Unit 3
Illumination Models & Surface Rendering Techniques
- Illumination model is used to calculate the intensity of light reflected from a point on
a surface.
- Surface rendering uses the intensity calculations from the illumination model to
determine the light intensity at all pixels in the image, by possibly, considering light
propagation between surfaces in the scene.
Light sources
Light sources are referred as light emitting object and light reflectors.
i. Point source: A point light source emits light equally in all directions from a
single point.
ii. Parallel source: Light rays are all parallel. May be modeled as a point source at
infinity (the sun).
iii. Distributed source: All light rays orginate at a finite area in space. A nearby
sources such as fluorescent light.
Types of Reflection
i. Diffuse Reflections: Surfaces appear equally bright from all viewing angles since
they reflect light with equal intensity in all directions. E.g. rough surfaces
- Position of viewer is not important.
, ii. Specular reflection: Light reflected with unequal intensity. E.g. shiny surface.
- Position of viewer is important.
Illumination model / Lighting model / Shading model
An illumination model is a formula in variables associated to the surface properties and light
conditions to calculate the intensity of light reflected from a point on a surface.
Based on standard lighting conditions in a scene, some illumination models are:
i. Ambient light:
Surface getting light from various reflected source i.e. light not coming directly from
a light source but coming after getting reflected from other surface.
- Ambient light has no spatial or directional characteristics and amount on each
object is a constant for all surfaces and all directions.
The reflected intensity 𝐼 due to ambient light of any point on the surface is:
𝐼 = 𝐼𝑎𝐾𝑎
Where, 𝐼𝑎= intensity of ambient light.
𝐾𝑎= ambient reflection coefficient, 0 ≤ 𝐾𝑎 ≤ 1.
ii. Diffuse reflection:
Light reflected with equal intensity in all direction. Amount of light seen by viewer
is independent of viewer direction. Brightness depends only on the angle 𝜃 between
light direction and the surface normal.
L=light source
N
V L
𝜃 N=normal to surface
O V=viewer direction
𝜃=angle betn S & N
The reflected intensity 𝐼 of any point on surface is
𝐼 = 𝐼𝑙 𝐾𝑑 𝑁⃗→ . 𝐿⃗→ = 𝐼𝑙 𝐾𝑑 𝑐𝑜𝑠𝜃
Where, 𝐾𝑑= coefficient of diffuse reflection
Net diffuse reflection= Diffuse reflection due to ambient light +Diffuse reflection
due to light source = 𝐼𝑎𝑚𝑏, 𝑑𝑖𝑓𝑓 + 𝐼𝑙, 𝑑𝑖𝑓𝑓 = 𝐼𝑎𝐾𝑎 + 𝐼𝑙𝐾𝑑𝑐𝑜𝑠𝜃
iii. Specular reflection and Phong model
In shiny surface we see highlight or bright spot from certain viewing directions called
specular reflection. (Light reflected with unequal intensity).
Illumination Models & Surface Rendering Techniques
- Illumination model is used to calculate the intensity of light reflected from a point on
a surface.
- Surface rendering uses the intensity calculations from the illumination model to
determine the light intensity at all pixels in the image, by possibly, considering light
propagation between surfaces in the scene.
Light sources
Light sources are referred as light emitting object and light reflectors.
i. Point source: A point light source emits light equally in all directions from a
single point.
ii. Parallel source: Light rays are all parallel. May be modeled as a point source at
infinity (the sun).
iii. Distributed source: All light rays orginate at a finite area in space. A nearby
sources such as fluorescent light.
Types of Reflection
i. Diffuse Reflections: Surfaces appear equally bright from all viewing angles since
they reflect light with equal intensity in all directions. E.g. rough surfaces
- Position of viewer is not important.
, ii. Specular reflection: Light reflected with unequal intensity. E.g. shiny surface.
- Position of viewer is important.
Illumination model / Lighting model / Shading model
An illumination model is a formula in variables associated to the surface properties and light
conditions to calculate the intensity of light reflected from a point on a surface.
Based on standard lighting conditions in a scene, some illumination models are:
i. Ambient light:
Surface getting light from various reflected source i.e. light not coming directly from
a light source but coming after getting reflected from other surface.
- Ambient light has no spatial or directional characteristics and amount on each
object is a constant for all surfaces and all directions.
The reflected intensity 𝐼 due to ambient light of any point on the surface is:
𝐼 = 𝐼𝑎𝐾𝑎
Where, 𝐼𝑎= intensity of ambient light.
𝐾𝑎= ambient reflection coefficient, 0 ≤ 𝐾𝑎 ≤ 1.
ii. Diffuse reflection:
Light reflected with equal intensity in all direction. Amount of light seen by viewer
is independent of viewer direction. Brightness depends only on the angle 𝜃 between
light direction and the surface normal.
L=light source
N
V L
𝜃 N=normal to surface
O V=viewer direction
𝜃=angle betn S & N
The reflected intensity 𝐼 of any point on surface is
𝐼 = 𝐼𝑙 𝐾𝑑 𝑁⃗→ . 𝐿⃗→ = 𝐼𝑙 𝐾𝑑 𝑐𝑜𝑠𝜃
Where, 𝐾𝑑= coefficient of diffuse reflection
Net diffuse reflection= Diffuse reflection due to ambient light +Diffuse reflection
due to light source = 𝐼𝑎𝑚𝑏, 𝑑𝑖𝑓𝑓 + 𝐼𝑙, 𝑑𝑖𝑓𝑓 = 𝐼𝑎𝐾𝑎 + 𝐼𝑙𝐾𝑑𝑐𝑜𝑠𝜃
iii. Specular reflection and Phong model
In shiny surface we see highlight or bright spot from certain viewing directions called
specular reflection. (Light reflected with unequal intensity).
