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Advanced Texturing Methods

Advanced Texturing Methods. Glenn G. Chappell CHAPPELLG@member.ams.org U. of Alaska Fairbanks CS 381 Lecture Notes Monday, December 8, 2003. Review: Quick & Dirty Texture Generation.

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Advanced Texturing Methods

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  1. Advanced Texturing Methods Glenn G. ChappellCHAPPELLG@member.ams.org U. of Alaska Fairbanks CS 381 Lecture Notes Monday, December 8, 2003

  2. Review:Quick & Dirty Texture Generation • An easy way to generate a small texture is to begin with a string array, then turn map characters into colors somehow. • For an example, see qdtexture.cpp, on the web page. CS 381

  3. Review:Procedural Texture [1/6] • Often, the trickiest part of texture mapping is generating the texture image. • Possible sources: • External images. • From photographs, various graphics programs. • Reading the frame buffer. • Use glReadPixels. • Program-generated texture. • This is what we look at now. • Texture generated “from scratch” by a program is called procedural texture. • We look at a technique for procedural-texture generation pioneered by Ken Perlin of New York University. CS 381

  4. Review:Procedural Texture [2/6] • Perlin’s idea is to begin with a special “noise” function. Properties: • Easy to generate. • Random-ish looking. • All variation is at about the same scale. • All variation has about the same amplitude. • Highest, lowest, and average values are about the same for all parts of the image. CS 381

  5. Review:Procedural Texture [3/6] • One type of noise we would like to be able to use is 1/f-noise. • All frequencies present, with amplitudes proportional to the inverse of the frequency. • So “the height of a hill is proportional to its width”. • 1/f-noise is the way the real world looks. • But it’s a pain to generate. • Perlin suggests simulating 1/f-noise: • Sum many copies of his noise function, each at twice the frequency and half the amplitude of the previous. CS 381

  6. Review:Procedural Texture [4/6] • Another idea of Perlin: “noise with cusps”. • Sum as before, but take the absolute value of each noise function before summing them all. • Good for cloudy-looking things. CS 381

  7. Review:Procedural Texture [5/6] • Perlin’s technique is aimed at 3-D textures. • Here we can make objects appear to be cut out of a 3-D block of material. • Essentially, let the texture coordinates be the same as the vertex coordinates. • Unfortunately, OpenGL 1.1 (which most of us seem to have) does not support 3-D texturing in a system-independent manner. • However, we can use Perlin’s techniques to generate 2-D texture simply by taking a slice of a 3-D noise function. CS 381

  8. Review:Procedural Texture [6/6] • I have created a package (“Pnoise”) to create a minor variation on Perlin’s noise function. • Major differences: • His is entirely deterministic. I use an externally seeded pseudo-random number generator. • I produce noise functions that “wrap”. • The images on the previous slides were generated by my code, not Perlin’s. • Specifically: pnoise3d(…), fnoise3d(…), filterfnoise3d(…, pnoise_abs). • See proctexture.cpp for sample code. CS 381

  9. More on Procedural Texture:Producing Colors [1/2] • One piece remains in order to produce interesting procedural textures: • Convert a noise function into an image. • We need to be able to map noise values (in [–1,1]) to colors (RGB). CS 381

  10. More on Procedural Texture:Producing Colors [2/2] • One versatile method: • Choose colors for specified values in [–1,1]. • Then determine the color associated with a value by lirping between the two nearest specified values. • EXAMPLE TIME. –1.0 –0.7 0.2 1.0 –1.0 –0.7 0.2 0.7 1.0 CS 381

  11. More on Procedural Texture:Other Methods • Other methods for generating procedural texture are possible (and are not really that difficult to come up with). For example: • A particle doing a random walk and leaving behind trails of color. • This technique was used to generate some of the 3-D textures used in BLUIsculpt. • Think of something else … CS 381

  12. Bump Mapping Demo • Bump Mapping • Like texturing, except look up normals instead of colors. • Use the bump-map normal to perturb the usual surface normal. • Result: bumpy-looking surface. • With a smooth silhouette. • Bump mapping requires per-fragment lighting. • And therefore it is not well suited to the OpenGL pipeline. • But bump mapping is not hard to add to a ray tracer. • What do you suppose is a good way to generate a bump map? CS 381

  13. Environment Mapping:Introduction • In environment mapping, we simulate mirror-like reflection using texturing. • Generate texture coordinates based on the direction that light, originating from the viewer, would reflect off the surface. • Need: • Viewing location (always (0,0,0) in OpenGL). • Vertex coordinates. • Surface normal. CS 381

  14. Environment Mapping:Sphere Map • A simple environment-mapping technique is to use a sphere map. • Take the reflected light direction, add 1 to z, normalize. • Use the resulting x, y as texture coordinates. • The texture is a fish-eye-lens picture of the environment. • E.g., see plate 21 in the red book. CS 381

  15. Environment Mapping:Chrome Mapping • Chrome mapping is a type of environment mapping in which we do not attempt to produce a realistic picture of the environment. • Just make mirror-like reflections of something. • Then objects look metallic, regardless of the incorrect reflections. • Note: Some people would not call chrome mapping a type of environment mapping. • I would. • In particular, it is legal on assignment #11. CS 381

  16. Environment Mapping:OpenGL • OpenGL includes automatic texture-coordinate generation. • One of the options does a sphere map. • In initialization (or whereever): glTexGeni(GL_S, GL_TEXTURE_GEN_MODE, GL_SPHERE_MAP); glTexGeni(GL_T, GL_TEXTURE_GEN_MODE, GL_SPHERE_MAP); • To enable: glEnable(GL_TEXTURE_GEN_S); glEnable(GL_TEXTURE_GEN_T); • Disabling is as usual. • Normals must be specified. glTexCoord* is not necessary. • How to generate the texture, though … ? CS 381

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