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More on Environment Mapping

More on Environment Mapping. Glenn G. Chappell CHAPPELLG@member.ams.org U. of Alaska Fairbanks CS 381 Lecture Notes Wednesday, December 10, 2003. Review: More on Procedural Texture [1/2]. One way to convert a noise function into an image: Choose colors for specified values in [–1,1].

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More on Environment Mapping

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1. More on Environment Mapping Glenn G. ChappellCHAPPELLG@member.ams.org U. of Alaska Fairbanks CS 381 Lecture Notes Wednesday, December 10, 2003

2. Review:More on Procedural Texture [1/2] • One way to convert a noise function into an image: • Choose colors for specified values in [–1,1]. • Then determine the color associated with a value by lirping between the two nearest specified values. • Create the image by mapping noise values to colors. –1.0 –0.3 0.4 1.0 –1.0 –0.3 –0.1 0.4 1.0 CS 381

3. Review:More on Procedural Texture [2/2] • Code for color generation will often look something like this: // mapcolor - map value in [-1,1] to an RGB color. void mapcolor(double value, GLubyte color[3]) { if (value < -0.3) { t = (value – (-1.)) / ((-0.3) – (-1.)); color[0] = (1.-t)*1.0*255 + t*0.8*255; // lirp between white (1.0, 1.0, 1.0) color[1] = (1.-t)*1.0*255 + t*0.6*255; // and brown (0.8, 0.6, 0.4) color[2] = (1.-t)*1.0*255 + t*0.4*255; } else if (value < 0.4) { t = (value – (-0.3)) / ((0.4) – (-0.3)); color[0] = (1.-t)*0.8*255 + t*0.9*255; // lirp between brown (0.8, 0.6, 0.4) color[1] = (1.-t)*0.6*255 + t*0.1*255; // and magenta (0.9, 0.1, 0.9) color[2] = (1.-t)*0.4*255 + t*0.9*255; } else … CS 381

4. Review:Environment Mapping [1/4] • 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 (transformed by model/view). • Surface normal (transformed by model/view). CS 381

5. Review:Environment Mapping [2/4] • 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

6. Review:Environment Mapping [3/4] • 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

7. Review:Environment Mapping [4/4] • 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. • In particular, all the glutSolid… functions work! CS 381

8. More on Environment Mapping:Introduction • We have seen how to create mirror-like reflections cheaply using a texture. • Disadvantages • No object-object reflections. • The reflected scene is considered to lie at infinity. • Often get errors at the edge of the silhouette. • With sphere mapping, anyway. • Creating the texture can be tricky. • We look at this now. CS 381

9. More on Environment Mapping:Creating the Texture [1/4] • The hard part of environment mapping is creating the texture itself. • When to Create • A sphere-map texture depends on: • The environment. • The viewing position. • Thus, the texture needs to be recomputed whenever these change. • It does not need to be computed when an environment-mapped object moves. • Another way to think about it: • Environment mapping is good for a stationary viewer viewing moving objects in a static environment. • Next: How to Create CS 381

10. More on Environment Mapping:Creating the Texture [2/4] • In environment mapping, given a reflected-ray direction in eye coordinates, texture coordinates are computed. • To make the texture do this backwards: • Look at each texel in turn. • For each, determine what reflected-ray direction it corresponds to. • Use the color of the environment, seen in that direction, for the color of the texel. • Procedure for reflected-ray determination: • Given texture coordinates a, b. • Scale a, b so they are in [–1,1]. • Find c 0 so that (a, b, c) is a unit vector. • Reflected-ray direction is (2ca, 2cb, 2c2–1). CS 381

11. More on Environment Mapping:Creating the Texture [3/4] • We want to write some code to generate the sphere-map texture. • Suppose that the image array is declared as usual: const int img_width = …; const int img_height = …; GLubyte the_image[img_height][img_width][3]; • Now suppose we have a function void envgetcolor(double x, double y, double z, GLubyte color[3]) • Given a unit vector (x, y, z). • Returns a color in the given array (RGB are 0..255). • Color is the color of the environment, see when looking in direction (x, y, z). CS 381

12. More on Environment Mapping:Creating the Texture [4/4] • Then here is code to make the sphere-map texture: void envmakeimage() { for (int i=0; i<img_width; ++i) for (int j=0; j<img_height; ++j) { double a = 2.*i/img_width-1.; double b = 2.*j/img_height-1.; double k = 1.-a*a-b*b; double c = ((k <= 0.) ? 0. : sqrt(k)); double x = 2.*c*a; double y = 2.*c*b; double z = 2.*c*c-1.; envgetcolor(x, y, z, the_image[j][i]); } } CS 381

13. More on Environment Mapping:The Environment Looks Like … • So, how do we determine the color of the environment, seen in a particular direction? • Answer 1: Render the environment in a buffer, and read this to determine colors. • Tricky … • Answer 2: Ray tracing. • Yes, this is slow, but it only needs to be done when the texture is computed. • Answer 3: Do it backwards. • Decide: The color in this direction is … • Then draw the environment as a large sphere, colored according to the above decisions. CS 381

14. More on Environment Mapping:EXAMPLE • We put together an example environment-mapped object (which actually reflected its environment) using the following pieces from envmapfuncs.cpp (on the web page): • Function envgetcolor was written to make an interesting background. • Function envmakeimage created a sphere-map texture using envgetcolor. • Function envdrawsphere drew the environment: a sphere colored using envgetcolor. • The sphere was drawn with a large radius, to simulate an environment at infinity. CS 381

15. More on Environment Mapping:Some Hints • When using environment mapping: • Disable lighting for environment-mapped objects. • Environment mapping is lighting … sort of. • Other objects may still need lighting. • Perfect reflectors are unrealistic; do not draw them. • Set GL_MODULATE mode, and give your reflecting objects a color other than full white. • Color (0.9, 0.9, 0.9) is not bad for silvery metal. • Lower B (and maybe lower G) gives gold/coppery colors. • Make your environment-mapped objects move. • This adds realism. • It is also cheap, since the texture does not need to be recomputed. CS 381

16. More on Environment Mapping:Cube Maps • We have discussed how to store a picture of the environment as a sphere map. Another way to store it is as a cube map. • Instead of a single, warped texture (as in a sphere map), we use 6 textures, forming the sides of a cube. • The 6 textures form a picture of the environment, as seen from the center of the cube. • Computations are simpler.  • Generating the images is much easier.  • Not supported by standard OpenGL.  • Although it is available in extensions. CS 381

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