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CS 445 / 645 Introduction to Computer Graphics

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## CS 445 / 645 Introduction to Computer Graphics

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**CS 445 / 645Introduction to Computer Graphics**Lecture 18 Antialiasing**Environment Mapping**• Used to model a object that reflects surrounding textures to the eye • Polished sphere reflects walls and ceiling textures • Cyborg in Terminator 2 reflects flaming destruction • Texture is distorted fish-eye view of environment • Spherical texture mapping creates texture coordinates that correctly index into this texture map**The q-coordinate**• Remember, texture coordinate interpolation is linear • Texture coordinates can contain an optional q-parameter • q scales texture coordinates by dividing each coordinate by q • This captures perspective transformations Imageplane Z – into the scene**Multitexturing**• Pipelining of multiple texture applications to one polygon • The results of each texture unit application is passed to the next texture unit, which adds its effects • More bookkeeping is required to pull this off**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**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**Line Segments**• Instead, quantize to many shades • But what sampling algorithm is used?**Area Sampling**• Shade pixels according to the area covered by thickened line • This is unweighted area sampling • A rough approximation formulated by dividing each pixel into a finer grid of pixels**Unweighted Area Sampling**• Primitive cannot affect intensity of pixel if it does not intersect the pixel • Equal areas cause equal intensity, regardless of distance from pixel center to area**Weighted Area Sampling**• Unweighted sampling colors two pixels identically when the primitive cuts the same area through the two pixels • Intuitively, pixel cut through the center should be more heavily weighted than one cut along corner**Intensity**W(x,y) x Weighted Area Sampling • Weighting function, W(x,y) • specifies the contribution of primitive passing through the point (x, y) from pixel center**Images**• An image is a 2D function I(x, y) that specifies intensity for each point (x, y)**Sampling and Image**• Our goal is to convert the continuous image to a discrete set of samples • The graphics system’s display hardware will attempt to reconvert the samples into a continuous image: reconstruction**Point Sampling an Image**• Simplest sampling is on a grid • Sample dependssolely on valueat grid points**Point Sampling**• Multiply sample grid by image intensity to obtain a discrete set of points, or samples. Sampling Geometry**Sampling Errors**• Some objects missed entirely, others poorly sampled**Fixing Sampling Errors**• Supersampling • Take more than one sample for each pixel and combine them • How many samples is enough? • How do we know no features are lost? 150x15 to 100x10 200x20 to 100x10 300x30 to 100x10 400x40 to 100x10**Unweighted Area Sampling**• Average supersampled points • All points are weighted equally**Weighted Area Sampling**• Points in pixel are weighted differently • Flickering occurs as object movesacross display • Overlapping regions eliminates flicker**How is this done today?Full Screen Antialiasing**• Nvidia GeForce2 • OpenGL: render image 400% larger and supersample • Direct3D: render image 400% - 1600% larger • Nvidia GeForce3 • Multisampling but with fancy overlaps • Don’t render at higher resolution • Use one image, but combine values of neighboring pixels • Beware of recognizable combination artifacts • Human perception of patterns is too good**GeForce3**• Multisampling • After each pixel is rendered, write pixel value to two different places in frame buffer**GeForce3 - Multisampling**• After rendering two copies of entire frame • Shift pixels of Sample #2 left and up by ½ pixel • Imagine laying Sample #2 (red) over Sample #1 (black)**GeForce3 - Multisampling**• Resolve the two samples into one image by computing average between each pixel from Sample 1 (black) and the four pixels from Sample 2 (red) that are 1/ sqrt(2) pixels away**GeForce3 - Multisampling**• No AA Multisampling**GeForce3 - Multisampling**• 4x Supersample Multisampling**ATI Smoothvision**• ATI SmoothVision • Programmer selects samping pattern**3dfx**• 3dfx Multisampling • 2- or 4-frame shift and average • Tradeoffs?