Advanced Computer Graphics James Gain Department of Computer

Advanced Computer Graphics: James Gain Department of Computer Science University of Cape Town jgain@cs. uct. ac. za Advanced Computer Graphics. Collaborative Visual Computing Laboratory

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. 18/09/2020 Advanced Computer Graphics 2

Aliasing Occurs when the sampling inherent in rendering does not contain enough information for an accurate image. Original Scene Scanline Luminosity 18/09/2020 Sampling Pixel Centres Sampled Signal Advanced Computer Graphics Rendered Image Luminosity Signal 3

Effects of Aliasing • Common aliasing errors (called artefacts) are: jagged profiles, disappearing or improper fine detail, and disintegrating textures. Jagged Profiles Loss of Detail 18/09/2020 Advanced Computer Graphics 4

Disintegrating Texture • • • 18/09/2020 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. Advanced Computer Graphics 5

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): • • • 18/09/2020 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 512 x 512. Sampling at three times the width and three times the height of the display resolution would yield 1536 x 1536 samples. A filter provides the weights used to compute the average. Advanced Computer Graphics 6

Prefiltering No Antialiasing 18/09/2020 Prefiltering Advanced Computer Graphics 7

Postfiltering Sampling: take jittered, regular or random samples. Jittered Regular Averaging: use an filter to assign weights to each sample. Sum all the weighted samples to obtain a pixel. 18/09/2020 Advanced Computer Graphics 8

Filtering Example 18/09/2020 Advanced Computer Graphics 9

Antialiasing Examples 18/09/2020 Advanced Computer Graphics 10

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