Illumination, Lighting and Shading Model

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Illumination, Lighting and Shading Model

• Pradondet Nilagupta
• Dept. of Computer Engineering
• Kasetsart University

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Definitions (1/2)

• Illumination the transport of energy (in
  particular, the luminous flux of visible light)
  from light sources to surfaces points
• Note includes direct and indirect illumination
• Lighting the process of computing the luminous
  intensity (i.e., outgoing light) at a particular
  3-D point, usually on a surface
• Shading the process of assigning colors to pixels

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Definitions (2/2)

• Illumination models fall into two categories
• Empirical simple formulations that approximate
  observed phenomenon
• Physically-based models based on the actual
  physics of light interacting with matter
• We mostly use empirical models in interactive
  graphics for simplicity
• Increasingly, realistic graphics are using
  physically-based models

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Lighting Model

• Many different models exist for simulating
  lighting reflections
• Most models break lighting into constituent parts
• ambient reflections
• diffuse reflections
• specular highlights

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Lighting Model Component

• Material Properties
• used to describe an objects reflected colors
• Surface Normals
• Light Properties
• used to describe a lights color emissions
• Light Model Properties
• global lighting parameters

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Components of Illumination

• Surface properties
• Reflectance spectrum (i.e., color of the surface)
• Geometric attributes
• Position
• Orientation
• Micro-structure

• Light sources (or emitters)
• Spectrum of emittance (i.e, color of the light)
• Geometric attributes
• Position
• Direction
• Shape
• Directional attenuation

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Ambient Light Sources

• Objects not directly lit are typically still
  visible
• E.g., the ceiling in this room, undersides of
  desks
• This is the result of indirect illumination from
  emitters, bouncing off intermediate surfaces
• Too expensive to calculate (in real time)

Ireflected kambient Iambient
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Directional Light Sources

• all rays of light from the source are parallel
• As if the source is infinitely far away from the
  surfaces in the scene
• A good approximation to sunlight
• direction is constant for all surfaces in the
  scene

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Point Light Sources

• A point light source emits light equally in all
  directions from a single point
• The direction to the light from a point on a
  surface thus differs for different points

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Other Light Sources

• Spotlights are point sources whose intensity
  falls off directionally.
• Supported by OpenGL
• Area light sources define a 2-D emissive surface
  (usually a disc or polygon)
• Good example fluorescent light panels

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Reflection

• Ambient Reflections
• Diffuse Reflection
• Specular Reflections

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Ambient Reflections

• Color of an object when not directly illuminated
• Think about walking into a room with the curtains
  closed and lights off

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Diffuse Reflections

• Color of an object when directly illuminated
• often referred to as base color
• The angle between the surface normal and the
  incoming light is the angle of incidence
• Idiffuse kd Ilight cos ?

• In practice we use vector arithmetic
• Idiffuse kd Ilight (n l)

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Specular Reflections

• Shiny surfaces exhibit specular reflection
• Polished metal
• Glossy car finish
• A light shining on a specular surface causes a
  bright spot known as a specular highlight

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Phong Lighting Model

• Using surface normal
• OpenGLs lighting model based on Phongs

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OpenGL Material Properties

• GL_AMBIENT
• GL_DIFFUSE
• GL_SPECULAR
• GL_SHININESS
• GL_EMISSION

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Computing Surface Normals

• Lighting needs to know how to reflect light off
  the surface
• Provide normals per
• face - flat shading
• vertex - Gouraud shading
• pixel - Phong shading
• OpenGL does not support Phong natively

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Face Normals

• Same normal for all vertices in a primitive
• results in flat shading for primitive

glNormal3f( nx, ny, nz ) glBegin( GL_TRIANGLES
) glVertex3fv( v1 ) glVetrex3fv( v2 )
glVertex3fv( v3 ) glEnd()
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Computing Face Normals ( Polygons )

• Were using only planar polygons
• Can easily compute the normal to a plane
• use a cross product

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Computing Face Normals ( Algebraic )

• For algebraic surfaces, compute
• where

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Vertex Normals

• Each vertex has its own normal
• primitive is Gouraud shaded basedon computed
  colors

glBegin( GL_TRIANGLES ) glNormal3fv( n1 )
glVertex3fv( v1 ) glNormal3fv( n2 )
glVetrex3fv( v2 ) glNormal3fv( n3 )
glVertex3fv( v3 ) glEnd()
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Computing Vertex Normals (Algebraic )

• For algebraic surfaces, compute

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Computing Vertex Normals ( Polygons )

• Need two things
• face normals for all polygons
• know which polygons share a vertex

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Sending Normals to OpenGL

• glNormal3f( x, y, z )
• Use between glBegin() / glEnd()
• Use similar to glColor()

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Normals and Scale Transforms

• Normals must be normalized
• non-unit length skews colors
• Scales affect normal length
• rotates and translates do not
• glEnable( GL_NORMALIZE )

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Why?

• Lighting computations are really done in eye
  coordinates
• this is why there are the projection and
  modelview matrix stacks
• Lighting normals transformed by the inverse
  transpose of the ModelView matrix

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Flat Shading

• The simplest approach, flat shading, calculates
  illumination at a single point for each polygon

• If an object really is faceted, is this accurate?
• No
• For point sources, the direction to light varies
  across the facet
• For specular reflectance, direction to eye varies
  across the facet

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Flat Shading

• We can refine it a bit by evaluating the Phong
  lighting model at each pixel of each polygon, but
  the result is still clearly faceted
• To get smoother-looking surfaceswe introduce
  vertex normals at eachvertex
• Usually different from facet normal
• Used only for shading (as opposed to what?)
• Think of as a better approximation of the real
  surface that the polygons approximate (draw it)

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Vertex Normals

• Vertex normals may be
• Provided with the model
• Computed from first principles
• Approximated by averaging the normals of the
  facets that share the vertex

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Gouraud Shading

• This is the most common approach
• Perform Phong lighting at the vertices
• Linearly interpolate the resulting colors over
  faces
• This is what OpenGL does
• Demo at http//www.cs.virginia.edu/cs551/vrml/tpo
  t.wrl
• Does this eliminate the facets?

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Phong Shading

• Phong shading is not the same as Phong lighting,
  though they are sometimes mixed up
• Phong lighting the empirical model weve been
  discussing to calculate illumination at a point
  on a surface
• Phong shading linearly interpolating the surface
  normal across the facet, applying the Phong
  lighting model at every pixel
• Same input as Gouraud shading
• Usually very smooth-looking results
• But, considerably more expensive

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An Aside Transforming Normals

• Irritatingly, the matrix for transforming a
  normal vector is not the same as the matrix for
  the corresponding transformation on points
• In other words, dont just treat normals as
  points

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Transforming Normals

• Some not-too-complicated affine analysis shows
• If A is a matrix for transforming points,then
  (AT)-1 is the matrix for transforming normals
• When is this the same matrix?
• What is homogeneous representation of a vector
  (as opposed to a point?)
• Can use this to simplify the problem only upper
  3x3 matrix matters, so use only it
• More detail in FvD Appendix A.5

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Texture Mapping Motivation

• Scenes created with diffuse lighting look
  convincingly three-dimensional, but are flat,
  chalky, and cartoonish
• Phong lighting lets us simulate materials like
  plastic and (to a lesser extent) metal, but
  scenes still seem very cartoonish and unreal
• Big problem polygons are too coarse-grained to
  usefully model fine surface detail
• Solution texture mapping

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Texture Mapping Motivation

• Adding surface detail helps keep CG images from
  looking simple and sterile
• Explicitly modeling this detail in geometry can
  be very expensive
• Zebra stripes, wood grain, writing on a
  whiteboard
• Texture mapping pastes images onto the surfaces
  in the scene, adding realistic fine detail
  without exploding the geometry

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Texture Mapping Examples
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Texture Mapping Fundamentals

• A texture is typically a 2-D image
• Image elements are called texels
• Value stored at a texel affects surface
  appearance in some way
• Example diffuse reflectance, shininess,
  transparency
• The mapping of the texture to the surface
  determines the correspondence, i.e., how the
  texture lies on the surface
• Mapping a texture to a triangle is easy (why?)
• Mapping a texture to an arbitrary 3-D shape is
  more complicated (why?)

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Texture Mapping Rendering

• Rendering uses the mapping
• Find the visible surface at a pixel
• Find the point on that surface corresponding to
  that pixel
• Find the point in the texture corresponding to
  that point on the surface
• Use the parameters associated with that point on
  the texture to shade the pixel

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Texture Mapping Basics

• We typically parameterize the texture as a
  function in (u, v)
• For simplicity, normalize u v to 0, 1
• Associate each triangle with a texture
• Give each vertex of the triangle a texture
  coordinate (u, v)
• For other points on the triangle, interpolate
  texture coordinate from the vertices
• Much like interpolating color or depth
• But theres a catch...