1. Computer Graphics Material Colours and Lighting
CO2409 Computer Graphics
Week 11

2. Lecture Contents Materials Shading Lighting Light Types Light Models
Applying Lighting

3. Materials A material defines the surface properties of a polygon:
Colour, shininess, texture, bumpiness, transparency, etc.
Have looked at material colours already
This lecture considers how material colour is affected by incident light (light hitting surface)
Base material colour can be defined as:
Face colours
Each polygon has a single colour
Vertex colours
Each vertex has a colour
The colour is blended across the polygon using the nearest vertex colours
Like labs so far

4. Shading
When drawing entire polygons / meshes, we can choose whether to blend the colours across the surface:
With hard edges:
All vertex colours in each polygon are the same so each polygon appears flat
Implies vertex duplication
Or use face colours
With soft edges
Vertex colours shared between polygons
Result is smooth
Originally called Gouraud shading
Adjust normal directions at the edges to get these effects
Artists do this to create hard or soft edges as required

5. Lighting Can improve realism of scenes by using lighting
Light colour interacting with any existing vertex / face colours
So far have assumed constant white light everywhere
so everything is perfectly clear
Lights can greatly improve the look of even the simplest model
Several types of light source
Point, directional and spotlights
Several light effects on surfaces
Ambient, diffuse, specular…
Don’t confuse these two concepts

6. Light Sources: Directional / Point
Three main source types of light:
Directional Lights
Considered to be infinitely far away
All the light comes from the same direction
No attenuation (see later)
Sunlight is the main example
Data: direction + colour
Point Lights
Light emitting in all directions from a single point
Light attenuates with distance
A light bulb is a good example
Data: point + colour

7. Light Sources: Spotlights
Like point lights:
Light emitting from single point
Light attenuates with distance
But also:
Light is constrained to a cone
Only emits in the direction bounded by the cone
Brightest at centre of the cone
Less bright towards the edges
Data: point, direction + colour

8. Light Attenuation
The light emitted from point lights and spotlights attenuates over distance
The light is diffused (i.e. scattered) by the atmosphere
Light from distant source is weaker than from near source
Several methods, physics suggests:
Attenuated Colour =Original Colour / Distance2
Usually get nicer looking result with:
Attenuated Colour =Original Colour / Distance
So in the following lighting equations, we use the attenuated light colour rather than the actual light colour
Calculation not shown to keep it simple

9. Light Effects: Diffuse Lighting
Diffuse lighting lights parts of the mesh that point towards the light source
We’ll consider the light hitting a single vertex
The diffuse light hitting a vertex can be approximated using a dot product:
Diffuse = LightD max(N • L , 0)
LightD is light colour (attenuated)
N is the vertex normal
L is a normal pointing from the vertex to the light
Notes:
Diffuse = LightD if normal points at the light
Diffuse = 0 if it points away (even a little bit)
max used to avoid negative result
This is called Lambertian diffuse

10. Light Effects: Specular Lighting
Another key light effect is specular lighting
Treats the surface as reflective resulting in a reflection of the light source becoming visible
The reflection is called a highlight
Can extend this idea and create a reflection of the entire scene in the surface, but lights will always be most prominently reflected.
On a shiny surface highlights are sharp and bright
The surface is smooth, so the reflection is focused
Other surfaces have more spread out highlights
Surface diffuses the reflection more

11. Specular Lighting – Blinn-Phong
A couple of mathematical models for specular light
The Blinn-Phong model is commonly used for a basic effect
The Blinn-Phong specular calculation for a vertex is:
Specular = LightS max(N • H , 0)P
LightS is the light colour (attenuated)
Can use different light colours for calculating diffuse and specular to get nicest result
N is the vertex normal
H is the halfway normal
Described on the diagram
= normalise(L+C)
P is the specular power
The spread of the highlight

12. Light Effects: Ambient Lighting
A final basic lighting effect is Ambient Light
A background light level, lighting everything evenly
An approximation for indirect light
Light that reaches a surface after reflecting off other surfaces
Without it shadows would be black
Ambient can be a constant colour for an entire scene
Or vary locally depending on lights
Ambient is adjusted for the type of scene (night, day, indoor)
Call the ambient level LightA
Often just added onto the diffuse light equation

13. Applying Lighting
Total light hitting a vertex is the sum of the three effects:
Incident Light = LightA + LightD max(N • L, 0) + LightS max(N • H, 0)P
We need to combine the result with the colour of the material itself
We use multiplicative blending
Common to define material colour as two components:
Diffuse material colour = basic colour of material
Specular material colour = shininess of material
Gives final result:
Colour = MaterialD (LightA + LightD max(N • L, 0)) +
MaterialS LightS max(N • H, 0)P

14. Blending with Vertex Colours
The final effect on a sphere:
Using typical material colours:
MaterialD = red
This is the sphere’s colour
MaterialA = 1.0 (= white)
The specular light is fully reflected
[The reflection isn’t tinted red]
Note that the calculation is performed separately for the red, green and blue components
If there are several lights then:
Accumulate the incident light for all of them
Then combine with the material colours

15. Normals & Matrix Transforms
We earlier covered the use of matrices to transform geometry
Start by using each model’s world matrix to transform its vertices from model space into world space (shown)
This process applies to normals too
They need to be put into world space for lighting
Lights are positioned in the world
Scaling a model can cause issues:
Will scale the normals – not normals any longer. Fix by renormalising

16. Programming Lighting Equations are calculated in a shader
Can use vertex or pixel shader
Will see difference in a lab later
Need to set up:
Light types, positions, directions, attenuation etc.
Textures and material colours (if used)
The given equations are not physically perfect, just approximations to reality
Other lighting models are available
Can use shader flexibility for alternative light models