Shading

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Shading

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Contents

• Definition
• Light sources
• Phong reflection model
• Polygonal shading
• Shading in OpenGL
• Global rendering

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Shading

• Determine a color for each filled pixel
• How to choose a color
• the color of the light source and of the surfaces
  determines the color of one or more pixels in the
  frame buffer

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Light and Matter

• Color is determined by multiple interactions
  among light sources and reflective surfaces
• recursive reflection of light between surfaces
  accounts for subtle shading

Rendering Equation
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Light-Material Interactions

• Specular surfaces mirror
• scatter reflected light in a narrow range of
  angle
• Diffuse surfaces chalk, clay
• scatter reflected light all directions
• Translucent surfaces glass, water
• refraction

specular surface
diffuse surface
translucent surface
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Light Sources

• Illumination function I (x, y, z, ?, f, ?)
• each point (x, y, z) on the surface
• direction of emission (?, f)
• wavelength ?

light source
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Modeling Light Sources

• Simple mathematical models
• ambient light
• point light
• directional light
• spot light

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Color Sources

• Three-color theory
• human perceive three primary colors red, green,
  and blue
• three-component intensity or luminance function
• independent calculations

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Ambient Light

• Uniform lighting
• identical at every point in the scene

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Point Sources (1/2)

• Emit light equally in all directions
• p0 point source location
• proportional to the inverse square distance

point source illuminating a surface
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Point Sources (2/2)

• Large finite size of most light sources
• umbra full shadow
• penumbra partial shadow
• Attenuation

shadows created by finite-size light source
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Spotlights

• Characterized by a narrow range of angle through
  which light is emitted
• ps apex of a cone
• ls direction of pointing
• ? angle to determine width
• Distribution of light
• concentrate in the center
• light intensity drop off

spotlight
attenuation
exponent
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Distant Light Sources

• far from the surface ? vector does not change
• location ? direction

parallel light source
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Phong Reflection Model

• Efficient and close enough approximation to
  physical reality to produce good renderings
• Use 4 vectors
• n normal at p
• v direction from p to the viewer or COP
• l direction from light source
• r direction of reflectance
• 3 types of material-light
• interactions
• ambient, diffuse, and specular

vectors used by the Phong model
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Computation

• Light source and reflection term
• Ex. red intensity
• Total Intensity

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Ambient Reflection

• Same at every point on the surface
• Ambient reflection coefficient
• amount reflected
• some of light source is absorbed and some is
  reflected
• three ambient coefficients , , and
• Ambient reflection term
• can be any of the individual light sources
• can be a global ambient term

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Diffuse Reflection (1/2)

• Characterized by rough surfaces
• perfectly diffuse surface
• ? so rough that there is no preferred
• angle of reflection
• Lamberts law
• only the vertical component of light source

rough surface
at noon
in the afternoon
Lamberts law
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Diffuse Reflection (2/2)

• Lamberts law (cont)
• Diffuse reflection term
• incorporate a distance term

Lamberts law
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Specular Reflection

• Specular surface is smooth
• highlight in the direction of the viewer
• Specular reflection term
• shininess coefficient
• a ? infinite mirror
• 100 lt a lt 500 metal
• a lt 100 broad highlight

specular highlights
specular surface
effect of shininess coefficient
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Phong Model

• Including the distance term

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Utha Teapots with Different Material Properties
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Computation of Vectors (1/2)

• Normal vectors
• in OpenGL
• Angle of reflection

n
glNormal3f(nx, ny, nz) glNormal3fv(pointer_to_nor
mal)
coplanar condition
?
?
by ? and ?
mirror
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Computation of Vectors (2/2)

• Halfway vector
• half-angle 2?f
• Transmitted light
• Snells law

halfway vector
perfect light transmission
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Polygonal Shading

• Shade a polygonal mesh
• flat shading
• interpolative or Gouraud shading
• Phong shading

polygonal mesh
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Flat Shading (1/2)

• Constant shading
• flat polygon
• ? n constant
• distant light source
• ? l constant
• distant viewer
• ? v constant
• One shading calculation for each polygon
• In OpenGL

distant source and viewer
glShadeModel(GL_FLAT)
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Flat Shading (2/2)

• Always be disappointing for a smooth surface
• lateral inhibition
• human visual system has a remarkable sensitivity
• Mach bands
• perceive the increases in brightness along the
  edges

flat shading of polygonal mesh
perceived and actual intensities at an edge
We need smoother shading techniques to avoid it!!
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Interpolative and Gouraud Shading

• In OpenGL
• One lighting calculation for each vertex
• bilinearly interpolate colors
• Vertex normal could be defined through
  interpolation

glShadeModel(GL_SMOOTH)
normals near interior vertex
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Wireframe
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Flat Shading
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Gouraud Shading
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Phong Shading

• May not prevent the appearance of Mach bands
• What if polygonal mesh is too coarse to capture
  illumination effects in polygon interiors?
• One shading calculation for each pixel ? off-line
• Compute vertex normal at each point

interpolate normals across each polygon
interpolation of normals
edge normals
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Visible-Line Determination
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Visible-Surface Determination with Ambient
Illumination Only
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Flat Shaded Polygons with Diffuse Reflection
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Gouraud Shaded Polygons with Diffuse Reflection
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Gouraud Shaded Polygons with Specular Reflection
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Phong Shaded Polygons with Specular Reflection
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Phong Shading of Curved Surfaces with Specular
Reflection
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Improved Illumination Model and Multiple Lights
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Texture Mapping