10-lighting-shading.ppt

Lighting

•

_{The other side of rendering}

•

Computing the light

– How much

– Which colour

arriving

– From a point in the scene

– To the eye

Lighting: relevant factors

light

eye

object

Trasmission (with rifraction)

absorption

Inner reflection

Absorption by the environment

(e.g. fog)

Sub-surface scattering

Additional light

blo

ck

er

shade

reflection

Lighting: relevant factors

Multiple reflections

(indirect Lighting)

light

Lighting: global VS local

– Only takes into account:

• Light conditions

– N. lights

– their position

– Their color

• Surface area to lite

– orientation (normal)

– Optical charcateristics

» E.g, color

– Anything else is ignored

– Multiple reflections

– shadows

– Sub-surface scattering

– refraction

– ...

Local lighting

Global lighting

Much easier

to do on our

Local lighting

light

eye

object

reflection

What can be easily accomplished

•

Local lighting:

– Light reflections on objects

• Using simplified optical properties

– wiht multiple light sources

• But simplifies: point-sized

•

Global lighting:

– Multiple reflections

• VERY simplified

– Absorption by the environment

• Simplified case (uniform fog)

– Everything else only “ad-hoc"

The 3 factors we take into account

total light

=

ambient

+

reflection

+

emission

For any factor we have the

R, G and B components.

Defined for both the

object

,

(vertex attributes)

And for each

light

used

The optical properties of the

object

,

(usually vertex attributes)

Collected together form the

"

material

"

Component

emission

•

_{LEDs, light bulbs...}

•

Not related to the lights

– Only to the object

•

It is only an addittive Component

– costant for R, G and B

•

Note: it does not send light to nearby objects

– It isn’t Global lighting

The 3 factors we take into account

total light

=

ambient

+

reflection

+

Component

ambient

•

It models (very approximately) the light hitting

the object after multiple reflections

•

Assumption:

“a small amount of light hits any surface from

every direction"

– Even surfaces in shadow

•

Small additive

constant

Component

ambient

•

_{Product of:}

– “

ambient

” color of the material ( R

_M

G

_M

B

_M

)

– “

ambient

” color of the light ( R

_L

G

_L

B

_L

)

•

_{Note: the RGB colors can be different}

Component

ambient

•

_{It models (very approximately) the light hitting}

the object after multiple reflections

without

The 3 factors we take into account

total light

=

ambient

+

reflection

+

emission

diffuse reflection

+

specular reflection

only

Component

The 4 factors we take into account

total light

=

ambient

+

diffuse reflection

+

specular reflection

+

Component

diffuse reflection

•

_{Effect shown by a few materiale (e.g.):}

– chalk

– wood (almost)

– Very opaque materials(not shiny)

•

Also known as

– diffuse reflection

– Lambertian reflection

Component

diffuse reflection

The light hitting a lambertian

surface is reflected in all

directions (in the half-sphere)

Component

diffuse reflection

The light hitting a lambertian

surface is reflected in all

directions (in the half-sphere)

Component

diffuse reflection

The light hitting a lambertian

surface is reflected in all

directions (in the half-sphere)

Component

diffuse reflection

•

_{It only depends on:}

– Surface orientation

• (the "normal")

– Light direction

Component

diffuse reflection

•

_{It only depends on:}

– Surface orientation

N

• (the "normal")

– Light direction

L

• (Light ray)



cos







_light

_diff

_material

_diff

diff

I

k

I

R, G, B

(usually white: 1,1,1)

R, G, B

(the "color" of the object)

property of the

"material"

(associated to

the object)

Component

diffuse reflection

•

_{It only depends on:}

– Surface orientation

N

• (the "normal")

– Light direction

L

• (Light ray)



cos







_light

_diff

_material

_diff

diff

I

k

Component

diffuse reflection

•

_{It only depends on:}

– Surface orientation

N

• (the "normal")

– Light direction

L

• (Light ray)



cos







_light

_diff

_material

_diff

diff

I

k

Component

diffuse reflection

•

_{It only depends on:}

– Surface orientation

N

• (the "normal")

– Light direction

L

• (Light ray)

)

Lˆ

Nˆ

(









I

_light

_diff

k

_material

_diff

Angle between

0⁰ and 90⁰

,

otherwise 0,

(object in its own shadow)



cos







_light

_diff

_material

_diff

diff

I

k

Component

diffuse reflection

L

N

Component

diffuse

small

⍬

=70⁰

L

N

Component

diffuse

large

⍬

=35⁰

L

N

Component

diffuse

largest

Component

diffuse reflection

L

N

Component

diffuse

ZERO

⍬

=90⁰

L

N

Component

diffuse

ZERO

⍬

>90⁰

Component

diffuse reflection

•

_Properties

– Accurate model of the optical characteristics

of a number of real materials



– But only a small number



– The model is physically coherent



• As an example, energy is conserved

The 4 factors we take into account

total light

=

ambient

+

diffuse reflection

+

specular reflection

+

Component

specular reflection

•

_{"Specular" reflection}

•

For shiny materials

– With bright reflections

– ("highlights")

Component

specular reflection

•

Basic idea:

light

is not

reflected

by

shiny materiale

Component

specular reflection

L: light ray

N: normal

R: reflected ray

V: view direction

N

L

_R

V

 

_

Component

specular reflection

•

_{Phong light model}

– by Bui-Tuong Phong, 1975

in 3D



cos







_light

_spec

_material

_spec

spec

I

k

Component

specular reflection

•

_{By using higher exponents for the cosine, we can}

Component

specular reflection

•

_{Phong light model}

– by Bui-Tuong Phong, 1975

in 3D

Belong to the "material"

(properties of the object)



n

spec

material

spec

light

spec

I

k

I







cos



cos







_light

_spec

_material

_spec

spec

I

k

Component

specular reflection

•

_{Phong light model}

– by Bui-Tuong Phong, 1975

in 3D



cos







_light

_spec

_material

_spec

spec

I

k

I

n

spec

material

spec

light

k

R

V

I





(

ˆ



ˆ

)





n

spec

material

spec

light

spec

I

k

Component

specular reflection

1



n

n



5

n



10

n



100

Component

specular reflection

• Blinn-Phong light model:

• Simplification of the Phong light model

• Similar results, different formula:

n

spec

material

spec

light

spec

I

k

R

V

I







(

ˆ



ˆ

)

phong:

blinn-phong:

I

_spec



I

_light

_spec



k

_material

_spec



(

H

ˆ



N

ˆ

)

n

N

L

_R

V

 

_

Component

specular reflection

• Blinn-Phong light model:

• Simplification of the Phong light model

• Similar results, different formula:

Jim Blinn

(MEGA-MEGA-GURU)

phong:

blinn-phong:

n

spec

material

spec

light

spec

I

k

R

V

I







(

ˆ



ˆ

)

n

spec

material

spec

light

spec

I

k

H

N

The 4 factors we take into account

total light

=

ambient

+

diffuse reflection

+

specular reflection

+

Complete lighting equation









n

specular

material

specular

light

k

H

N

I

(

)









k

(

L

N

)

I

_light

_diffuse

_material

_diffuse





_material

_ambient

ambient

light

k

I

emission

material

k



tot

I

property of the

Lighting equation: lights modeling

emission

materiale

k



tot

I

property of the light

L

V

V

L

ˆ

ˆ

)

ˆ

ˆ

(













n

specular

material

specular

light

k

H

N

I

(

)









k

(

L

N

)

I

_light

_diffuse

_material

_diffuse





_material

_ambient

ambient

light

k

Lights modeling

•

_{How does L change ?}

– constant in the scene: "

directional

" light sources

• Used for light sources very far away, e.g. the sun

– Variable in the scene: "

positional

" light sources

Lights modeling: positional lights

•

_{The positional lights, have their}

_intensity

dimmed according to the

distance

•

_{In theory (according to physics)}

intensity = 1 / distance

2

















₂

L

1

d

c

Lights modeling: positional lights

•

In practice, it leads to light attenuations that are

too quick

•

We rather use:



















min

1

₂

,

1

L

3

L

2

1

c

d

c

d

Lighting equation



tot

I



f

light

attenuatio

n



















min

1

₂

,

1

L

3

L

2

1

c

d

c

d

c

f

_light

_attenuatio

_n

n

specular

material

specular

light

k

H

N

I





(



)









k

(

L

N

)

I

_light

_diffuse

_material

_diffuse





_material

_ambient

Kind of lights

•

_{Kind of lights:}

Spotlights

Lighting equation

effect

spotlight

f





direction

cutoff

Angle

beam

width



effect

spotlight

L

spot

spot

spot

f



f

,

,

,



tot

I

light

attenuatio

n

f



n

specular

material

specular

light

k

H

N

I





(



)









k

(

L

N

)

I

_light

_diffuse

_material

_diffuse





_material

_ambient

ambient

light

k

I

emission

material

k





















min

1

₂

,

1

L

3

L

2

1

c

d

c

d

Lighting: Where?

Lighting & Shading

Computer Graphics Pagina 49

y

z

v

v

v

v

v

v

Final pixels

The 4 factors we take into account

total light

=

ambient

+

diffuse reflection

+

specular reflection

+

Lighting equation

n

specular

material

specular

light

k

H

N

I





(

ˆ



ˆ

)









k

(

L

ˆ

N

ˆ

)

I

_luight

_diffuse

_material

_diffuse





_material

_ambient

ambient

light

k

I

emission

material

k





tot

I



f

light

attenuatio

n



















min

1

₂

,

1

L

3

L

2

1

c

d

c

d

c

f

_light

_attenuatio

_n

effect

spotlight

f





direction

cutoff

Angle

beam

width



effect

spotlight

L

spot

spot

spot

f



f

,

,

,

properties of the light

properties of the

material

^

Normal of a triangle

•

_{That is its orientation in space}

N

v

₀

v

₂

v

₁

)

(

)

(

v

₁

v

₀

v

₂

v

₀

N









|

|

ˆ

Lighting face by face

"flat shading"

1.

Initial geometry

2.

For each face,

Compute the normal

3.

Definition

•

_Shading

_:

–

Recipe to apply

lighting

•

_{As an example:}

flatshading

1. Apply lighting to each face normal

- (obtaining a color)

Flat shading: problem

When we approximate curved surfaces with

triangles and apply flat shading the result is:

– Edges very visible on curved surfaces

bad

artifact

!

Flat shading: problem

Flat shading: problem

•

Increasing the number

of faces, makes the

problem less visible

>10.000 faces,

and we still see

the artifact

edges

Flat shading: problem

To make things worse:

the

Mach-band optical effect

The contrast between different

uniform areas catches our eye.

(even when areas are a lot,

and their difference is small).

The brain increases the

contrast between uniform

areas.

Idea

"Gouraud" Shading

To apply the lighting, I need to have the

normal!

The normal is defined for a face, not for a

vertex

by Henri Gouraud,

1971

Idea

•

_{Use color interpolation within a face}

1- Apply lighting to the 3 vertices of each triangle

• (obtain a color)

Vertex normals

•

_{In many cases, the vertex normals are defined}

at the same time of the 3D model.

– As an example,

• When we model a sphere, a cylinder, a cone...

• When we compute the surface of a volume

• When we a triangulated surface sampling a parametric surface

• ...

Vertex normals

Normal of a triangle:

v

₁

v

₂

v

₁

×

v

₂

v

N

ˆ

ˆ

N

ˆ

N

ˆ

N

ˆ

N

ˆ

N

ˆ

N

Normal of a vertex

Shared by

n

triangles:

n

N

N

N

N



ˆ

₁



ˆ

₂



...



ˆ

Where do we compute lighting?

x

y

z

v

v

v

v

v

v

Final pixels

Fundamental decision

•

_{In our rendering paradigm, the (vertex) normal:}

• Is NOT

computed in the pipeline

• Is introduced as an

ATTRIBUTE

of each

VERTEX

•

The normale “is in the model"

• Exactly similar to vertex position

• When necessary, the normals computation is tipically a

pre-processing

Final pixels

Gouraud shading

How to improve the result

•

_{Rather than interpolating the}

_color

_{after the}

lighting

, we interpolate the

normal

before of the

lighting

!

•

_{Be careful:}

interpolating two normal vectors, the result is

not always a normal:

How to improve the result

•

_{Rather than interpolating the}

_color

_{after the}

lighting

, we interpolate the

normal

before of the

lighting

!

"Phong" Shading

_{by Bui-Tuong Phong ,}

1973

1- Interpolate the normal in the face

2- Normalize

3- Compute lighting

* Do not confuse Phong Shading with the Phong Lighting Model

Final pixels

(filling of the screen-buffer)

fr

a

gm

e

nt

s

(c

an

di

da

te

p

ix

el

s)

V

e

rt

ic

e

s

(p

oi

nt

s

in

R

)

P

ro

je

ct

ed

ve

rt

ic

es

(p

oi

nt

s

in

R

)

V

er

te

x

c

om

pu

ta

tio

ns

F

ra

gm

en

t

co

m

pu

ta

tio

ns

set-up

set-up

set-up

rasterizer

triangoli

Segments

rasterizer

rasterizer

punti

Triangles

rasterizer

Points

rasterizer

Phong shading

including:

property

of the

material

and

normal

Projection

of

vertices

and

normals

Normal

interpol

ation

including:

interrpolate

d normal

including:

Transforme

d normal

Normalize

and compute

lighting

To obtain the

Gouraud vs Phong shading

•

Goraud Shading

-

lighting per vertex much less

computationally expensive:

Compute the lighting once per vertex!

•

Phong Shading

-

lighting per fragment offers better results

Especially suited for bright and small highlights (high shiness)

Gouraud and Phong shading

flat shading

Goraud shading

Gouraud and Phong shading

•

_{Goraud and Phong suited for curved surfaces}

– They hide the artifact edges

– Also hide the REAL edges

Gouraud or Phong shading?

•

_{The OpenGL specifications}

_{do not recommend}

which one should be used

– In most cases (at least now) Gouraud

•

_{It dipends on the HW implementation}

Its use in OpenGL

•

_{Turn on lighting:}

( currbet color – that is

glColor3f

–

is now ignored. Only the current material!)

glEnable(GL_LIGHTING);

Normals

•

_{We set the “current normal". Similarly to colors:}

glNormal3d(x,y,z);

What happens to the normals?

world Coordinates

x

y

z

v

v

v

y

-z

v

v

v

view Coordinates

(a.k.a. eye Coordinates)

y

x

-z

v

v

v

v

v

v

Normalized Device

Coordinates

1

-1

1

-1

x

x

y

z

v

v

v

object Coordinates

modeling

v

ie

w

p

ro

je

ct

io

n

v

v

v

screen Space

viewport

Modeling + View transformations:

(keep the angles)

Projection:

All angles are modified

Are normals preserved in the Transform?

•

_{Only when the modeling-view transformations}

are rigid

modeling-view = V ‧ M

rotations,

translations

(always rigid)

rotations,

translations

Are normals preserved in the Transform?

•

_{Note: if the ModelView includes}

_scaling

_we

need to

normalize

the normals before Lighting

•

Tell OpenGL to normalize:

or not to do it:

(default)

glEnable(GL_NORMALIZE);

Normals as attributes

•

_{Exactly like the color:}

glBegin(GL_TRIANGLES);

glNormal3fv( n );

glVertex3fv( v0 );

glVertex3fv( v1 );

glVertex3fv( v2 );

glBegin(GL_END);

glBegin(GL_TRIANGLES);

glNormal3fv( n0 );

glVertex3fv( v0 );

glNormal3fv( n1 );

glVertex3fv( v1 );

glNormal3fv( n2 );

glVertex3fv( v2 );

glBegin(GL_END);

flat shading !

Normals as attributes

•

_Shortcut:

– When we set:

glShadeModel(GL_FLAT);

glShadeModel(GL_SMOOTH);

OpenGL: lights!

•

_{We have N lights available}

– remember: their effects

(ambient + diffuse + specular) add up

•

_{How many ?}

– Dependent on the specific OpenGL implementation

– The OpenGL standard requires at least 8

– The exact number is the value of the constant

OpenGL: lights!

•

_{Each light can be on or off}

•

where

GL_LIGHT0

,

GL_LIGHT1

etc are costants

– note:

GL_LIGHT

k

is

GL_LIGHT0

+

_k

.

useful for the

_for

•

By default, the light 0 is the only one on

OpenGL: lights!

•

_{For each light, we set the colors}

glLightfv(GL_LIGHT0, GL_DIFFUSE, v);

glLightfv(GL_LIGHT0, GL_AMBIENT, v);

OpenGL: lights!

•

_{For each light, we can also set:}

– The spotlight effect:

glLightfv(GL_LIGHT0,GL_SPOT_DIRECTION,v);

glLightf (GL_LIGHT0,GL_SPOT_CUTOFF,v);

glLightf (GL_LIGHT0,GL_SPOT_EXPONENT,v);

default:

(0,0,-1)

180

0.0

glLightf(GL_LIGHT0,GL_CONSTANT_ATTENUATION,a);

glLightf(GL_LIGHT0,GL_LINEAR_ATTENUATION,b);

glLightf(GL_LIGHT0,GL_QUADRATIC_ATTENUATION,c);

– distance attenuation:

default:

OpenGL: lights!

•

_{For each light, we set the position:}

glLightfv(GL_LIGHT0,GL_POSITION,v);

– If it is a positional light,

v = {x,y,z,1}

– If it is a directional light, (“infinite distance")

v = {x,y,z,0}

OpenGL: lights!

•

_{For each light, we set the position :}

•

_{Note: the position of the lights is multiplied by}

the MODEL_VIEW matrix

glLightfv(GL_LIGHT0,GL_POSITION,v);

Lighting equation

n

specular

material

specular

light

k

H

N

I





(

ˆ



ˆ

)









k

(

L

ˆ

N

ˆ

)

I

_luight

_diffuse

_material

_diffuse





_material

_ambient

ambient

light

k

I

emission

material

k





tot

I



f

light

attenuatio

n



















min

1

₂

,

1

L

3

L

2

1

c

d

c

d

c

f

_light

_attenuatio

_n

effect

spotlight

f





direction

cutoff

Angle

beam

width



effect

spotlight

L

spot

spot

spot

f



f

,

,

,

properties of the light

properties of the

material

OpenGL: materials!

•

Let us set the parameters of the materials:

– the colors:

glMaterialfv(

face

, GL_AMBIENT, colorvec);

glMaterialfv(

face

, GL_EMISSION, colorvec);

glMaterialfv(

face

, GL_DIFFUSE, colorvec);

glMaterialfv(

face

, GL_SPECULAR, colorvec);

glMaterialf (

face

, GL_SHININESS, intval);

– The specular exponent:

"GL_FRONT"

shortcut: we can use

asdasdsadasdasd

To set both colors to the same

value

Other option: Color - Material

glEnable(GL_COLOR_MATERIAL);

activate:

use:

glColorMaterial(

face

,

mode

);

glColorMaterial(GL_FRONT, GL_DIFFUSE);

e.g.: when we set

The diffuse color of the material will be the

(otherwise ignored) current color

Two-sided Lighting

glLightModeli(GL_LIGHT_MODEL_TWO_SIDE,1);

glColorMaterial3f(

GL_BACK

,

...

);

glColorMaterial3f(

GL_FRONT

,

...

);

glColorMaterial3f(

GL_FRONT_AND_BACK

,

...

);

activation: