Advanced Computer Graphics:

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Advanced Computer Graphics

• James Gain
• Department of Computer ScienceUniversity of Cape
  Town
• jgain_at_cs.uct.ac.za

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Objectives

• To show the problems caused by sampling in
  computer graphics (aliasing).
• To describe a variety of methods designed to
  combat these effects (antialiasing).
• Slides taken from ACM SIGGRAPH Educators Slide
  Set.

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Aliasing

• Occurs when the sampling inherent in rendering
  does not contain enough information for an
  accurate image.

Sampling Pixel Centres
Rendered Image
Original Scene
Scanline Luminosity
Sampled Signal
Luminosity Signal
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Effects of Aliasing

• Common aliasing errors (called artefacts) are
  jagged profiles, disappearing or improper fine
  detail, and disintegrating textures.

Jagged Profiles
Loss of Detail
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Disintegrating Texture

• The checkers on a plane should become smaller
  with distance.
• But aliasing causes them to become larger and/or
  irregular.
• Increasing resolution only moves the artefact
  closer to the horizon.

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Antialiasing

• Developed to combat aliasing.
• Applies to all types of aliasing PSC, RT and
  temporal.
• Prefiltering
• Treat a pixel as an area. Calculate overlap of
  polygon with pixel. Costly
• Postfiltering (supersampling)
• Compute pixel colour by averaging multiple
  samples. Preferred Method.
• Sample the scene at n times the display
  resolution. For example, suppose the display
  resolution is 512x512. Sampling at three times
  the width and three times the height of the
  display resolution would yield 1536x1536 samples.
  
• A filter provides the weights used to compute the
  average.

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Prefiltering
No Antialiasing
Prefiltering
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Postfiltering

• Sampling take jittered, regular or
  random samples.
• Averaging use an filter to
  assign weights to each sample.
  Sum all the weighted samples to
  obtain a pixel.

Jittered
Regular
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Filtering Example
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Antialiasing Examples
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