CAP 4703 Computer Graphic Methods

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CAP 4703Computer Graphic Methods

• Prof. Roy Levow
• Chapter 6

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Light and Matter

• Surface may
• emit light
• reflect light
• Rendering equation
• Recusrively applying the effect from a source to
  the viewer yields the
• Equation cannot be solved in general
• Approximate approaches can be computationally
  intensive

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Approximations

• Ray tracing
• trace light from source to eye
• most rays dont reach the eye
• Radiosity
• Treat surfaces as small regions each emitting
  fixed light
• Calculate emission independent of viewer
• Simplified ray tracing
• Single interaction between light source and each
  surface

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Light and Surfaces
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Interaction with Surfaces

• Specular surface
• Similar to mirror
• Light leaves atsame angle as entry

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Interaction with Surfaces (cont)

• Diffuse Surface
• Light leaves inall directions

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Interaction with Surfaces (cont)

• Translucent Surface
• some light penetrates surface
• leaves at modified angle refraction
• some reflected

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Light Sources

• Six variable illumination function
• I(x,y,x,theta, phi, lambda)
• Generally too complex to compute for all points

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Light Sources

• Consider four simpler light sources
• Ambient lighting
• Omnidirectional constant everywhere
• Point source
• Radiates in all directions
• Spot lights
• Small cone of constant light
• Distant light
• rays are parallel

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Color Sources

• Describe color with 3-dimensional RGB vector
• (Ir, Ig, Ib)T

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Characteristics of Light Sources

• Ambient light
• every point in scene receives same light in every
  direction
• Point source
• Intensity decreases with inverse square of
  distance
• Often approximate with form
• (a bd cd2)-1 where d is distance to soften
  effect

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Characteristics of Light Sources

• Spot light
• Light limited to cone of angle
• diminishes toward edges of cone as
• cose(theta) for some value of e
• intensity as for point source
• Distant source
• constant intensity
• fixed direction

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Phong Reflection Model

• Described by 4 vectors
• n normal to p
• v to viewer
• l to source
• r reflected ray

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Phong Reflection Model

• Supports
• ambient
• diffuse
• specular
• or combination of these
• Uses 3 x 3 matrix
• columns for r g b
• rows for ambient, diffuse, specular
• Color source and reflection matrices

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Phong Reflection Model

• resulting intensity is sum or products of
  corresponding terms
• Iir RiraLiraRirdLirdRirsLirs
• IiraIirdIirs
• for red, etc
• Sum over all points for complete effect

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Ambient Reflection

• Ra ka, 0 lt ka lt 1
• a fraction of light is reflected
• one coefficient for each color

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Diffuse reflection

• Lambertian surfaces
• reflect only vertical component
• Rd proportional to cos(theta)
• Id kd (l . n) Ld / (a bd cd2)

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Specular Reflection

• Phong model equation
• Is ks Ls (R . v)a
• ks is shininess coefficient

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OpenGL Shading

• Polygonal shading
• Generate shading on individual polygons
• Flat shading
• glShadeModel(GL_FLAT)
• select one vertex of polygon for normal
• same color throughout polygon
• tends to produce artificial looking surfaces with
  distinctly colored regions
• eye is very sensitive to change
• Mach Bands

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Flat Shading
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Interpolative and Gourand Shading

• GL_SMOOTH
• average normals
• example plates 4 5