8 - Illumination (week 4)

Question 1

Light sources:

Answer 1

• Point lights ( e.g. a candle flame)
• Directed lights ( infinite distance e.g. the sun)
• Spot lights (e.g. torch)
• Distributed light sources (e.g. bar-type light)

Question 2

What is attenuation of light

Answer 2

Intensity goes down with distance

1/d^2

Directed light has no attenuation

Question 3

What 4 things can happen when a ray of light hits a surface?

Answer 3

-Absorption (no re-emission)
-Reflection (diffuse, specular)
-Refraction (where the light is refracted, potentially at a different angle, through another medium)
-Fluorescence (RARE)

Question 4

Reflection types:

Answer 4

• Diffuse: reflections are distributed equally
• Specular :
  perfect
  where reflection is just in one direction
  imperfect
  where it is in multiple directions
  some light is scattered away from the principal reflected direction

Question 5

Global Illumination Modelling

Answer 5

Global illumination modelling considers all interactions of light in the scene – both directly from its source and indirect lighting.

Question 6

Why do we not follow the rays of each light for global illumination modelling?

Answer 6

Way too complicated and computationally expensive. We just follow the ray of light until it hits the first object.

Question 7

Local Illumination Modelling

Answer 7

considers the interaction of light between the object and the light sources in the scene;

The effect of light from other objects in the scene is ignored.

Question 8

Diffuse Reflection: Lambert’s Cosine Law

Answer 8

Light intensity observed from an object’s surface that is diffusely reflecting is directly proportional to the cosine of the angle θ between the observer’s line of sight and the surface normal.

FORMULA:
I intensity of incident light
L direction to light
n surface normal

L = normalise(Plight - Ppoint)

I = Ik cos = Ik n . L

kd is the reflective property of the surface (diffuse parameter)

Question 9

Specular Reflection variables:

Answer 9

I intensity = the specular reflected intensity
r ‘mirror’ direction
v viewing direction
L direction to light
d distance to the light
n surface normal
k is the specular reflection intensity parameter; its value depends on the material

Ω=omega

Note that r is the reflected direction, and can be calculated using n and L

Question 10

phong model formula

Answer 10

I = Ik (r⋅v)^n = Ik (cosΩ)^n

if n is small it’s a dull surface if its larger it’s shiny
if its infinite its a mirror (perfect reflection)

Question 11

What happens in the Phong model if the point light is at infinity vs at a given position?

Answer 11

I and L are constant

L = normalise(Plight - Ppoint)

Question 12

Blinn Phong model

Answer 12

Simplified Phong model
Calculate vector H (halfway) vector between viewer and light source
If H = N, Is is maximum.
Is = Ik(N.H)^n

Question 13

When is Blinn Phong most efficient:

Answer 13

• Both the viewers and the light source are far from the surface
• V and L are constant (so H is also constant).

Question 14

Shading 3 types:

Answer 14

Flat,
Gouraud,
Phong

Question 15

Flat shading:

Answer 15

a single intensity is calculated for each polygon.

The shade is determined by the polygon’s normal and colour at the “centre”.

FAST

Question 16

How do we calculate the normal of a triangular face?

Answer 16

Cross product of any of the two vectors in the triangle

Question 17

How are vertex normals calculated?

Answer 17

From the connected faces the normals are calculated and averaged (must be normalized)

Question 18

Gouraud shading

Answer 18

Calculate vertex normals and shades,

Bilinearly interpolate LIGHT INTENSITY between the vertices

BASIC, GOOD FOR DIFFUSE,
POOR FOR SPECULAR

Interpolate intensities between vertices:
Calculates shades at vertices and interpolates to interior pixels

However, specular reflections get muted in this representation;

Question 19

Phong

Answer 19

Calculate vertex normals, Bilinearly interpolate

NORMALS between the vertices then calculate shade

SLOW, GOOD

Interpolate normals:
Interpolate normal direction to interior pixels then calculate shade

Question 20

Best use Flat Phong and gourad

Answer 20

Flat : Basic Applications

Gourad : Diffuse surfaces ks ≈ 0

Phong : Surfaces with specular reflection large ks

Question 21

Properties use Flat Phong and gourad

Answer 21

• Flat: Fast; avoids interpolation

Gourad :
* Basic shading, good for diffuse reflections
* Poor rendering of highlights and specular reflections

Phong:
* Highest quality without volumetric rendering
* 4-5 slower than Gouraud

Question 22

When would you use Phong and Gouraud together

Answer 22

Use Phongf for surfaces with specular reflection (large ks)
Use Gouraud for diffuse (ks=0)

Question 23

Phong explain:

Answer 23

Calculates each pixel individually.

Takes into account the normals at each pixel of the surface allowing to better display curled surfaces and accurately simulate the change of direction of light.

Question 24

Gourad

Answer 24

Calculates the average colour for each vertex in the polygon and interpolates them across its surface.

Can result in a loss of accuracy with complex curvatures leading to a faceted appearance.

Incapable of accurate highlights.