COMP471

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COMP471 Computer Graphics

• OpenGL Let There Be Light

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Whats Light?

• Light is energy
• A given surface can emit, reflect, etc this
  energy.
• Light bulb is emitting surface
• Some other surface reflecting base on properties
  and material of that surface
• White light contains all the color spectrum

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Light Sources

• Ambient
• Point
• Spotlight
• Distant

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Ambient Light

• Uniform illumination of the objects in a given
  environment.
• Coming from nowhere and going to nowhere.
• Its a diffuse light w/o a direction
• Good to highlight a little a dark scene

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Point Light
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Spotlight

• GL_SPOT_CUTOFF ?

?
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Distant Light Source

• Parallel
• The source is very-very far (like Sun)
• Directional vector determines such a parallel
  light source

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Material

• Surfaces can have different material properties.
• These properties determine how a given surface
  will reflect a light.

Diffuse
Secular (shiny)
Translucent
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OpenGL and Light

• Light sources
• Spotlight
• Distant
• Emissive
• Material

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Light Sources

• Enable Lighting (another OpenGL state)
• glEnable(GL_LIGHTING)
• Enable one of the light sources
• glEnable(GL_LIGHT0)
• Define light source atttibutes
• glLight(GL_LIGHT0, .)

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Material

• GLfloat ambient 0.2,0.2,0.2,1.0
• glMaterialfv(GL_FRONT,GL_AMBIENT, ambient)

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Light Positioning

• Fixed
• Moving around stationary objects
• Moving along with the viewpoint
• NOTE Position and direction of light are
  affected by the current Model-View matrix.

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Fixed Light Position

• glMatrixMode(GL_MODELVIEW)
• glLoadIdentity()
• glLightfv(, GL_POSITION, pos)
• other transformations

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Independently Moving Light

• Set viewpoint coordinates
• gluLookAt()
• glPushMatrix()
• glTranslate() glRotate()
• glLight()
• glPopMatrix()
• draw objects

These transformations will apply only to the
light source.
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Moving Light Source Along With the Viewpoint

• Set light source position pos
• glMatrixMode(GL_MODELVEIW)
• glLoadIdentity()
• glLightfv(..., GL_POSITION, pos)
• Change the viewpoint

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Normals

• For the light to highlight proper side of an
  object, a normal has to be defined for that side.
• Use glNormal() right before you draw the polygon
  for it to be lit properly.
• There are default normals however, they might
  not be what you want them to be, so when dealing
  with light and textures mapping explicitly state
  the normals.

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Normals (2)
void Cubedraw() // Front
glNormal3f(0.0, 0.0, 1.0) drawFace(0, 3, 2,
1, 0) // Right glNormal3f(1.0, 0.0,
0.0) drawFace(2, 3, 7, 6, 1) // Bottom
glNormal3f(0.0, -1.0, 0.0) drawFace(0, 4,
7, 3, 2)
// Top glNormal3f(0.0, 1.0, 0.0)
drawFace(1, 2, 6, 5, 3) // Back
glNormal3f(0.0, 0.0, -1.0) drawFace(4, 5, 6,
7, 4) // Left glNormal3f(-1.0, 0.0,
0.0) drawFace(0, 1, 5, 4, 5)
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References

• Jianxiang Dong Slides from Fall 2001