Computer Graphics Blending CO2409 Computer Graphics Week 14.
Computer Graphics Shading - Arizona Computer … · Computer Graphics Copyright Gotsman, Elber, ......
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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
Transcript of Computer Graphics Shading - Arizona Computer … · Computer Graphics Copyright Gotsman, Elber, ......
Shading
Page 1
Computer Graphics
Copyright Gotsman, Elber, Barequet, Karni, Sheffer Computer Science, Technion
The Physics
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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
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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
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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.
Also defines object color
Shading
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Computer Graphics
Copyright Gotsman, Elber, Barequet, Karni, Sheffer Computer Science, Technion
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
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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
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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)
Specular Reflection
N L R V θ θ α
Shading
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Computer Graphics
Copyright Gotsman, Elber, Barequet, Karni, Sheffer Computer Science, Technion
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Specular Reflection (Phong Model) Illumination equation:
ks - Specular reflection coefficient n - Specularity exponent
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Specular Reflection (cont’d) Exponent n of cosine controls decay factor of attenuation function:
No physical basis but looks good:
Retroreflector " sends light back where it came from regardless of the angle
of insidence
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For multiple light sources:
" Ip of all light sources are added together
" Precautions should be taken from overflows
More on Illumination Equation shadingmodel
Shading
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Computer Graphics
Copyright Gotsman, Elber, Barequet, Karni, Sheffer Computer Science, Technion
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For distance/atmospheric attenuation sources:
dp - distance between surface and light source and/or distance between surface and viewer (heuristic atmospheric attenuation)
Even More on Illumination Equation
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Flat Shading
Applied to piecewise linear polygonal models
Simple surface lighting approximated over polygons
Illumination value depends only on polygon normal ⇒ each polygon is colored with a uniform intensity
Looks non-smooth (worsened by Mach band effect)
Normal per Vertex
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Gouraud Shading
Compute illumination intensity at vertices using those normals
Interpolate intensity over polygon interior
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Gouraud Shading
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Phong Shading Interpolate (at the vertices in image space) normal vectors
instead of illumination intensities Apply the illumination equation for each interior pixel with
its own (interpolated) normal
Shading
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Computer Graphics
Copyright Gotsman, Elber, Barequet, Karni, Sheffer Computer Science, Technion
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Comments on Shading Phong shading is more expensive (why?) but well
worth the effort
Can achieve good looking specular highlight effects
Both the Gouraud and Phong shading schemes are performed in the image plane and fit well into a polygonal scan-conversion fill scheme
Both the Gouraud and Phong are view dependent
Can cause artifacts during animation as they are transformation dependent
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Comparison
