CSPC 352: Computer Graphics

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CSPC 352 Computer Graphics
Chapter 6 Lighting and Shading
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Overview

• Local and global illumination
• Phong reflectance model (local illumination)
• Flat, Gouraud, and Phong
• Shading on a polygonal mesh
• Surface subdivisions
• Shading in OpenGL

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Perspective

• Lighting and shading are accomplished by modeling
  the world and simulating the laws of physics
• Short story Stanislaw Lem, The Cyberiad The
  constructor Trurl creates a tiny simulation of a
  kingdom in a box to make a deposed, exiled despot
  happy. Trurls friend thinks that is terrible
• There are those who say that we exist in the mind
  of God. What do you think of that idea?
• Pascal, Pensées The arithmetical machine
  produces effects which come closer to thought
  than anything which animals can do but it can do
  nothing which might lead us to say that it
  possesses free will, as the animals have.

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Need for shading

• Was it hard to make the 3D flower (first program)
  look 3D?
• Shading that is appropriate for the lighting is
  the primary cue to 3D appearance
• What are some other cues?

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Illumination models

• General approach
• model the world
• simulate physics
• Global illumination models (ray tracing,
  radiosity) determine shading by bouncing light
  around an entire environment (too slow for
  interactive graphics)
• Local illumination models consider only the
  surface, light direction, and viewing direction

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Local illumination

• To make lighting fast enough, we will initially
  restrict our attention to
• Light source, one surface, and viewer (ignore
  inter-object reflections, shadows)
• Ambient, diffuse, and specular reflection (ignore
  transparency, refraction, reflection, )

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Light sources

• In general, a light source is a rather
  complicated thing. It can emit different amounts
  of light for each
• Location (x, y, z)
• Direction (?, f)
• Wavelength (l)
• Illumination functionI(x, y, z, ?, f, l)
• Examples ambient, point, area, spot,distant,

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Colored lights

• Intensity of emitted light can also be a function
  of wavelength
• We usually model as I Ir, Ig, Ib components
• Some experiments have been done with a whole
  spectrum of color values, giving more realistic
  results in some cases

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Ambient light

• Intensity doesnt vary with x, y, z, ?, f
• I Iar, Iag, Iab

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Point lights

• Point lights have a location (so farther objects
  receive less light) but are not directional
• I(p0) Ir(p0), Ib(p0), Ig(p0)
• How would you compute the illumination at point
  p?
• Illumination proportional to inverse square of
  distance
• I(p, p0) (1/d2) Ir(p0), Ib(p0), Ig(p0)

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Limitations of point lights

• Usually result in artificiallyhigh-contrast
  images
• Can generate umbra (full shadow) but
  notpenumbra (partial shadow)
• Area lights generate softershadows, but are
  usuallyused only in raytracing or radiosity

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Distant (directional) lights

• Light point lights, but
• Without attenuation based on the distance
• Without difference in direction (parallel rays)
• Location of light source becomes x, y, z, 0
  noattenuation
• More efficient to computethan point sources

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Spotlights

• Directional, i.e. light is emitted in a narrow
  range of angles, q
• More realistic spotlights wouldmodel a gradual
  fall-off of light
• E.g. cose f (s l)e if s is direction
  ofsource, l direction to source, both unit
  vectors

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Illumination and shading

• How do these light sources affect brightness of a
  surface point?
• Most commonly used model for interactive
  graphics Phong Illumination Model
• Involves terms
• Ambient
• Diffuse
• Specular
• It is a (simplified) model of the physics of
  reflection

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Vectors used by Phong model

• The directions used by the phong model
• n surface outward normal
• v direction to viewer
• l direction to light source
• r reflection direction
• Since these are directions, theyare unit
  vectors.

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Ambient term of Phong model

• An object has an ambient reflectivity
  coefficient, ka
• A light source gives off a certain amount of
  ambient light, La
• Total ambient illumination Ia ka La
• (For colored light, we repeat this computation
  for R, G, and B ambient light values and
  reflectivity coefficients)

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Diffuse term

• A perfectly diffuse reflector is so rough that it
  scatters light equally in all directions
• But note that when thelight comes in at an
  angle,the same energy is spreadout over larger
  area
• Such surfaces are calledLambertian surfaces
  (obeying Lamberts Law)

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Diffuse shading

• At noon, illum. is 1
• As the angle q (u infigure) decreases,
  illumination goes to zero
• Illumination is proportional to cos(q)
  (Lamberts law)
• cos(q) l n
• Id kd l n Ld

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Specular Term

• Specular term adds highlights in the reflection
  direction
• Note that the smoother and shinier the object,
  the tigher and brighter thehighlight
• Highlight power falls as viewer v moves away from
  reflection dir, r. (cos f v r)
• Modeled as cosa f, a is shininess coefficient
  (1..200)
• Is ks Ls (r v)a

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Phong illumination model

• Phong illumination model
• I Ambient Diffuse Specular
• Ia Id Is
• ka La kd Ld l n ks Ls (r v)a
• May add light attenuation term
• 1/(abdcd2) ( ka La kd l n Ld) ks Ls (r
  v)a
• Parameters needed
• Light La, Ld, Ls for each light
• Surface ka, kd, ks, a
• Repeat for each color component, light source
• How hard to calculate?

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Polygon shading

• How do you use the Phong Illumination Model to
  render an object with shading?
• Consider a polygonal sphere approximation
• How do you find the normals to the faces?
• Shade a face with a constant color?
• glShadeModel(GL_FLAT)
• Called flat shading or Constant shading
• How much computation would this require
• Per pixel?
• Per vertex?

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Flat shading drawbacks

• The human visual system enhances edges
• We see stripes (known as MachBands) along edges
• Much like aconvolution!
• How to avoid?

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Gouraud shading

• Gouraud shading
• Define vertex normals as averageof surrounding
  faces
• Compute lighting equation at each vertex
• Interpolate colors across polygon
• glShadeModel(GL_SMOOTH)
• Computation required
• Per pixel?
• Per vertex?
• Very fast! Especially with reasonably large
  polygons and hardware color interpolation

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Gouraud drawbacks

• Drawbacks of Gouraudshading?
• Polygon edges are still visible
• Brightness is modelled asa linear function, but
  thatsnot really accurate
• Real highlights are smalland bright and drop off
  sharply
• If polygons are too large, highlights get
  distorted and dimmed (notice the funny shape)
• How to avoid these artifacts?

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Phong shading

• To eliminate artifacts, interpolate normals
• Results better shading, much nicer highlights
• Computation required per pixel?
• This is still too expensive to do in hardware, in
  general

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Shading summary

• Dont confuse Phong Illumination Model and Phong
  Shading
• Gouraud shading compute illumination model at
  each vertex. Interpolate colors. (Often done in
  hardware)
• Phong shading interpolate vertex normals.
  Compute illumination model at each vertex

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Specifying lights in OpenGL

• OpenGL supports those four light types
• Point, directional lights
• GLfloat light0_pos 1.0, 2.0, 3.0, 1.0
• GLfloat light0_pos 1.0, 2.0, 3.0, 0.0
• Diffuse, Ambient, Specular coefficients
• GLfloat diffuse0 1, 0, 0, 1
• GLfloat ambient0 1, 0, 0, 1
• GLfloat spedular0 1, 1, 1, 1
• glEnable(GL_LIGHTING)

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Enabling lights

• Can enable at least 8 lights
• glEnable(GL_LIGHT0)
• glLightfv(GL_LIGHT0, GL_POSITION, light0_pos)
• glLightfv(GL_LIGHT0, GL_AMBIENT, ambient0)
• glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuse0)
• glLightfv(GL_LIGHT0, GL_SPECULA, specular0)
• Spotlights set more light parameters as above
• GL_SPOT_DIRECTION
• GL_SPOT_EXPONENT
• GL_SPOT_CUTOFF

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Specifying Materials

• Material properties are part of the drawing
  state, specified by glMaterialfv
• GLfloat ambient 0.2, 0.2, 0.2, 1.0
• GLfloat diffuse 1.0, 0.8, 0.0, 1.0
• GLfloat specular 1.0, 1.0, 1.0, 1.0
• glMaterialfv(GL_FRONT, GL_AMBIENT, ambient)
• glMaterialfv(GL_FRONT, GL_DIFFUSE, diffuse)
• glMaterialfv(GL_FRONT, GL_SPECULAR, specular)
• glMaterialf(GL_FRONT, GL_SHININESS, 100.0)
• GLfloat emission0.0, 0.3, 0.3, 1.0
• glMaterialfv(GL_FRONT, GL_EMISSION, emission)
• Use GL_FRONT_AND_BACK for two-sided faces

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OpenGL Gouraud Shading

• OpenGL needs to know vertex normals as well as
  locations
• glNormal3fv(n)
• glVertex3fv(p)
• How to compute vertex normals?
• Cross product for face normals
• Average normals of surrounding faces
• How to find neighboring faces?

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Virtual Trackball shading

• Flat shading
• Compute a normal for each face
• Gouraud shading
• Compute a normal for each vertex as average of
  adjacent faces
• Defect you may not want to smooth-shade across
  every edge how should it really be done?
• What would you have to do to handle material
  properties, surface colors, etc?

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Surface subdivision

• In real modelers
• one can usually define smooth curved surfaces
• Eg spheres, quadrics, NURBS, smoothed polygons
• Modeler renders with smoothness setting
• Recursively split polygons into smaller pieces,
  with new vertices on the smooth surface
• Splitting a triangle can be done by
• Bisecting angles
• Computing the centrum
• Bisecting sides
• Of course, smoother surfaces take longer to draw

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Chapter summary

• Phong illumination model has ambient, diffuse,
  and specular terms
• It can be used for Flat, Gouraud, and Phong
  shading
• OpenGL supports eight ambient, point, distant,
  and spot lights
• You must specify light and material properties
  with many OpenGL function calls
• Curved surfaces are modeled with polygon
  subdivisions