Procedural texture
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A procedural texture is a computer generated image created using an algorithm intended to create a realistic representation of natural elements such as wood, marble, granite, metal, stone, and others.
Usually, the natural look of the rendered result is achieved by the usage of fractal noise and turbulence functions. These functions are used as a numerical representation of the “randomness” found in everything that surrounds us.
In general, these noise and fractal functions are simply used to “disturb” the texture in a natural way such as the undulations of the veins of the wood. In other cases, like marbles' textures, they are based on the graphical representation of fractal noise.
[edit] Example of a procedural marble texture:
(Taken from The Renderman Companion Book, by Steve Upstill)
/* Copyrighted Pixar 1988 */
/* From the RenderMan Companion p.355 */
/* Listing 16.19 Blue marble surface shader*/
/*
* blue_marble(): a marble stone texture in shades of blue
* surface
*/
blue_marble(
float Ks = .4,
Kd = .6,
Ka = .1,
roughness = .1,
txtscale = 1;
color specularcolor = 1)
{
point PP; /* scaled point in shader space */
float csp; /* color spline parameter */
point Nf; /* forward-facing normal */
point V; /* for specular() */
float pixelsize, twice, scale, weight, turbulence;
/* Obtain a forward-facing normal for lighting calculations. */
Nf = faceforward( normalize(N), I);
V = normalize(-I);
/*
* Compute "turbulence" a la [PERLIN85]. Turbulence is a sum of
* "noise" components with a "fractal" 1/f power spectrum. It gives the
* visual impression of turbulent fluid flow (for example, as in the
* formation of blue_marble from molten color splines!). Use the
* surface element area in texture space to control the number of
* noise components so that the frequency content is appropriate
* to the scale. This prevents aliasing of the texture.
*/
PP = transform("shader", P) * txtscale;
pixelsize = sqrt(area(PP));
twice = 2 * pixelsize;
turbulence = 0;
for (scale = 1; scale > twice; scale /= 2)
turbulence += scale * noise(PP/scale);
/* Gradual fade out of highest-frequency component near limit */
if (scale > pixelsize) {
weight = (scale / pixelsize) - 1;
weight = clamp(weight, 0, 1);
turbulence += weight * scale * noise(PP/scale);
}
/*
* Magnify the upper part of the turbulence range 0.75:1
* to fill the range 0:1 and use it as the parameter of
* a color spline through various shades of blue.
*/
csp = clamp(4 * turbulence - 3, 0, 1);
Ci = color spline(csp,
color (0.25, 0.25, 0.35), /* pale blue */
color (0.25, 0.25, 0.35), /* pale blue */
color (0.20, 0.20, 0.30), /* medium blue */
color (0.20, 0.20, 0.30), /* medium blue */
color (0.20, 0.20, 0.30), /* medium blue */
color (0.25, 0.25, 0.35), /* pale blue */
color (0.25, 0.25, 0.35), /* pale blue */
color (0.15, 0.15, 0.26), /* medium dark blue */
color (0.15, 0.15, 0.26), /* medium dark blue */
color (0.10, 0.10, 0.20), /* dark blue */
color (0.10, 0.10, 0.20), /* dark blue */
color (0.25, 0.25, 0.35), /* pale blue */
color (0.10, 0.10, 0.20) /* dark blue */
);
/* Multiply this color by the diffusely reflected light. */
Ci *= Ka*ambient() + Kd*diffuse(Nf);
/* Adjust for opacity. */
Oi = Os;
Ci = Ci * Oi;
/* Add in specular highlights. */
Ci += specularcolor * Ks * specular(Nf,V,roughness);
}
This article was taken from The Photoshop Roadmap with written authorization
