Lighting (đồ họa máy TÍNH SLIDE)

Lighting


Calculating the color of objects

•

The incident light

–
–
–
•

Direction
Color

The object

–
•

Position of light source

Reflectance

Viewer

–

Position

2


Overview


Illumination: how to calculate the colour?



Shading: how to colour the whole surface?

3


Illumination

•

Simple 3 parameter model

–

The sum of 3 illumination terms:

•
•
•

Ambient : 'background' illumination
Specular : bright, shiny reflections
Diffuse : non-shiny illumination and shadows

+

=

+

Ambient

Diffuse

Specular

(colour)

(directional)

(highlights)

R

4

c


Ambient Lighting

•
•
•

Light from the environment

–
–
–

Light reflected or scattered from other objects
Coming uniformly from all directions
and then reflected equally to all directions

Ex:

–

Backlighting in a room has a large ambient component, since most
of the light that bounced off many surfaces

Example: sphere

Result: globally uniform colour for object

I = kaIa
I = resulting intensity
Ia = light intensity

Object

ka = reflectance
5


Diffuse Lighting

•

•

Light reflected to all directions

–
–
–

the light that comes from one direction
considers the angle of incidence of light on surface
scattered equally in all directions, no matter where the eye is located

Result: lighting varies over surface with orientation to light
Example: sphere
(lit from left)
Infinite point

I = Ip kd cosθ
Ip : Light Intensity

N

light source

Ln

V

θ

θ : the angle between the normal vector and
direction toward the light

Object

No dependence on camera
angle!

kd : diffuse reflectivity
6


Specular Lighting

•
•

Direct reflections of light source off shiny object

–

specular intensity n = shiny reflectance of object

Result: specular highlight on object

I = Ip ks cosnα

Infinite point

N

light source

R (Reflection)

Ip: light intensity
I : output color

Ln

θ

θ

α
V

R : reflection vector
V : direction towards the camera
α : angle between R and V

No dependence on object colour

Object

7


Specular Light

•

Specular light with different n values

8


Combined Lighting Models

•

Summing it altogether : Phong Illumination Model

n
Iλ = Ia ka + Ip (kd cosθ + ks cos α )

+

=

+

Ambient

Diffuse

Specular

(colour)

(directional)

(highlights)

R
c

9


When you implement it …

•

Use dot product of the vectors instead of calculating the angles

Iλ = Ia ka + Ip (kd N• L + ks (V• R)n)
V : Vector from the surface to the viewer
N : Normal vector at the colored point
R : Normalized reflection vector

L : Normalized vector from
the coloured point
towards the light source
Infinite point

N

light source

R (Reflection)
Ln

θ

θ

α
V

Object

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Phong Principles

•

Lighting simulates how objects reflect light

–
–
–

material composition of object
light’s color and position
global lighting parameters

•
•

•

ambient light
two sided lighting

available in both color index
and RGBA mode

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Types of Shading

•

There are several well-known / commonly-used shading methods

–
–

–

Flat shading
Gouraud shading
Phong shading


Shading Comparison


How OpenGL Simulates Lights

•

Phong lighting model

–

•

Computed at vertices

Lighting contributors

–
–
–

Surface material properties
Light properties

Lighting model properties

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Adding Lighting to Your Scene

•

These are the steps required to add lighting to a scene:

1.
2.
3.
4.

Define normals for all vertices of all objects
Create, select, and position one or more lights
Create and select a lighting model
Define material properties of the objects in the scene


Surface Normals

•

Normals define how a surface reflects light

–
–

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glNormal3f(x, y, z)
Current normal is used to
compute vertex’s color
Use unit normals for proper lighting
scaling affects a normal’s length

glEnable(GL_NORMALIZE)
or
glEnable(GL_RESCALE_NORMAL)

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Material Properties

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Define the surface properties of a primitive
glMaterialfv(face, property, value);

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face: separate materials for front and back by specifying either GL_FRONT or GL_BACK,
or for both faces simultaneously using GL_FRONT_AND_BACK.

–

The OpenGL material properties are:

GL_DIFFUSE

Base color

Khuyếch tán

GL_SPECULAR

Highlight Color

Phản chiếu

GL_AMBIENT

Low-light Color

Xung quanh

GL_EMISSION

Glow Color

Phát sáng

GL_SHININESS

Surface Smoothness

Độ bóng

17


Material Properties Example

float mat_diffuse[] = { 1.0f, 1.0f, 1.0f, 1.0f };
float mat_specular[] = { 1.0f, 1.0f, 1.0f, 1.0f };
float mat_shininess[] = { 50.0f };

gl.glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse);
gl.glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
gl.glMaterialfv(GL_FRONT, GL_SHININESS, mat_shininess);

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Light Material Demo

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Light Properties

•

OpenGL approximates light and lighting as if light can be broken into red, green,

and blue components.

–

•

Thus, the color of a light source is characterized by the amounts of red, green, and blue light
it emits

glLightfv(light, property, value);

–
–

light specifies which light

•

multiple lights, starting with GL_LIGHT0

glGetIntegerv(GL_MAX_LIGHTS, n);

property

•
•
•

colors
position and type

attenuation

22


Light Sources (cont.)

•

Light color properties

–
–
–

GL_AMBIENT
GL_DIFFUSE
GL_SPECULAR

float light_ambient[] = { 0.0, 0.0, 0.0, 1.0 };
float light_diffuse[] = { 1.0, 1.0, 1.0, 1.0 };
float light_specular[] = { 1.0, 1.0, 1.0, 1.0 };
float light_position[] = { 1.0, 1.0, 1.0, 0.0 };
glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient);
glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse);
glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular);
glLightfv(GL_LIGHT0, GL_POSITION, light_position);

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24


Turning on the Lights

•

Flip each light’s switch

–

•

glEnable( GL_LIGHTn );

Turn on the power

–

glEnable( GL_LIGHTING );

25


Types of Lights

•

OpenGL supports two types of Lights

–
–

•

Local (Point) light sources
Infinite (Directional) light sources

Type of light controlled by w coordinate

w light
= 0source
Infinite
Local
w≠0

Light directed along ( x

Local Light positioned at

float light_position[] = { 1.0f, 1.0f, 1.0f, 1.0f };

gl.glLightfv(GL_LIGHT0, GL_POSITION,light_position);

(xw

y

Infinite light source

float light_position[] = { 1.0f, 1.0f, 1.0f, 0.0f };
gl.glLightfv(GL_LIGHT0, GL_POSITION, light_position);

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z)

y
w

z

w

)