How do we draw a picture

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How do we draw a picture?

• Define geometry. Now what?
• We can draw the edges of the faces. Wireframe.
• We can only draw the edges of faces that are
  visible.
• We can fill in the faces. Giving each object a
  colour (constant shading).

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Flat Shading

• Assumes the object is faceted, and light and
  viewer at infinity.
• Illumination model is applied only once per
  polygon.
• What if the object represents a curved surface?

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Mach Bands

• Perceived intensity change at edges are
  exaggerated by receptors in our eyes, making the
  dark facet look darker and the light facet look
  lighter.

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Gouraud Shading

• Also called smooth shading, intensity
  interpolation shading or color interpolation
  shading.
• Discontinuities eliminated by interpolating
  intensity.
• Almost removes Mach bands (high curvature can
  still suffer).
• OpenGL implements Gouraud shading.
• Easily implemented in hardware.

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How it works
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Work out lighting
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Average across polygon
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Inconsistencies
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Sometimes want sharp edge
Do you have more than one normal at a vertex?
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Phong Shading
Not Phong Illumination

• Also called normal-vector interpolation shading.
• Interpolates the surface normals instead of the
  intensity values.
• Calculate illumination at every pixel, as well as
  a vector normalization. Much slower than Gouraud.
• Gives better results, especially for highlights.
• Can handle highlights within a polygon.

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Interpolate normal
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Do lighting
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Interpolation Shading Problems

• Must store normals at each vertex.
• Silhouette edges not smoothed.
• Interpolation can mask regular changes.
• Interpolation in image space ignores perspective
  distortion.
• Crease edges should not have smooth shading.
• Orientation dependence.

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Fixes for Interpolation Shading

• Use triangles.
• Use more triangles.
• Use multiple normals (to get sharp edges)

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