Lighting and Shading

Question 1

3 major shading techniques

Answer 1

?

Question 2

interactions between light and materials can be classified into the 3 groups

Answer 2

• Specular surfaces
• Diffuse surfaces
• Translucent surfaces

Question 3

Specular surfaces

Answer 3

appear shiny because most of the light that is reflected or
scattered is in a narrow range of angles close to the angle of reflection.Mirrors
are perfectly specular surfaces

Question 4

Diffuse surfaces

Answer 4

characterized by reflected light being scattered in all
directions. Walls painted with matte or flat paint are diffuse reflectors.
Perfectly diffuse surfaces scatter light equally in all directions,

Question 5

Translucent surfaces

Answer 5

allow some light to penetrate the surface and to emerge

from another location on the object

Question 6

Polygonal Mesh

Answer 6

A polygonal mesh comprises many flat polygons, each of which has a well-defined normal

Question 7

3 ways to shade polygons

Answer 7

• Flat shading
• Gouraud Shading
• Phong Shading

Question 8

Flat shading

Answer 8

(constant shading)
The shading calculation is carried out only once for each polygon, and each point on the polygon is assigned the same shade. Flat shading will show differences in shading among adjacent polygons

Question 9

Gouraud shading

Answer 9

(smooth shading)
(Vertex shader)
The lighting calculation is done at each vertex using the material properties and the vectors 𝒏, 𝒗, and 𝒍 . Thus, each vertex will have its own colour that the rasterizer can use to interpolate a shade for each fragment

Question 10

Phong shading

Answer 10

(Fragment shader)
Instead of interpolating vertex intensities (colours), we interpolate normals across each polygon. We can thus make an independent lighting calculation for each fragment. We implement Phong shading in the fragment shader.

Question 11

4 types of light sources

Answer 11

• Ambient Light
• Point Light (source)
• Spotlights
• Distant Light (source)

Question 12

Ambient Light

Answer 12

Ambient illumination is characterized by an intensity, 𝐼a, that is identical at every point in the scene. (In many rooms, such as class rooms, the lights have been designed and positioned to provide uniform illumination throughout the room )

Question 13

Point source

Answer 13

An ideal point source emits light equally in all directions. The intensity of illumination received from a point source is proportional to the inverse square of the distance between the source and surface, called the distance term

Question 14

Spotlight

Answer 14

Spotlights are characterized by a narrow range of angles (a cone) through which light is emitted. More realistic spotlights are characterized by the distribution of light within the cone – usually with most of the light concentrated in the centre of the cone.

Question 15

Distant Light Source

Answer 15

All rays are parallel and we replace the location (point) of the light source with the direction (vector) of the light.

Question 16

Local Lighting Models

Answer 16

• Phong
• Model source from light reflected once off a surface towards the eye
• Indirect light is included with an ad hoc “ambient” term which is normally constant across the scene

Question 17

Global Lighting Models

Answer 17

• Raytracing or radiosity
• Try to measure light propagation in the scene
• Model interaction between objects and other objects and
  objects and their environment

Question 18

Phong Reflection Model: 4 vectors used for calculations

Answer 18

• n: the normal at p
• v: direction from p to the COP/viewer
• l: direction from p to an arbitrary point on the light source
• r: direction that a perfectly reflected ray would take from

Question 19

Phong Reflection Model: Ambient

Answer 19

The intensity of ambient light 𝐼a is the same at every point on the surface. Some of this light is absorbed and some are reflected. The amount reflected is given by the ambient reflection coefficient

Question 20

Phong Reflection Model: Ambient

Answer 20

The intensity of ambient light 𝐼a is the same at every point on the surface. Some of this light is absorbed and some is reflected. The amount reflected is given by the ambient reflection coefficient

Question 21

Phong Reflection Model: Lamberts Law

Answer 21

• we see only the vertical component of the incoming light
• 𝑅𝑑∝cos𝜃
• where 𝜃 is the angle between the normal at the point of interest 𝒏 and the direction of the light source 𝒍
• If both 𝒍 and 𝒏 are unit-length vectors, then
• cos𝜃=𝒍∙𝒏

Question 22

Phong Reflection Model: Diffuse Reflection

Answer 22

• add in a reflection coefficient 𝑘d (0≤𝑘d≤1) representing the fraction of incoming diffuse light that is reflected we have the diffuse reflection term
• 𝐼d=𝑘d𝐿d(𝒍∙𝒏)