Mohammad Shaker

mohammadshaker.com

@ZGTRShaker

2014

OpenGL Graphics

L04-Lighting

What we’ve learnt so far..

RULETO SEE A 3D SCENE YOU SHOULD SET UP:

CAMERA (Singleton, for all objects)

PROJECTION (Singleton, for all objects)

WORLD MATRIX (For each object separately)

Lighting

How We See Things?

How We See Things?

Light and Color Systems

Additive Colors Subtractive Colors

Colors in OpenGL

• glColor3f(red, green, blue) [0,1]

• glColor4f(red, green, blue, alpha) [0,1]

Colors in OpenGL

• glColor3f(red, green, blue) [0,1]

• glColor4f(red, green, blue, alpha) [0,1]– alpha for transparency [0; full,1: no]

Color Filling Mode

• Filling Mode

– glShadeModel(GL_FLAT);

• Fill the polygon with solid color (the same as last vertex color)

– glShadeModel(GL_SMOOTH);

• Interpolate colors between polygon edges

Light’s Type

• DIFFUSE color's alpha value actually determines the transparency of the polygon

Only ambient light only specular lightonly diffuse light

The Modified Phong Model

• Computes a color or shade for each vertex using a lighting model (the modified

Phong model) that takes into account

– Diffuse reflections

– Specular reflections

– Ambient light

– Emission

• Vertex shades are interpolated across polygons by the rasterizer

Lighting - Types

• Ambient Light: doesn’t come from any particular direction. It has an original

source somewhere, but the rays of light have bounced around the room or scene

and become directionless.

Lighting - Types

• Diffuse Light: The diffuse part of an OpenGL light is the directional component

that appears to come from a particular direction and is reflected off a surface with

an intensity proportional to the angle at which the light rays strike the surface.

Thus, the object surface is brighter if the light is pointed directly at the surface

than if the light grazes the surface from a greater angle.

Lighting - Types

• Specular light: Like diffuse light, specular light is a highly directional property, but

it interacts more sharply with the surface and in a particular direction.

The Modified Phong Model

• The model is a balance between simple computation and physical realism

• The model uses

– Light positions and intensities

– Surface orientation (normals)

– Material properties (reflectivity)

– Viewer location

• Computed for each source and each color component

Material Properties

• Define the surface properties of a primitive

• you can have separate materials for front and back

Property Description

Diffuse Base object color

Specular Highlight color

Ambient Low-light color

Emission Glow color

Shininess Surface smoothness

Material Properties

• Define the surface properties of a primitive

• you can have separate materials for front and back

Property Description

GL_DIFFUSE Base color

GL_SPECULAR Highlight Color

GL_AMBIENT Low-light Color

GL_EMISSION Glow Color

GL_SHININESS Surface Smoothness

Color Theory

Color Theory

Lighting in OpenGL

Lighting in OpenGLLight Source and Object Material

Lighting in OpenGLLight Source and Object Material

Turning on the Lights

• For any type of light, you should first

– Flip each light’s switch

glEnable(GL_LIGHT0);

– Turn on the power

glEnable(GL_LIGHTING);

Lighting in OpenGL – Light Source

• For the light source:

– Set light properties:float Light_Ambient [ ] = {0.2,0.2,0.2,1};

float Light_Diffuse [ ] = {1,1,1,1};

float Light_Specular [ ] = {1,1,1,1};

float Light_Position [ ] = {0,0,20,0};

– Enable a specific light (GL_LIGHT0):glEnable(GL_LIGHTING);

glEnable(GL_LIGHT0);

– Bind the light properties with the specific light (GL_LIGHT0):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);

Lighting in OpenGL – Object Material

• For the object material:

– Set material properties:float Cone_Ambient [ ] = {0.1,0.1,0.1,1};

float Cone_Diffuse [ ] = {0.2,0.1,0.9,1};

float Cone_Specular [ ] = {1,1,1,1};

float Cone_Shininess = 100;

– Bind the material properties with a specific object:glMaterialfv(GL_FRONT_AND_BACK , GL_AMBIENT , Cone_Ambient);

glMaterialfv(GL_FRONT_AND_BACK , GL_DIFFUSE , Cone_Diffuse);

glMaterialfv(GL_FRONT_AND_BACK , GL_SPECULAR , Cone_Specular);

glMaterialf (GL_FRONT_AND_BACK , GL_SHININESS , Cone_Shininess);

• Now draw whatever you want.GLUquadric *quadric = gluNewQudric();

gluCylinder(quadric, 4, 1, 7, 32, 32);

Lighting ExampleComplete Code

Lighting Example

SpotLight Lighting ExampleComplete Code

Lighting Example

Normals for Lighting

Why Normals are essential for lighting?

Why Normals are essential for lighting?

Normals for Lighting

• Vertex Normals– Prefixed w/ “vn” (Wavefront)

– Contains x,y,z of normal

– Not necessarily unit length

– Not necessarily in vertex order

– Indexed as with vertices(x0,y0,z0)

(a0,b0,c0)

(u0,v0)

(x1,y1,z1)

(a1,b1,c1)

(u1,v1)

(x2,y2,z2)

(a2,b2,c2)

(u2,v2)

Surface Normals

• Normals define how a surface reflects light

– Application usually provides normals as a vertex atttribute

– Current normal is used to compute vertex’s color

– Use unit normals for proper lighting

• scaling affects a normal’s length

Vector Normalization

Vector NormalizationWhy to?

Advanced Lighting, Spotlight

Shadowing

Shadowing

• How to make a shadow?

– Draw the whole object again, projecting it on the shadow plane (the plane in which the

shadow will appear)

Shadowing