Advanced Computer Graphics:

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Presentation transcript:

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

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

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 18/09/2018 Advanced Computer Graphics

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 18/09/2018 Advanced Computer Graphics

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. 18/09/2018 Advanced Computer Graphics

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. 18/09/2018 Advanced Computer Graphics

Advanced Computer Graphics Prefiltering No Antialiasing Prefiltering 18/09/2018 Advanced Computer Graphics

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 18/09/2018 Advanced Computer Graphics

Advanced Computer Graphics Filtering Example 18/09/2018 Advanced Computer Graphics

Antialiasing Examples 18/09/2018 Advanced Computer Graphics

Advanced Computer Graphics 18/09/2018 Advanced Computer Graphics

Advanced Computer Graphics 18/09/2018 Advanced Computer Graphics

Advanced Computer Graphics 18/09/2018 Advanced Computer Graphics