Real World Lighting

1
Real World Lighting
Incident Light
Reflected Light
Surface

• If only green is reflected, red and blue are
  absorbed, then object appears green

2
Lights Reflected Towards Viewer

• Emissive Light
• Originate from objects that glow by themselves
• Reflected Light
• Reflect part of the incoming light from light
  sources towards the viewer

3
OpenGL Lighting

• Light source is characterized by the amount of
  RGB emitted
• Material of surface is characterized by the
  amount of RGB reflected towards the viewer

4
Different Kinds of Light Sources

• Distant (directional) light source
• Located infinitely far away
• Specified as w0
• Point (positional) light source
• Specified as w1
• By default, radiate in all directions
• Spotlight, radiate within a cone

5
Defining Light in OpenGL

• glLightifv (Glenum light, Glenum pname, TYPE
  param)
• light GL_LIGHT0, , GL_LIGHT7
• pname e.g. GL_AMBIENT, GL_DIFFUSE, GL_POSITION
• param a pointer to RGBA value, position of
  light, etc

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Multiple Lights

• Can support at least 8 lights
• Each light is identified by constants GL_LIGHTn,
  where n 0, 1, , 7

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pname in glLight

• GL_AMBIENT
• Specifies the ambient RGBA intensities of light
  source
• GL_DIFFUSE
• Specifies the diffuse RGBA intensities of light
  source

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pname in glLight (Cont)

• GL_SPECULAR
• Specifies the specular RGBA intensities of a
  light source
• GL_POSITION
• Specifies the (x, y, z, w) position of a light
  source

9
Constructing Spotlights

• glLightif (lightNo, GL_SPOT_CUTOFF, degree)
• glLightifv (lightNo, GL_SPOT_DIRECTION,
  spot_direction)

10
Switch on the Lights

• glEnable(GL_LIGHTING)
• Enable light calculations
• glEnable(GL_LIGHT0)
• Turn on a light source
• glDisable(GL_LIGHT0)
• Turn off a light source

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Defining Material in OpenGL

• glMaterialifv (GLenum face, GLenum pname, TYPE
  param)
• face GL_FRONT, GL_BACK, GL_FRONT_AND_BACK
• pname GL_AMBIENT_AND_DIFFUSE, GL_SPECULAR,
  GL_SHININESS
• param a pointer to the value RGBA value

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pname in glMaterial

• GL_AMBIENT
• Specifies fraction of ambient light reflected
• GL_DIFFUSE
• Specifies fraction of diffuse light reflected
• GL_SPECULAR
• Specifies fraction of specular light reflected
• GL_SHININESS
• Specifies the specular exponent (shininess)

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Material Properties

• Ambient light
• Light being scattered by the environment so much
  that its direction is impossible to determine

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Material Properties

• Diffuse
• Defines the fraction of light scattered into all
  directions
• Defines the color of the surface

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Material Properties

• Specular
• Defines the fraction of light reflected near the
  mirror reflection direction
• The shinier the object, the more focused is the
  highlight

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Rendering Polygonal Mesh

• Polygonal mesh A collection of
• Vertices
• Edges
• Faces

• Edge

• Face

Normal Vector
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Normal Vector

• A vector which is perpendicular to a surface at a
  point
• Determines how the point reflects the incident
  light

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Normal Vector (Cont)

• Planar surface (e.g. cube faces)
• Same normal for entire surface
• Curved surface
• Different normals at different points

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Finding Vertex Normals

• Compute from analytical representation (e.g.
  Bezier surfaces)
• Average face normals
• Special effects, e.g.bump mapping

20
Hints for Building Polygonal Surfaces

• Consistent polygon orientation
• Counterclockwise when looking from outside
• Avoid non-triangular polygons
• Because they may not be planar
• Display speed vs quality
• Small polygons better quality
• Avoid T-intersections

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Polygonal mode

• glPolygonMode(GL_FILL)
• Shade the interior of polygon
• glPolygonMode(GL_LINE)
• Draw the edges of polygon
• For drawing wireframe model

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Shading Model

• Flat shading
• glShadeModel(GL_FLAT)
• With a single color for each polygon
• Smooth shading (Gourand shading)
• glShadeModel(GL_SMOOTH)
• Interpolate colors at vertices to find color for
  interior pixels

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Rendering Polygon

• glShadeModel(GL_FLAT)
• glBegin(GL_POLYGON)
• glNormal3fv(n1)
• glVertex3fv(v1)
• glVertex3fv(v2)
• glVertex3fv(v3)
• glVertex3fv(v4)
• glEnd()

v3
v4
n1
v2
v1
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Rendering Polygon (Cont)

• glShadeModel(GL_SMOOTH)
• glBegin(GL_POLYGON)
• glNormal3fv(n1)
• glVertex3fv(v1)
• glNormal3fv(n2)
• glVertex3fv(v2)
• glNormal3fv(n3)
• glVertex3fv(v3)
• glNormal3fv(n4)
• glVertex3fv(v4)
• glEnd()

n3
n2
n4
n1