Lighting, Shading Models

1
Lighting, ShadingModels
Abdennour El Rhalibi
2
Illumination Models

• Discuss how to shade surfaces based on position,
  orientation, and characteristics of the surfaces
  and the light sources illuminating them
• Light components
• Illumination models

3
Definitions

• Illumination
• The transport of energy from a light source to a
  surface
• Lighting
• The process of computing the luminous intensity
  (i.e., outgoing light) at a particular 3D point,
  usually on a surface
• Shading
• Assigning colors to pixels

4
Classification of Lights

• Lighting is divided into 4 independent
  components
• Ambient light
• Diffuse light
• Specular light
• Emissive light

5
Ambient Light (1/3)

• Ambient illumination is light thats been
  scattered so much by the environment that its
  direction is impossible to determine it seems
  to come from all directions
• Backlighting in a room has a large ambient
  component since most of the light reaching your
  eye has first bounced off many surfaces
• A spotlight outdoors has a tiny ambient component
  since most of the light travels in the same
  direction

6
Ambient Light (2/3)

• Illumination equation resulting intensity at
  each point on the object is
• I Ia ?ka
• Ia intensity of the ambient light (light
  property)
• ka ambient-reflection coefficient, ranging
  between 0 and 1. (material property)

7
Ambient Light (3/3)
A sphere lit by a directional light
A sphere lit by an ambient light

• Viewpoint not important
• Light position not important
• Surface angle not important

8
Diffuse Light

• The diffuse component is the light that comes
  from one direction,
• so its brighter if it comes squarely down on a
  surface than if it barely glances off the surface
• Once it hits the surface, its scattered equally
  in all directions,
• so it appears equally bright from all viewing
  positions

9
Lamberts Law

• Brightness depends on the angle ? between the
  light direction and the surface normal

Illumination equation I Ip ? kd ? cos?
Ip point light intensity kd materials
diffuse-reflection coefficient
10
Light-Source Attenuation

• Lambertian reflection does not take into account
  distance between light source and surface points
• We introduce a light-source attenuation factor
  fatt
• The energy from a point light source that reaches
  a given part of a surface falls off as the
  distance to this part lengthens.
• c1, c2, c3 are user-defined constants associated
  with the light source

I fatt ? Ip ? kd ? cos?
11
Specular Light

• Specular light comes from a particular direction
  and tends to bounce the surface in a preferred
  direction
• A well-collimated laser beam bouncing off a
  mirror produces almost 100 specular reflection
• Shiny surface has a high specular component
• Chalk and carpet have almost no specular component

12
Phongs Law
I Is ? ks ? cosn?

• Specular reflection depends on viewpoint max
  when ? 0 and falls off as ? increases
• ks ? 0, 1 specular-reflection coefficient, a
  material property
• n ? 1, 100s specular-reflection exponent, a
  material property

13
Putting It All Together
lights

• Itotal kaIa ?fatt,jIj(kdcos?j kscosn?j)

j1
Ambient component
Diffuse component
Specular component
attenuation factor

• Usually called Phong Lighting Model
• What about colored light???

14
Shading Models

• Surface shading
• apply shading model to each point of curved
  surface
• approximate curved surfaces by plane surfaces
  and then shade the plane surfaces
• Constant shading, Gouraud shading, Phong shading

15
Constant Shading (Flat Shading)

• infinitely distant light source (constant )
  result in constant diffuse reflection
• constant and infinitely distant viewpoint
  (constant ) result in constant specular
  reflection
• abrupt change in surface orientation of adjacent
  surfaces produce an unrealistic effect

16
Smooth shading

• Two popular methods
• Gouraud shading (used by OpenGL)
• Phong shading (better specular highlight, not in
  OpenGL)

17
Normal vector of a vertex
18
Gouraud Shading

• Compute vertex illumination (color) before the
  projection transformation
• Shade interior pixels color interpolation
  (normals are not needed)

C1
for all scanlines
Ca lerp(C1, C2)
Cb lerp(C1, C3)
C3
C2
lerp linear interpolation
Lerp(Ca, Cb)
19
Gouraud Shading Problem

• Lighting in the polygon interior can be inaccurate

20
Mach band effect

• These Mach Bands are not physically there.
  Instead, they are illusions due to excitation and
  inhibition in our neural processing.

The bright bands at 45 degrees (and 135 degrees)
are illusory. The intensity of each square is the
same.
21
Mach band effect
Count the Black Dots. ?
22
Phong Shading

• Surface normals are interpolated
• Shades are computed at each point using the
  interpolated normal vector
• The shading computed by Phong shading is C1
  continuous.
• Fix the mach band effect remove edge
  discontinuity

23
Phong Shading

• Normal interpolation
• Slow not supported by OpenGL and most graphics
  hardware

24
Problems with Interpolated Shading

• Polygon silhouette
• Perspective distortion
• Orientation dependence
• Shared edges
• Unrepresentative vertex
• normals

25
Refractions (Transparent Surfaces)

• ? diffuse refraction
• partially transparent object (e.g. frosted glass)
  penetrating light is diffused
• decrease light intensity, spread intensity
  contribution of each point onto a finite area on
  the refracting surface
• expensive, seldom used

26
Specular Refraction (Snells law)

• N
• L R
• T
• , index of refraction of each material
• (averaged over wavelengths)

27
Specular Refraction

• Path shifts are ignored for thin objects
• From Snells law, we can obtain the unit
  transmission vector T in the direction

28
Interpolated Transparency
transmission coefficient (0 for opaque
objects, 1 for totally transparent
objects)
2
1
line of sight