1. Advanced Computer Graphics:
James Gain
Department of Computer Science University of Cape Town
Advanced Computer GraphicsCollaborative Visual Computing Laboratory

2. Advanced Computer Graphics
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 Educator’s Slide Set.
18/09/2018
Advanced Computer Graphics

3. Aliasing Original Scene Sampling Pixel Centres Rendered Image
Occurs when the sampling inherent in rendering does not contain enough information for an accurate image.
Original Scene
Sampling Pixel Centres
Rendered Image
Scanline Luminosity
Sampled Signal
Luminosity Signal
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Advanced Computer Graphics

4. Advanced Computer Graphics
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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Advanced Computer Graphics

5. 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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Advanced Computer Graphics

6. Advanced Computer Graphics
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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Advanced Computer Graphics

7. Advanced Computer Graphics
Prefiltering
No Antialiasing
Prefiltering
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Advanced Computer Graphics

8. Advanced Computer Graphics
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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Advanced Computer Graphics

9. Advanced Computer Graphics
Filtering Example
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Advanced Computer Graphics

10. Antialiasing Examples
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Advanced Computer Graphics

11. Advanced Computer Graphics
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12. Advanced Computer Graphics
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Advanced Computer Graphics

13. Advanced Computer Graphics
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Advanced Computer Graphics