Lighting Review

1
Lighting Review Example

• Glenn G. ChappellCHAPPELLG_at_member.ams.org
• U. of Alaska Fairbanks
• CS 381 Lecture Notes
• Monday, November 17, 2003

2
ReviewThe Phong Model

• Here is a summary of the three components of the
  Phong Model, along with the recommended colors to
  use for each type of light reflection.

Type of Light
Type of Reflection
Phong Model
Ambient Light
Diffuse Reflection
Ambient Component
ColorPaint color
ColorDim version of direct light colorOR
black??
Diffuse Component
Direct Light
Specular Reflection
Specular Component
ColorWhite??
ColorDirect light color
3
ReviewThe Phong Model 2/4

• To compute the ambient component of the Phong
  Model, we need
• The light color of the ambient light (AR, AG,
  AB).
• The paint color of the material of the surface
  the vertex lies on (MR, MG, MB).
• The resulting color is found via componentwise
  multiplication(ARMR, AGMG, ABMB).

4
ReviewThe Phong Model 3/4

• The diffuse component works much like the
  ambient, except that it matters at what angle the
  light hits the surface.
• Do the lighting computation as for ambient.
• Use the direct-light color, not the ambient-light
  color.
• Multiply R, G, B the Lambert cosine k.
• This is the dot product of the normal and
  light-direction vectors, assuming these are unit
  vectors.
• If we are doing attenuation, multiply R, G, B by
  1/d2.
• For the specular component, both the illumination
  and viewing directions matter.
• Do the lighting computation as for ambient.
• Use the direct-light color, as with diffuse.
• And use the specular-reflection color, not the
  paint color.
• Multiply R, G, B by (RV)s, where V is the
  viewing direction, and s is the shininess.
• If desired, do attenuation as for diffuse.

5
ReviewThe Phong Model 4/4

• We have seen how the Phong Model computes the
  effects of ambient, diffuse, and specular
  reflection.
• Now, what is the color of the vertex?
• How about just adding these up?
• Final red ambient red diffuse red specular
  red.
• Similarly for green blue.
• But this may result in RGB components greater
  than 1.
• Solution If any of R, G, B ends up being greater
  than 1, set it equal to 1.
• This is not entirely satisfactory, but at least
  it gives legal results.

6
ReviewBasic OpenGL Lighting

• OpenGL implements the Phong Model.
• We still generally need to compute normals
  ourselves.
• We set up lights and materials OpenGL does the
  rest.
• How it works
• Set up and enable one or more lights (light
  sources).
• Use glLight, glEnable.
• Various other lighting properties can be set.
• With glLightModel, glShadeModel, etc.
• Enable/disable lighting as appropriate.
• Enable with glEnable(GL_LIGHTING).
• When drawing, set material properties.
• Use glMaterial.
• Forget about glColor, for now.
• Before each glVertex command, specify a normal
  vector.
• Use glNormal.

7
ReviewNormal Vectors

• When we do lighting, each polygon vertex needs an
  associated normal vector.
• This should be a unit vector pointing straight
  out from the surface at the vertex.
• In OpenGL
• We specify a normal with glNormal, inside
  glBegin-glEnd.
• A glNormal call comes before the associated
  glVertex.
• ExampleglNormal3d(0., 0., 1.)glVertex3d(1.,
  2., 3.)
• The tricky part is computing the normals. We
  discussed three methods.
• Facet normals using cross products.
• See the code fragment drawtriangle.cpp, on the
  web page.
• Normals based on the mathematical formula for the
  surface.
• Other methods.

8
Moving Lights ExampleLights and Model/View

• A light is something to be positioned within the
  world, just like a polygon.
• Therefore, it is natural that light positions
  pass through the model/view transformation.
• The matrix used is the current one at the time
  when glLightfv(, GL_POSITION, ) is called.
• Not the current one when a lit object is drawn!
• Therefore, you probably want to position your
  lights in the display function
• If you want a light to move.
• If you want to make a light source visible by
  drawing an object at its location.
• If you have any other reason to be interested in
  exactly where your lights are.
• You can still set a lights colors in the
  initialization, then set its position in the
  display function.

9
Moving Lights ExampleAn Object at the Light
Position

• Suppose we want to draw an object at the light
  position.
• It should appear to be the light source.
• We want to position the object using model/view.
• If we use this same model/view matrix for setting
  the light position, then we will specify the
  light position as
• (0, 0, 0, 1).
• We probably want the color of the object to match
  the color of the direct light.
• Make the light color a global?
• OpenGL material specs add an emission color to
  the Phong model.
• Its effect on the vertex color ignores
  light-source colors and angles.
• In short, its rather like glColor, except that
  it is added to the other components in the Phong
  Model.
• Use GL_EMISSION to set the emission color.

10
Moving Lights ExampleExample

• Animate the following scene
• A rotating object in the center.
• A moving light revolving about the object.
• The light should be visible.