards − 𝑑 ⋅ tan𝜃

2⋅ up

𝑝2 = 𝑝0 + 𝑑 ⋅ towards + 𝑑 ⋅ tan𝜃

2⋅ up

𝑑

towards𝑝0

up

𝜃



𝑝 𝑖 = 𝑝1 +𝑖 + 0.5

height⋅ 𝑝2 − 𝑝1



𝑝1 = 𝑝0 + 𝑑 ⋅ towards − 𝑑 ⋅ tan𝜃

2⋅ up

𝑝2 = 𝑝0 + 𝑑 ⋅ towards + 𝑑 ⋅ tan𝜃

2⋅ up

𝑝2

𝑝1

𝑑

towards𝑝0

up

𝑝[𝑖]

𝜃

2⋅𝑑⋅tan

𝜃/2


• 2D Example: The ray passing through the 𝑖-th pixel is defined by:

Ray 𝑡 = 𝑝0 + 𝑡 ⋅ Ԧ𝑣 𝑝0: camera position

Ԧ𝑣: direction to the 𝑖-th pixel:

Ԧ𝑣 =𝑝 𝑖 − 𝑝0𝑝 𝑖 − 𝑝0

𝑝[𝑖]: 𝑖-th pixel location:

𝑝 𝑖 = 𝑝1 +𝑖 + 0.5

height⋅ 𝑝2 − 𝑝1

𝑝1 and 𝑝2 are the endpoints of the view plane:𝑝1 = 𝑝0 + 𝑑 ⋅ towards − 𝑑 ⋅ tan 𝜃/2 ⋅ up𝑝2 = 𝑝0 + 𝑑 ⋅ towards + 𝑑 ⋅ tan 𝜃/2 ⋅ up

𝑝2

𝑝1

𝑑

towards𝑝0

up

𝑝[𝑖]

𝜃

2⋅𝑑⋅tan

𝜃/2


Figuring out how to do this in 3D is assignment 2

Note: Given the vertical field of view angle, 𝜃𝑣

𝜽𝒗




And the aspect ratio, 𝑎𝑟 =ℎ𝑒𝑖𝑔ℎ𝑡

𝑤𝑖𝑑𝑡ℎ

heig

ht

𝜽𝒗




And the aspect ratio, 𝑎𝑟 =ℎ𝑒𝑖𝑔ℎ𝑡

𝑤𝑖𝑑𝑡ℎ

The horizontal field of view angle, 𝜃ℎ, satisfies:sin 𝜃𝑣/2

sin 𝜃ℎ/2= 𝑎𝑟

heig

ht

𝜽𝒉

𝜽𝒗

Ray Casting

Where?




}return image;

}

Ray Casting

What?




}return image;

}

Ray-Scene Intersection

• Intersections with geometric primitives Sphere

Triangle

Ray-Sphere Intersection

Ray: 𝑝 𝑡 = 𝑝0 + 𝑡 ⋅ Ԧ𝑣, (0 ≤ 𝑡 < ∞)Sphere: Φ 𝑝 = 𝑝 − 𝑐 2 − 𝑟2 = 0

𝑝0

𝒗

𝑐

𝑝

𝑟

𝑝′



Substituting for 𝑝, we get:

Φ 𝑡 = 𝑝0 − 𝑡 ⋅ Ԧ𝑣 − 𝑐 2 − 𝑟2 = 0

Solve quadratic equation:

𝑎 ⋅ 𝑡2 + 𝑏 ⋅ 𝑡 + 𝑐 = 0where:

𝑎 = 1𝑏 = 2 Ԧ𝑣, 𝑝0 − 𝑐𝑐 = 𝑝0 − 𝑐 2 − 𝑟2

𝑝0

𝒗

𝑐

𝑝

𝑟

𝑝′




Φ 𝑡 = 𝑝0 − 𝑡 ⋅ Ԧ𝑣 − 𝑐 2 − 𝑟2 = 0

Solve quadratic equation:

𝑎 ⋅ 𝑡2 + 𝑏 ⋅ 𝑡 + 𝑐 = 0where:

𝑎 = 1𝑏 = 2 Ԧ𝑣, 𝑝0 − 𝑐𝑐 = 𝑝0 − 𝑐 2 − 𝑟2

𝑝0

𝒗

𝑐

𝑝

𝑟

𝑝′

Generally, there are two solutions to

the quadratic equation, giving two

points of intersection, 𝑝 and 𝑝′.Want to return the first positive hit.


𝑝0

Ԧ𝑣

𝑐

𝑝 𝑟

𝑛

• Need normal vector at intersection

for lighting calculations:

𝑛 =𝑝 − 𝑐

𝑝 − 𝑐


• More generally, if the shape is given as the set of

points 𝑝 satisfying:

Φ 𝑝 = 0for some function Φ:ℝ3 → ℝ, then the normal of

the surface will be parallel to the gradient.

Ray-Scene Intersection

• Intersections with geometric primitives Sphere

» Triangle

Ray-Triangle Intersection

1. Intersect ray with plane

2. Check if the point is inside the triangle

𝑝

𝑝0

Ԧ𝑣

Ray-Plane Intersection

Ray: 𝑝 𝑡 = 𝑝0 + 𝑡 ⋅ Ԧ𝑣, (0 ≤ 𝑡 < ∞)Plane: Φ 𝑝 = 𝑝, 𝑛 − 𝑑 = 0


Φ 𝑡 = 𝑝0 + 𝑡 ⋅ Ԧ𝑣, 𝑛 − 𝑑 = 0

Solution:

𝑡 = −𝑝0, 𝑛 − 𝑑

Ԧ𝑣, 𝑛 𝑛

𝑝

𝑝0

Ԧ𝑣

Algebraic Method

What are the implications of Ԧ𝑣, 𝑛 = 0?

Ray-Triangle Intersection I

• Check for point-triangle intersection algebraically: Generate planes through the ray

source and each edge

Check if the point of intersection is

above each of these planes

𝑝0

𝑣2

𝑣3

For each edge:

𝑛𝑖 = (𝑣𝑖+1−𝑝0) × (𝑣𝑖 − 𝑝0)if ( 𝑝 − 𝑝0, 𝑛𝑖 < 0 )

return FALSE;

𝑛1

𝑣1

𝑝

Ray-Triangle Intersection II

• Check for point-triangle intersection parametrically

In general, given 𝑝 ∈ ℝ3 and given

three points 𝑣1, 𝑣2, 𝑣3 ⊂ ℝ3

(in general position) we can

solve for 𝛼, 𝛽, 𝛾 ∈ ℝ such that:

𝑝 = 𝛼𝑣1 + 𝛽𝑣2 + 𝛾𝑣3

𝑝 is in the plane spanned by 𝑣1, 𝑣2, 𝑣3 iff.:

𝛼 + 𝛽 + 𝛾 = 1𝑝 is inside the triangle with vertices {𝑣1, 𝑣2, 𝑣3} iff.:

𝛼, 𝛽, 𝛾 ≥ 0

𝑝

𝑣1𝑣2

𝑣3

𝛼 𝛽

𝛾







𝑝 = 𝛼𝑣1 + 𝛽𝑣2 + 𝛾𝑣3

To get 𝛼, 𝛽, 𝛾, solve the system:

𝑣1𝑥 𝑣2

𝑥 𝑣3𝑥

𝑣1𝑦

𝑣2𝑦

𝑣3𝑦

𝑣1𝑧 𝑣2

𝑧 𝑣1𝑧

𝛼𝛽𝛾

=

𝑝𝑥

𝑝𝑦

𝑝𝑧⇔

𝛼𝛽𝛾

=

𝑣1𝑥 𝑣2

𝑥 𝑣3𝑥

𝑣1𝑦

𝑣2𝑦

𝑣3𝑦

𝑣1𝑧 𝑣2

𝑧 𝑣11

−1𝑝𝑥

𝑝𝑦

𝑝𝑧

𝑝𝛼 𝛽

𝛾

𝑣1𝑣2

𝑣3







𝑝 = 𝛼𝑣1 + 𝛽𝑣2 + 𝛾𝑣3


𝑣1𝑥 𝑣2

𝑥 𝑣3𝑥

𝑣1𝑦

𝑣2𝑦

𝑣3𝑦

𝑣1𝑧 𝑣2

𝑧 𝑣1𝑧

𝛼𝛽𝛾

=

𝑝𝑥

𝑝𝑦

𝑝𝑧⇔

𝛼𝛽𝛾

=

𝑣1𝑥 𝑣2

𝑥 𝑣3𝑥

𝑣1𝑦

𝑣2𝑦

𝑣3𝑦

𝑣1𝑧 𝑣2

𝑧 𝑣11

−1𝑝𝑥

𝑝𝑦

𝑝𝑧

𝑝𝛼 𝛽

𝛾

𝑣1𝑣2

𝑣3







𝑝 = 𝛼𝑣1 + 𝛽𝑣2 + 𝛾𝑣3


𝑣1𝑥 𝑣2

𝑥 𝑣3𝑥

𝑣1𝑦

𝑣2𝑦

𝑣3𝑦

𝑣1𝑧 𝑣2

𝑧 𝑣3𝑧

𝛼𝛽𝛾

=

𝑝𝑥

𝑝𝑦

𝑝𝑧⇔

𝛼𝛽𝛾

=

𝑣1𝑥 𝑣2

𝑥 𝑣3𝑥

𝑣1𝑦

𝑣2𝑦

𝑣3𝑦

𝑣1𝑧 𝑣2

𝑧 𝑣3𝑧

−1𝑝𝑥

𝑝𝑦

𝑝𝑧

𝑝𝛼 𝛽

𝛾

𝑣1𝑣2

𝑣3

This will fail if the vertices 𝑣1, 𝑣2, 𝑣3lie in a plane through the origin.


• Check for point-triangle intersection parametricallyEmbrace the problem case by translating the whole

system to the origin:

𝑝

𝑣1 𝑣2

𝑣3

𝛼𝛽

𝛾

𝑝 − 𝑣1

𝑣2 − 𝑣1

𝑣3 − 𝑣1

𝛼𝛽

𝛾

𝑣1𝑥 𝑣2

𝑥 𝑣3𝑥

𝑣1𝑦

𝑣2𝑦

𝑣3𝑦

𝑣1𝑧 𝑣2

𝑧 𝑣3𝑧

𝛼𝛽𝛾

=

𝑝𝑥

𝑝𝑦

𝑝𝑧

0 𝑣2𝑥 − 𝑣1

𝑥 𝑣3𝑥 − 𝑣1

𝑥

0 𝑣2𝑦− 𝑣1

𝑦𝑣3𝑦− 𝑣1

𝑦

0 𝑣2𝑧 − 𝑣1

𝑧 𝑣3𝑧 − 𝑣1

𝑧

𝛼𝛽𝛾

=

𝑝𝑥

𝑝𝑦

𝑝𝑧




𝑝

𝑣1 𝑣2

𝑣3

𝛼𝛽

𝛾

𝑝 − 𝑣1

𝑣2 − 𝑣1

𝑣3 − 𝑣1

𝛼𝛽

𝛾

𝑣1𝑥 𝑣2

𝑥 𝑣3𝑥

𝑣1𝑦

𝑣2𝑦

𝑣3𝑦

𝑣1𝑧 𝑣2

𝑧 𝑣3𝑧

𝛼𝛽𝛾

=

𝑝𝑥

𝑝𝑦

𝑝𝑧

𝑣2𝑥 − 𝑣1


𝑥

𝑣2𝑦− 𝑣1


𝑦

𝑣2𝑧 − 𝑣1


𝑧

𝛽𝛾

=

𝑝𝑥

𝑝𝑦

𝑝𝑧




𝑝

𝑣1 𝑣2

𝑣3

𝛼𝛽

𝛾

𝑝 − 𝑣1

𝑣2 − 𝑣1

𝑣3 − 𝑣1

𝛼𝛽

𝛾

𝑣1𝑥 𝑣2

𝑥 𝑣3𝑥

𝑣1𝑦

𝑣2𝑦

𝑣3𝑦

𝑣1𝑧 𝑣2

𝑧 𝑣3𝑧

𝛼𝛽𝛾

=

𝑝𝑥

𝑝𝑦

𝑝𝑧

𝑣2𝑥 − 𝑣1


𝑥

𝑣2𝑦− 𝑣1


𝑦

𝑣2𝑧 − 𝑣1


𝑧

𝛽𝛾

=

𝑝𝑥

𝑝𝑦

𝑝𝑧

This is an over-constrained

system!

This is an over-constrained system!

In general, we can’t express a 3D point as the

linear combination of two 3D points.

This is not the general case!

A solution exists since 𝑝 is in the plane spanned by {𝑣1, 𝑣2, 𝑣3}

After solving for 𝛽 and 𝛾, we can set:

𝛼 = 1 − 𝛽 − 𝛾

Other Ray-Primitive Intersections

• Cone, cylinder, ellipsoid: Similar to sphere

• Box Intersect 3 front-facing planes, return closest

• Convex polygon» Find the intersection of the ray with the plane

» Check that the intersection is above every triangle

generated by the ray source and polygon edge.

• Concave polygon Same plane intersection

More complex point-in-polygon test

Documents

3D Rendering and Ray Castingmisha/Fall19/06.pdf3D Rendering and Ray Casting Michael Kazhdan (601.457/657) HB Ch. 13.7, 14.6 FvDFH 15.5, 15.10 Announcements • We have a new CA: Cindy