LIGHTING

SET09115 Intro Graphics Programming

Breakdown

Working with Colour Basics of Lighting

Why Lighting? Ambient Light

Surface Lighting Diffuse Light Specular Light

Other Lighting Types

Recommended Reading

Real Time Rendering Chapter 5 – Visual Appearance

Course Text Chapter 6 – Lighting and Shading

Colours

Colour Representation

Modern graphics work with 32 bit colour Typically 4 values of 8 bits used

Red, green, blue, alpha (transparency) Other formats exist

We could work with values 0..255 Better to work with floats 0.0f ..1.0f

0% to 100%

AlphaRed Green Blue

32 bits

8 bits

Working with Colour

Treat colour as a value you can work with

Add colours together Red + Green = Yellow

(1.0, 0.0, 0.0) + (0.0, 1.0, 0.0) = (1.0, 1.0, 0.0)

Multiply colours together Magenta * Yellow = Red

(1.0, 0.0, 1.0) * (1.0, 1.0, 0.0) = (1.0, 0.0, 0.0)

Vertex Colours

Attaching colours to vertices is what we have been doing up until this point Blending occurs over surfaces between colour

points OpenGL follows the RGBA format

Image formats may not be in RGBA DirectX can support other formats

These colours can be used in lighting calculations Typically, we use textured objects to provide

more detail

Blending

The ability to blend colours in this way is useful Multi-texturing Terrain

We will revisit blending colours when working with textures and terrain

Working with colour

Questions?

Basics of Lighting

Why Lighting?

Lighting adds depth to a scene Unlit objects are

flat and lack detail

Lighting adds realism to objects Specular detail Light reflection

How Lighting Works

Light works in rays from a graphics point of view Light comes from

a source Light is reflected

off of objects Light eventually

makes it into the eye

Original light source

Light reflected from red box to

blue sphere

Light reflected from red box into

eye

Light reflected from blue sphere

into eye

Materials

Light rays are just one part of the equation

Light itself can have a colour Objects also have a colour

The type of light they reflect into the eye rather than absorb

The light that an object reflects is commonly described in its material Material may have other properties

Main Lighting Types

Generally, an object has three lighting effects affecting it at any time Ambient Diffuse Specular

The colour of an object are these three lighting effects combined Plus any other effectx

What is Ambient Light?

Ambient light is the light that affects everything equally in a scene Has no direction Has no distance Doesn’t care about which part of the object

is being calculated Outside, daylight is an ambient light

effect

Ambient Light Equation

Ambient light calculation is the simplest

The ambient light of a particular vertex is just the ambient material colour of the object multiplied by the ambient light colour of the scene

LightMaterialAmbient

Example

Why lighting?MaterialsAmbient light

Questions?

Lighting Surfaces

Lighting a Surface

Ambient light is something affecting all geometry equally

Still gives a flat appearance to a scene Objects reflect light based on how their

surfaces are aligned with the light source Occluded objects / surfaces get no light Surfaces perpendicular to light source get

full light

Working with Normals

We use surface normals to determine how light should be reflected off a surface Andrew will go

into the mathematics of lighting next week

Normals are very important for lighting

What is Diffuse Light?

Diffuse light is concerned with directionality of the light Geometry facing a

light source is lit Geometry facing

away from the light source unlit

Think sunlight outside

How does Diffuse Light Work? Diffuse light is concerned with the light

direction against the surface normal of the piece of geometry in question If angle = 90 then full light If angle 0 to 90, or 90 to 180, then shaded If angle < 0 or > 180 then not lit

Think about how this would work if you shone a light on a table

Lambert’s Cosine Law

A simple way to calculate the diffuse light of an object is using Lambert’s Cosine Law Cos(0) = 0 Cos(90) = 1 Cos(180) = 0

The cosine graph shows how this works

Diffuse Light Algorithm

Using Lambert’s Cosine Law, we can generate the diffuse lighting algorithm

Remember, dot product of two vectors is equal to the cosine of the angle between them

We would normally have to transform the surface normal if the model is transformed

Diffuse alpha should always be 1. We will have to set this

)(

)0.0,max(

LightMaterialpowerDiffuse

NormalLightVecpower

Example

What is Specular Light?

Specular light refers to the point on the object where the light strikes Specular reflection

This provides a shiny look to objects

Also indicates where the light is coming from

How does Specular Light Work?

Specular light requires three pieces of information Light direction Surface normal Eye position

Specular intensity is based on eye position

Specular Light Algorithm

Specular light is the most complicated algorithm

LightMaterialpowerSpecular

werspecularPotoEyereflectpowpower

NormalLightVecreflectreflection

tionvertexPosieyePosnormalizetoEye

)),0.0,(max(

),(

)(

Example

Surface lightingDiffuse lightingSpecular lighting

Questions?

Other Lighting Types

Other Lighting Types

Ambient, Diffuse, and Specular are considered the main environmental lighting types Object materials usually deal with these

three types There are of course other lighting types

which affect a scene Dynamic lighting Point lights Spot lights

Point Lights

A point light is a light that emanates from a particular point in the scene

Use the same equations, but calculate light vector based on vertex position and light position

Distance to light affects the strength of the light

Spot Lights

Spot lights also work using light position and distance

The direction of the light is also important Objects behind the

spot light aren’t lit Light also has an

angle associated with it Beam width at

distance

Phong Shading

Phong Shading is a popular per-pixel approach to performing lighting calculations

Normals are interpolated across the surface of a polygon

Provides a smoother shading look Vertex lighting as discussed in this lecture is sometimes

referred to as Gourand shading

Point LightsSpot LightsPhong Shading

Questions?

To do this week…

Investigate some of the different forms of light that can affect a scene In particular, try and find their lighting

values Consider the type of lighting you may

want in your coursework OpenGL has light functionality built in

without the use of shaders Shaders provide more controllable lighting

Summary

Lighting is an important concept to provide depth and realism to your scene

Lighting is about the colour of the light, and how an object reflects this light

Three main types of lighting Ambient Diffuse Specular

You should make yourself familiar with how to work these out