人机接口与图形学(双语)10-Antialiasing.ppt

Antialiasing in Action A Closer Comparison Aliasing in Time Our animations can also experience temporal aliasing ? the motion of objects does not appear as it should Consider a spoked wagon wheel ? suppose it rotates 7/8 around in 1 second ? and we take a picture of it every second ? it appears to be rotating backwards! Aliasing in Time In principle, this is the same phenomenon as spatial aliasing ? converting from continuous to discrete representation ? unlucky choice of discrete samples will give us bad results Can solve the problem in the same way as spatial aliasing ? temporal supersampling and average ? in other words, motion blur Final Thoughts on Aliasing Unfortunately, we can never really win ? no matter how much we supersample, aliasing remains ? it just gets less and less apparent ? and at some point, humans can’t detect it anymore We’ve skipped over vast amounts of theory ? can develop rigorous theory of (anti)aliasing – based upon signal processing theory (see Foley) ? in this course, focus on simple antialiasing in practice Fractal:分形 * Antialiasing Department of CS E, East China University of Science Technology What is a pixel? A pixel is not… A box A disk A teeny tiny little light A pixel is a point It has no dimension It occupies no area It cannot be seen It can have a coordinate A pixel is more than a point, it is a sample Samples Most things in the real world are continuous Everything in a computer is discrete The process of mapping a continuous function to a discrete one is called sampling The process of mapping a continuous variable to a discrete one is called quantization Rendering an image requires sampling and quantization Samples Samples Line Segments We tried to sample a line segment so it would map to a 2D raster display We quantized the pixel values to 0 or 1 We saw stair steps, or jaggies Where Do Jaggies Come From? Our primitives don’t evenly cover all the pixels they touch ? higher reso