RenderMan for Artist 03 – Let’s shade a sphere.: “Flat” to “Plastic

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RenderMan For Artists #03

Wanho Choi

(wanochoi.com)

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RenderMan for Artists wanochoi.com

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(Review) RIB Generation from Maya (Method#1)

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(Review) RIB Generation from Maya (Method#1)

Display "result.jpg" "file" "rgb" Projection "perspective" "fov" 30 WorldBegin

ReadArchive "./sphere.rib" WorldEnd

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• rman genrib  perspShape_Final.0001.rib

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Sine & Cosine Function

30

60

1

3

2 3 3 ) 30 tan( 2 1 ) 30 sin( 2 3 ) 30 cos(       3 ) 60 tan( 2 3 ) 60 sin( 2 1 ) 60 cos(      

45

1

2

1 ) 45 tan( 2 2 ) 45 sin( 2 2 ) 45 cos(         cos  sin    cos sin tan 

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Scalar vs Vector

• Scalar

–

Only has magnitude

–

Ex. Mass: 1Kg, Temperature: 25 ºC, Speed: 100 km/h

• Vector (2D, 3D, 4D, …)

–

Has magnitude & direction (=arrow, ≠point)

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• Addition

• Subtraction

• Multiplication with scalar

–

Resultant vector is

a

times as long.

Basic Operations of Vectors

-A

B

=

C

A

×

2 =

C

+

A

B

=

C

) , , ( ) , , ( ) , , (A_x A_y A_z  B_x B_y B_z  A_xB_x A_yB_y A_z B_z   A B C ) , , ( ) , , ( ) , , (A_x A_y A_z  B_x B_y B_z  A_xB_x A_yB_y A_z B_z   A B C



ABBA





ABBA



) 1 , 1 ( ) 1 , 0 ( ) 0 , 1 (     A B C ) 0 , 1 ( ) 1 , 0 ( (1,1) ) 1 , 1 ( ) 1 , 0 ( ) 0 , 1 (      A B C ) 0 , 1 ( ) 1 , 0 ( ) 1 , 1 (  ) 1 , 1 ( ) 1 , 1 ( 1 1 ) 5 . 0 , 5 . 0 ( ) 1 , 1 ( ) 5 . 0 ( ) 5 . 0 ( ) 2 , 2 ( ) 1 , 1 ( 2 2                  A D A D A C ) 0 , 1 ( ) 1 , 0 ( ) 1 , 1 ( ) 2 , 2 ( ) 1 , 1 (  ) 5 . 0 , 5 . 0 ( ) , , (A_x A_y A_z    A C

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Example – Vector Subtraction



3,1,0





1,3,0





5,1,0





3,3,0





2,2,0



x y 0



2,2,0





2,2,0



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Length & Normal of Vectors

2 2 2 2 / 1 2 2 2 ) (A_x A_yA_z  A_x A_y A_z  A                 2 2 2 2 2 2 2 2 2 , , / z y x z z y x y z y x x n A A A A A A A A A A A A A A A

• Length (=magnitude)

–

• Normal

–

※ normalize, normalization

2 1 1 ) 1 , 1 (  2 2   A ) 0 , 1 ( ) 1 , 0 ( (1,1) 2         2 1 , 2 1 / A A A_n ) 0 , 1 ( ) 1 , 0 ( (1,1)       2 1 , 2 1

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Dot Product (=Inner Product, 내적)



cos

B

A

B

A





)

(

2

1

2 2 2

C

B

A







)

(

2

1

2 2 2

B

A

B

A









 





2 2 2 2 2 2 2 2 2



)

(

)

(

)

(

)

(

)

(

2

1

z z y y x x z y x z y x

a

b

a

b

a

b

a















z z y y x x

b

a

b

a

b

a





B

A

C

B

A

B

A











2

2 2 2

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Dot Product (=Inner Product, 내적)

• Result: scalar

• When A & B are unit vector, .

• When A & B are orthogonal, .

• When A & B are in same direction, .

• When A & B are in opposite direction, .

• “Dot Product” is useful for the shading, collision detection, etc.



cos

B

A

B

A





z z y y x x

b

a

b

a

b

a







cos





B

A



A



B







A



B









arccos

cos

1



0 



B

A

0

90 cos

cos











0 



B

A

0 



B

A

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Shader

• A small program to be used to determine the final surface properties.

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• shader

myShader.sl



myShader.slo

• prman

test.rib



result.jpg

Display "result.jpg" "file" "rgb" Projection "perspective" "fov" 30 Translate 0 0 5

WorldBegin

LightSource "ambientlight" 1 "intensity" [0.1]

LightSource "pointlight" 2 "from" [-2 2 -2] "intensity" [7] Color [1 1 1] Surface "myShader" Sphere 1 -1 1 360 WorldEnd surface myShader() { Oi = Os; Ci = Oi * Cs; }

2-01_sphere_basic

test.rib myShader.sl

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.rib File Structure (Example #1)

Display “sphere.tiff” “file” “rgb” Projection “perspective”

WorldBegin Translate 0 0 2

Surface “myShader” “color surfaceColor” [1 0 0] Sphere 1 -1 1 360

WorldEnd

.rib

surface myShader(

color surfaceColor = color(1,1,1); color surfaceOpacity = color(1,1,1); ) { Oi = surfaceOpacity; Ci = Oi * surfaceColor; } .sl

Setup Part

Drawing Part

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Surface Shader – Basic Structure

• O

is for opacity

–

Os

: opacity input provided by the RIB stream

–

Oi

: opacity output, which are then used by the renderer in the rest of its processing

• C

is for color

–

Cs

: color input provided by the RIB stream

–

Ci

: opacity output, which are then used by the renderer in the rest of its processing

• Globals

–

Predefined shader variables

–

Os, Oi, Cs, Ci, ….

myBasic.slo

Os

Cs

Oi

Ci

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Display "result.jpg" "file" "rgb" Projection "perspective" "fov" 30 Translate 0 0 5

WorldBegin

LightSource "ambientlight" 1 "intensity" [0.1]

LightSource "pointlight" 2 "from" [-2 2 -2] "intensity" [7] Color [1 1 1]

Surface "myShader" "color surfaceColor" [1 0 0]

Sphere 1 -1 1 360 WorldEnd

surface myShader( color surfaceColor = color(1,1,1);

color surfaceOpacity = color(1,1,1); ) { Oi = surfaceOpacity; Ci = Oi * surfaceColor; }

2-02_sphere_constant

.rib .sl

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Illumination

• Global illumination

–

Considers the interaction of light

between all surfaces

in a scene

• Local illumination

–

Considers the interaction of light between

one light source

and

one object surface

.

–

Usually, it is approximated as the summation of three components.

:

Ambient, Diffuse, Specular

• Ambient reflection

–

Crude approximation to global effects of light

–

Accounts for the general brightness from light scattering in all directions from all surfaces.

• Diffuse reflection

–

Due to the „roughness‟ of a surface at the microscopic level

–

Accounts for the light hitting a surface and then scattering evenly in all directions

–

Depends on “surface normal”

• Specular reflection

–

Due to the „shininess‟ of a material

–

Accounts for the highlights

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• Ideal diffuse reflection

–

An

ideal diffuse reflector

, at the microscopic level, is a very rough surface.

(real-world example: chalk)

–

Because of these microscopic variations, an incoming ray of light is equally likely to be

reflected in any direction over the hemisphere.

• What does the reflected intensity depend on?

(Lambert‟s cosine law)

–

The energy reflected by a small portion of a surface from a light source in a given direction

is proportional to the cosine of the angle between that direction and the surface normal.

• These are often called

Lambertian surfaces

• Note that the reflected intensity is independent of the viewing direction, but does

depend on the surface orientation with regard to the light source.

I

_diffuse

= k

_d

I

_light

cos



= k

_d

I

_light

(n • l)

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Phong Shading Model

= Phong reflection model = Phong illumination model = Phong Ligting model

• An empirical model of local illumination

• K

_a

: ambient reflection constant

• K

_d

: diffuse reflection constant

• K

_s

: specular reflection constant

• α : shininess constant for this material (=1/roughness)



























lights

i

s

d

a

p

k

I

k

I

N

L

k

I

R

V

I



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Specular Shiness



























lights

i

s

d

a

p

k

I

k

I

N

L

k

I

R

V

I



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Blinn-Phong Shading Model

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surface myShader( uniform float Ka = 1.00;

color surfaceColor = color(1,1,1); color surfaceOpacity = color(1,1,1); ) { Oi = surfaceOpacity; Ci = Oi * surfaceColor * ( Ka * ambient() ); }

2-03_sphere_ambient

.sl

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surface myShader( uniform float Kd = 1.00;

color surfaceColor = color(1,1,1); color surfaceOpacity = color(1,1,1); ) { normal Nn = normalize(N); color Cdiff = 0; illuminance( P, Nn, PI/2 ) { vector Ln = normalize(L); Cdiff += Cl * max( 0, Nn.Ln ); } Oi = surfaceOpacity; Ci = Oi * surfaceColor * ( Kd * Cdiff ); }

2-04_sphere_diffuse

.sl

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surface myShader( uniform float Kd = 1.00;

color surfaceColor = color(1,1,1); color surfaceOpacity = color(1,1,1); ) { normal Nn = normalize(N); Oi = surfaceOpacity; Ci = Oi * surfaceColor * ( Kd * diffuse(Nn) ); }

2-05_sphere_diffuse

.sl

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surface myShader( uniform float Ks = 1.00; uniform float roughness = 0.20;

color surfaceColor = color(1,1,1); color surfaceOpacity = color(1,1,1); color specularColor = color(1,1,1); ) { normal Nn = normalize(N); vector V = -normalize(I); color Cspec = 0; illuminance( P, Nn, PI/2 ) { vector Ln = normalize(L); vector H = normalize( Ln + V );

Cspec += Cl * pow( max( 0, Nn.H ), 1/roughness ); } Oi = surfaceOpacity; Ci = Oi * specularColor * ( Ks * Cspec ); }

2-06_sphere_specular

.sl

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surface myShader( uniform float Ks = 1.00; uniform float roughness = 0.20;

color surfaceColor = color(1,1,1); color surfaceOpacity = color(1,1,1); color specularColor = color(1,1,1); ) { normal Nn = normalize(N); vector V = -normalize(I); Oi = surfaceOpacity; Ci = Oi * specularColor * ( Ks * specular(Nn,V,roughness) ); }

2-07_sphere_specular

.sl

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surface myShader( uniform float Ka = 1.00; uniform float Kd = 0.85;

color surfaceColor = color(1,1,1); color surfaceOpacity = color(1,1,1); ) {

normal Nn = normalize(N); Oi = surfaceOpacity;

Ci = Oi * surfaceColor * ( Ka * ambient() + Kd * diffuse(Nn) ); }

2-08_sphere_lambert

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surface myShader( uniform float Ka = 1.00; uniform float Ks = 1.00; uniform float roughness = 0.20;

color surfaceColor = color(1,1,1); color surfaceOpacity = color(1,1,1); color specularColor = color(1,1,1); ) { normal Nn = normalize(N); vector V = -normalize(I); Oi = surfaceOpacity; Ci = Oi * ( surfaceColor * ( Ka * ambient() ) + specularColor * ( Ks * specular(Nn,V,roughness) ) ); }

2-09_sphere_metal

.sl

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surface myShader( uniform float Ka = 1.00; uniform float Kd = 1.00; uniform float Ks = 1.00; uniform float roughness = 0.20;

color surfaceColor = color(1,1,1); color surfaceOpacity = color(1,1,1); color specularColor = color(1,1,1); ) {

normal Nn = normalize(N); vector V = -normalize(I); Oi = surfaceOpacity;

Ci = Oi * ( surfaceColor * ( Ka * ambient() + Kd * diffuse(Nn) ) + specularColor * ( Ks * specular(Nn,V,roughness) ) ); }

2-10_sphere_plastic

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Surface Shader – Surface Normal (N vs Ng)

• N

: surface shading normal

• Ng

: Surface geometric normal

• Surface shaders are evaluated after the displacement shaders.

• This means that if the displacement shader altered the position of P and the direction

of N, then those values will be used by the surface shader.

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References

• Cook, Robert L., Loren Carpenter, Edwin Catmull, The Reyes Image Rendering Architecture, SIGGRAPH 87.

• Saty Raghavachary, Rendering for Beginners: Image synthesis using RenderMan, Focal Press, 2004

• Rudy Cortes, Saty Raghavachary, The RenderMan Shading Language Guide, Thomson Course Technology, 2007 • Ian Stephenson, Essential RenderMan Fast, Springer, 2003

• Anthony A. Apodaca, Larry Gritz, Advanced RenderMan: Creating CGI for Motion Pictures, Morgan Kaufmann, 1999

• http://en.wikipedia.org/wiki/Phong_shading