Shading Definitions

1
Shading Definitions

• Light emanating from small surfaces is called a
  Point light source e.g. the sun (not really small
  though!)
• Light emanating from a large surface is call a
  Distributed light source e.g. fluorescent tube
• Multiple reflections of light from nearby objects
  combine to produce uniform illumination called
  Ambient light e.g. walls

2
ShadingDefinitions

• Diffuse reflection occurs when light hits a
  dull surface and the surface re-emits some of
  the light in all directions
• Specular reflection is the direct reflection of
  light by a shiny surface

3
ShadingAmbient Light

• Ambient light can be considered to be of uniform
  intensity in all directions
• It is cloudy day light, no specific origin so
  no shadows
• It is usually modelled as a constant within a
  scene
• The intensity of a surface exposed to ambient
  light depends on the reflectivity of the surface
  and the ambient light level

4
ShadingDiffuse reflection

• The intensity of a surface due to a point light
  source is calculated from Lamberts cosine law,
  so objects appear brighter when light is almost
  behind your viewpoint
• It also depends on the distance of the object
  from the light source and the reflectivity of the
  surface

5
ShadingSpecular reflection

• At certain points a shiny surface reflects all
  incident light.
• This produces a spot of reflected light that is
  the same colour as the incident light

R
N
R
N
L
L
q
q
q
q
Dull surfaces have a wider reflection range
Shiny surfaces have a narrow reflection range
6
ShadingFlat

• Each surface intensity is calculated using
  ambient and diffuse reflections
• The whole surface is coloured with this one flat
  colour
• Objects look faceted
• Mach banding occurs
• Very quick to calculate

7
ShadingGouraud

• Assumes that uniform shading is not desired
• It interpolates the brightness over the surface
• Intensity at each vertex is calculated and then
  linearly interpolated across the surface
• Smoother appearance, in fact it can smooth hard
  edges too much
• Not good at incorporating specular reflection

8
ShadingPhong
From a lecture by B.Wyvill

• Phong Shading
• Extends Gourauds method but instead of
  interpolating intensities it interpolates the
  normals and calculates the intensity at every
  point on the surface
• It renders specular reflections properly
• It is very expensive in computer time

9
Shadows (Hard and Soft)
umbra
penumbra
10
Umbra-Penumbra
Penumbra
Umbra

• Umbra -penumbra dependence on the size of the
  light source

11
Shadows

• Shadows can be
• used to anchor objects
• used to emphasize the changing direction of the
  light source
• hard edged or soft edged
• contain both an umbra and penumbra
• Shadows are a quality add on
• No one particular shadow algorithm

12
Shadows

• For static scenes, shadows are fixed and do not
  change as viewpoint changes
• If relative positions of objects and light
  sources change the shadows have to be
  recalculated
• There is a high computational overhead