Shading Page 1 Computer Graphics Copyright Gotsman, Elber, Barequet, Karni, Sheffer Computer Science, Technion The Physics 2 3 Local vs. Global Illumination Models Local model – direct and local interaction of each object with the light. Global model: interactions and exchange of light energy between different objects. Example Ambient Diffuse Specular Final Image 5 Light Sources Point source (A): All light originates at a point Rays hit planar surface at different incidence angles Parallel source (B): All light rays are parallel Rays hit a planar surface at identical incidence angles May be modeled as point source at infinity Also called directional source Area source (C): Light originates at finite area in space. Inbetween the point and parallel sources Also called distributed source A B C 6 Assume non-directional light in the environment Object illuminated with same light everywhere Looks like silhouette The Illumination equation I = I a k a Ambient Light I a - ambient light intensity k a - fraction of ambient light reflected from surface. Also defines object color
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Local model – direct and local interaction of each object with the light.
Global model: interactions and exchange of light energy between different objects.
Example
Ambient Diffuse
Specular Final Image
5
Light Sources Point source (A): All light originates at a point
" Rays hit planar surface at different incidence angles
Parallel source (B): All light rays are parallel " Rays hit a planar surface at identical incidence angles " May be modeled as point source at infinity " Also called directional source
Area source (C): Light originates at finite area in space. " Inbetween the point and parallel sources " Also called distributed source A B C
6
Assume non-directional light in the environment Object illuminated with same light everywhere
" Looks like silhouette
The Illumination equation I = Iaka
Ambient Light
" Ia - ambient light intensity
" ka - fraction of ambient light reflected from surface.
Diffuse Light Dull surfaces such as solid matte plastic reflects incoming light
uniformly in all directions. Called diffuse or Lambertian reflection Understand intensity as the number of photons per inch2. If a flow
of m photos passing each second through an inch2 window orthogonal to the flow, is hitting the red surface, how many photons hit an inch2 of the surface ?
Let θ is the angle between the direction of incoming light and normal to surface, and let L, N be corresponding unit vectors.
L N
Diffused Ball
Moon Paradox
10
Diffuse Reflection
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Assume object is a perfect mirror.
Lights emits the object in direction V only if V=R
Reflection from a perfect mirror N L R
V θ θ α
α=0
12
Shiny objects (e.g. metallic) reflect light in preferred direction R determined by surface normal N.
Most objects are not ideal mirrors – also reflect in the immediate vicinity of R
Phong Model – approximate attenuation by the form of cosnα (no real physical basis)