Texture Mapping - Lecture Slides | CS 5501, Study notes of Computer Science

Download Texture Mapping - Lecture Slides | CS 5501 and more Study notes Computer Science in PDF only on Docsity! 1 David Luebke 5/6/2000 CS 551 / 645: Introductory Computer Graphics Texture Mapping II David Luebke 5/6/2000 Administrivia ● Assignment 5: just kidding David Luebke 5/6/2000 Recap: Texture Mapping ● Adding surface detail helps keep CG images from looking simple and sterile ● Explicitly modeling this detail in geometry can be very expensive – Zebra stripes, wood grain, writing on a whiteboard ● Texture mapping pastes images onto the surfaces in the scene, adding realistic fine detail without exploding the geometry David Luebke 5/6/2000 Recap: Texture Mapping Recap: Texture Mapping David Luebke 5/6/2000 Recap: Texture Coordinates ● Give each vertex of the triangle a texture coordinate (u, v) ● For other points on the triangle, interpolate texture coordinate from the vertices ● Problem: interpolating in screen-space (a la Gouraud shading) is incorrect – Perspective foreshortening should compress the texture image on distant regions of the surface – Demo at http://graphics.lcs.mit.edu/classes/6.837/F98/Lecture21/Slide06.html 2 David Luebke 5/6/2000 Recap: Perspective-Correct Interpolation ● Skipping some math… – Rather than interpolating u and v directly, interpolate u/z and v/z ■ These do interpolate correctly in screen space ■ Also need to interpolate z and multiply per-pixel – Problem: we don’t know z anymore – Solution: we do know w ≈ 1/z – So…interpolate uw and vw and w, and compute u = uw/w and v = vw/w for each pixel ■ This unfortunately involves a divide per pixel David Luebke 5/6/2000 Recap: Texture Map Aliasing ● Naive texture mapping looks blocky, pixelated – Problem: using a single texel to color each pixel: int uval = (int) (u * denom + 0.5f); int vval = (int) (v * denom + 0.5f); int pix = texture.getPixel(uval, vval); – Actually, each pixel maps to a region in texture ■ If the pixel is larger than a texel, we should average the contribution from multiple texels somehow ■ If the pixel is smaller than a texel, we should interpolate between texel values somehow ■ Even if pixel size ≈ texel size, a pixel will in general fall between four texels – An example of a general problem called aliasing David Luebke 5/6/2000 Recap: Texture Map Antialiasing ● Use bilinear interpolation to average nearby texel values into a single pixel value (Draw it) – Find 4 nearest texture samples ■ Round u & v up and down – Interpolate texel values in u – Interpolate resulting values in v ● Also addresses the problem of many pixels projecting to a single texel (Why?) David Luebke 5/6/2000 Recap: Texture Map Antialiasing ● What if a single pixel covers many texels? – Problem: sampling those texels at a single point (the center of the pixel): ■ Produces Moire patterns in coherent texture (checkers) ■ Leads to flicker or texture crawling as the texture moves – Approach: blur the image under the pixel, averaging the contributions of the covered texels ■ But calculating which texels every pixel covers is way too expensive, especially as the texture is compressed – Solution: pre-calculate lower-resolution versions of the texture that incorporate this averaging David Luebke 5/6/2000 MIP-maps ● For a texture of 2n x 2n pixels, compute n-1 textures, each at ½ the resolution of previous: ● This multiresolution texture is called a MIP-map Original Texture Lower Resolution Versions David Luebke 5/6/2000 Generating MIP-maps ● Generating a MIP-map from a texture is easy – For each texel in level i, average the values of the four corresponding texels in level i-1 ● If a texture requires n bytes of storage, how much storage will a MIP-map require? ● Answer: 4n/3