Lecture Eighteen Sampling

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Lecture EighteenSampling Anti-aliasing
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Two basic problems

• Loss of detail
• items which fall between two sample points
• Inaccurate representation of sampled data, e.g.

or
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Scan Conversion
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Scan conversion routine identifies pixels which
form part of the line and sets those to the line
colour. Other pixels play no part in the line
description
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Aliasing

• The jagged edges (staircase effect) on straight
  line segments
• a consequence of sampling a continuous signal at
  discrete locations

Signal Value
Sample Points
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Aim of anti-aliasing

• The removal of jagged edges and the correct
  representation of sub-pixel detail
• Anti-aliasing in hardware
• pixel phasing finer positioning of the CRT
  electron beam - not suitable for most
  applications
• Anti-aliasing in software
• 3 approaches...

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Supersampling

• 1st approachIncrease the sample points,
  resulting in a greater number of smaller steps.
• expensive solution
• doubling resolution in x and y quadruples the
  memory cost and scan conversion time
• does not address the basic flaw in point sampling
• detail which does not coincide with sample points
  is missed

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Postfiltering

• 2nd approachApply a smoothing filter to the
  completed image.
• removes jagged edges, but as the filter is
  applied globally to the image, the appearance of
  other objects is affected
• does nothing to resolve the problem of lost detail

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Prefiltering

• 3rd approach - 2 ideas (Crow 77 81)
• (i) treat a pixel as bounding a finite area and
  illuminate according to the amount of the area
  covered by the object being drawn
• proportional reduction of intensity
• (ii) the effect of objects influencing a pixel is
  spread over neighbouring pixels
• using an intensity matrix centred on the pixel
  and decreasing towards the edges

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Intensity related to area

• 100 coverage gt full intensity white (say)
• 75 coverage gt light grey
• 50 coverage gt mid grey
• 25 coverage gt dark grey
• 0 coverage gt black (the background colour)
• Gives a soft-edging effect...

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Example
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Idea (ii) - Spread the effect
1 1 2 2 2 2 1 1 1 2 3 4 4 3 2 1 2 3 5 6 6 5 3 2 2
4 6 8 8 6 4 2 2 4 6 8 8 6 4 2 2 3 5 6 6 5 3 2 1 2
3 4 4 3 2 1 1 1 2 2 2 2 1 1 0 1 2 3 4 5 6 7
7 6 5 4 3 2 1 0