Lighting in OpenGL

1
Lecture 10

• Lighting in OpenGL

2
Sources of light

• GLfloat myLightPosition 3.0, 6.0, 5.0, 1.0
• GLLightfv(GL_LIGHT0, GL_POSITION,
  myLightPosition)
• glEnable(GL_LIGHTING) //enable lighting
• glEnable(GL_LIGHT0) //enable this particular
  source
• Infinitely remote source (directional light)
• GLfloat myLightPosition 3.0, 6.0, 5.0, 0.0

3
RGB Values for Lights and Materials

• For light the numbers correspond to a percentage
  of full intensity for each color
• If the R, G, and B values for lights color are
  all 1.0, the light is the brightest possible
  white.
• If the values are 0.5, the color is still white,
  but only half intensity, so it appears gray.
• If RG1 and B0, light appears yellow.

4
RGB Values for Lights and Materials

• For materials, the number corresponds to
  reflected proportion of those colors.
• So, if R1, G0.5, and B0 for a material, that
  material reflects all the incoming green, and
  none of the incoming red light, half the incoming
  green, and none of the incoming blue light.

5
RGB Values for Lights and Materials

• Hence, OpenGL light has components (LR, LG, LB),
  and material has corresponding components (MR,
  MG, MB), then ignoring all other reflective
  effects, the light arrives at the eye is given by
  ( LRMR, LGMG, LBMB).
• Similarly, if we have two light source that sends
  (R1, G1, B1) and (R2, G2, B2) to the eye, OpenGL
  adds the components (R1R2, G1G2, B1B2).
• If a color intensity is bigger than 1, OpenGL
  makes it 1.

6
Lighting Terms

• Ambient Light
• is the light that has a source, but is evenly
  spread around the room. Objects that have
  ambient light are evenly lit and always visible.
• Light is evenly distributed that there is no way
  to tell its source. For example, due to
  reflection of light from walls.

7
Lighting Terms

• Diffuse Light
• is a light that comes from a particular direction
  and reflects evenly off the surface. Points
  where the light is directly normal
  (perpendicular) from the light source to the
  surface appear brighter.

8
Lighting Terms

• Specular Light
• is similar to the Diffuse light where the light
  comes from a particular direction, but the
  reflection is much more dynamic. This effect can
  usually cause a shine or a glare.

9
Lighting Terms in OpenGL

• GLfloat amb0 0.2, 0.4, 0.6, 1.0
• GLfloat diff0 0.8, 0.9, 0.5, 1.0
• GLfloat spec0 1.0, 0.8, 1.0, 1.0
• glLightfv(GL_LIGHT0, GL_AMBIENT, amb0)
  //attach them to LIGHT0
• glLightfv(GL_LIGHT0, GL_DIFFUSE, diff0)
• glLightfv(GL_LIGHT0, GL_SPECULAR, spec0)

10
Normals

• As far as the API is concerned, a square, or any
  object, is just a collection of points to form an
  object.
• Even with simple objects, OpenGL knows nothing of
  the properties of that object.
• In order to give more information to OpenGL about
  an object and light, a normal should be defined.

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Normals

• A normal vector is a line perpendicular to a
  plane in three-dimensional space.
• OpenGL uses normalized normals a normal vector
  of length 1, to calculate how light may reflect
  off an object.

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Normals

• Imagine any triangle ABC on the xy-plane (z
  values 0). The coordinates starting from top
  left and going counter-clock-wise are as follows
• ABC 0, 1, 0 , -1, -1, 0 , 1, -1, 0
  
• Normal vectors are only calculated at vertices,
  and the normals for the triangle are
• Normals ABC 0, 0, 1

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Normals

• Triangle ABC is co-planar (actually, any three
  points that form a triangle define a plane), so
  the normal to be calculated is the same anywhere
  on the triangle. At any angle of light, this
  vertex reflects the light symmetrically through
  the normal vector. This is how light is able to
  reflect off an object.
• This example was easy because the triangle lies
  completely on the xy-plane, but what about other
  triangles that lie anywhere in space?

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Calculating Normals

• Objects may have analytic surfaces and
  non-analytic surfaces
• I. Calculating normals for analytic surfaces
• Analytic surfaces are surfaces that are
  non-coplanar, such as spheres.
• Because calculating normals are only done at
  vertices, it becomes incredibly complicated to
  determine where the vertices are for an object
  such as a sphere.
• If the object being drawn is a predefined GLU
  shape, then using the gluQuadricNormals()
  function, will generate the normals for you
• GLUT predefined shapes automatically calculate
  normals).

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Calculating Normals

• II. To find a normal of a surface that is
  non-analytic
• Means that the surface lies on the same plane, or
  is co-planar.
• This means that the normal vector on this surface
  is the same on all vertices.
• We have to get two vectors v1, v2 that are
  non-collinear and share a tail.
• The result of the cross product of the two
  vectors is a vector normal to the surface.

16
Materials

• OpengGL lighting model makes the approximation
  that a materials color depends on the percentage
  of the incoming red, green, and blue light it
  reflects.
• For example, a perfectly red ball reflects all
  the incoming red light and absorbs all the green
  and blue light that strikes it.
• When such a ball is viewed in a white light
  (composed of equal amounts of red, green and blue
  light), all the red is reflected and you see a
  red ball.
• If the ball is viewed in a pure green light, it
  appears black (all the green is absorbed and
  there is no incoming red, so no light is
  reflected).

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Materials

• Like lights, materials have different ambient,
  diffuse and specular colors, which determine the
  ambient, diffuse, and specular reflectances.
• A materials
• ambient reflectance is combined with the ambient
  component of each incoming light source,
• the diffuse reflectance with the lights
  diffuse component
• specular reflectance with the lights specular
  component.

18
Materials

• In addition to ambient, diffuse, and specular
  colors, materials have an emissive color, which
  simulates light originating from an object.
• In OpenGL, the emissive color of a surface adds
  intensity to the object, but is unaffected by any
  light source.
• Materials are what we use to give an object a
  certain type of reflective (or non-reflective)
  quality.

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Materials

• Example
• glEnable( GL_COLOR_MATERIAL )
• float red 0.94, 0.2, 0.15, 1.0
• glMaterialfv( GL_FRONT, GL_AMBIENT_AND_DIFFUSE,
  red )
• // draw some objects
• glDisable( GL_COLOR_MATERIAL )

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Materials

• Enabling the material properties with a call to
  glEnable allow the materials to be manipulated.
• Next, it is needed to set which side the material
  colors it to be used for.
• The only three possible choices are GL_FRONT,
  GL_BACK, or GL_GL_FRONT_AND_BACK.

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Materials

• The next parameter allows you to set the types of
  light to set (GL_AMBIENT, GL_DIFFUSE,
  GL_SPECULAR, or GL_AMBIENT_AND_DIFFUSE).
• One thing to note is that in most cases the
  ambient and diffuse materials are set to the same
  values, and this is why OpenGL has an option for
  combining the two in one line.
• The last parameter is an array (vector) of the
  intensities of the Red, Green, Blue and Alpha
  values that define the (in our case) ambience and
  diffuse properties (the second parameter).

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Materials

• Another thing to note is our first time use of
  the alpha color. If you recall from color
  section, it was said that the alpha value is used
  for special effects. For use with lighting, we
  always set the alpha value to 1.0.
• Finally a call to glDisable() may speed up
  graphic intensive operations if the machine
  doesnt have to check for material colors anymore.