Lecture computer graphics and virtual reality slides lesson 10 illuminiation and shading

Lesson 10

Illuminiation and
shading

Content

1. Overview
2. Illumination
3. Shading

1.
Overview

How lights can be seen?

■ The eye works like a camera
■ Lots of photo sensors at the back of the eye
■ Sensing the amount of light coming from
different directions
■ Similar to CMOS and CCDs

Information will be sent
to brain for processing

What colour is this
dress?

Object’s colour

■ What Affects the Color of a Point on the

Object?

□ position of the sample point
□ position of the light
□ color and intensity of the light
□ camera vector
□ normal vector of the surface at the vertex
□ physical characteristics of the object

(reflectance model, color)

Colour’s illusion

Lighting and shading

■ Lighting

□ How to compute the color of objects according to
the position of the light, normal vector and camera
position

■ Shading

□ Different methods to compute the color of the
entire surface

2.
Lighting

Factors affecting lighting

Light source
Surface properties

Light source

■ Light’s properties

□ Spectrum of light source (i.e. light colour)
□ Geometry properties

▫ Position
▫ Direction
▫ Shape
□ Decline in intensity

Surface properties

■ Surface properties

□ Spectrum (i.e. surface’s colour)
□ Geometry properties

▫ Position
▫ Direction
▫ Microscope structure

Lighting

■ Assuming we have a polygon and a point of view

in 3D space, we can specify a pixel correspondingly.
■ The task in lighting is that we have to specify the
colour of that pixel

■ If we want to make the scene look *right*, we
have to simulate the lighting process on the surfaces
inside the scene

Lighting models

■ Two types of lighting models

□ Empirical: Use simple formulas to create similar
observable result

□ Physics simulation: The lighting formulas will be
based on physics formulas in lighting

■ In most cases, experimental models will be used
in interactive computer graphics applications.
■ In modern computer graphics applications,
some physic simulation models have been more
frequently used.

Phong lighting model

■ By Bui Tuong Phong (Utah University) is a
lighting model which is a combination of both
experimental and physics simulation
■ Is the sum of 3 illumination terms

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

Ambient light

Ambient light

■ Light from the environment
■ Light reflected or scattered from other objects
■ Coming uniformly from all directions and then
reflected equally to all directions
■ A precise simulation of such effects requires a
lot of computation

➢Use a simple “trick” called ambient light

Ambient light

■ Simple approximation to complex 'real-world‘
process

➢Result: globally uniform color for object

Ambient light

■ For each lighting process, the ambient light
reflected from object surface only depends on

□ Surface’s properties
□ Intensity of the ambient light source (is a constant

with every point on object surface)

E.g. a sphere

Ambient light

I = ka Ia

□ I = resulting intensity
□ Ia = light intensity
□ ka = reflectance

E.g. a sphere