BlinnPhong Shading Model and Halfvector

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BlinnPhong Shading Model and Halfvector

• ???

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BlinnPhong Shading Model

• Also called BlinnPhong reflection model or
  modified Phong reflection model.
• Trade visual precision for computing efficiency.
  
• Default shading model used in OpenGL.

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Differences (ref wikipedia)

• The halfway angle is smaller than the angle we
  want in Phong's model, but considering that Phong
  is using, we can set an exponent so that is
  closer to the former expression.
• The size of the specular highlights can be
  matched in this way very closely to a
  corresponding Phong reflection, but they will
  always retain a subtly different shape.

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Half-Vector in GLSL

• Built-in Uniform

Property of a (directional) light!!
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Halfvector in GLSL

• Lighting computation is computed in eye space.
• L (unit vector to light)
• the same for every fragment for directional light
• Eye is always (0,0,1) in eye space (camera at
  origin, looking to Z)
• Halfvector can be computed and stored for
  directional light

See LocalViewer
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Example
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GL_LIGHT_MODEL_LOCAL_VIEWER

• params is a single integer or floating-point
  value that specifies how specular reflection
  angles are computed. If params is 0 (or 0.0),
  specular reflection angles take the view
  direction to be parallel to and in the direction
  of the -z axis, regardless of the location of the
  vertex in eye coordinates. Otherwise specular
  reflections are computed from the origin of the
  eye coordinate system. The default is 0.

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Summary (Local Viewer)

• For simplified specular computation (local viewer
  0)
• Eye (0,0,1)

• For more sophisticated lighting (local viewer
  1)
• Eye -ecposition

z
z
H
Eye coord. system
V
L