/* I renamed the shader to MBVenus.sl -- tal AT renderman DOT org */
/*
* venus.sl - surface for a very cloudy planet like Venus.
*
*
* DESCRIPTION:
* When put on a sphere, sets the color to look like a densely
* clouded planet, very much like the real Venus appears in UV.
* The shader works by creating a fractal turbulence function over
* the surface to simulate the clouds. Strong Coriolis forces are
* simulated to give the twisting of clouds that is typically seen
* on Venus.
*
*
* PARAMETERS:
* Ka, Kd - the usual meaning
* offset, scale - control the linear scaling of the cloud value.
* twist - controls the twisting of the clouds due to Coriolis forces.
* omega - controls the fractal characteristics of the clouds
* octaves - the number of octaves of noise to sum for the clouds.
* radius - radius of planet
*
*
* AUTHOR: Ken Musgrave.
* Conversion to Shading Language and minor modifications by Larry Gritz.
* Planet radius param added by Mark Beckwith.
*
* REFERENCES:
* _Texturing and Modeling: A Procedural Approach_, by David S. Ebert, ed.,
* F. Kenton Musgrave, Darwyn Peachey, Ken Perlin, and Steven Worley.
* Academic Press, 1994. ISBN 0-12-228760-6.
*
*
* HISTORY:
* ???? - Venus texture developed by F. Ken Musgrave.
* Feb 1994 - Conversion to Shading Language by L. Gritz
* Dec 1996 - Added radius of planet as a parameter by Mark Beckwith
* (mark AT intrig DOT com )
*/
#define TWOPI (2*PI)
/* Use signed noise on [-1,1] */
#define snoise(x) ((2*noise(x))-1)
surface
MBVenus (float Ka = 1, Kd = 1;
float offset = 1;
float scale = 0.6;
float twist = 0.22;
float omega = 0.65;
float octaves = 8;
float radius = 1)
{
point Ptexture; /* the shade point in texture space */
point PtN; /* normalized version of Ptexture */
point PP; /* Point after rotation by coriolis twist */
float rsq; /* Used in calculation of twist */
float angle; /* Twist angle */
float sine, cosine; /* sin and cos of angle */
float l, o, a, i; /* Loop control for fractal sum */
float value; /* Fractal sum is stored here */
/* Transform to texture coordinates and map to nit sphere */
Ptexture = transform ("shader", P) / radius;
/* Calculate Coriolis twist, yielding point PP */
PtN = normalize (Ptexture);
rsq = xcomp(PtN)*xcomp(PtN) + ycomp(PtN)*ycomp(PtN);
angle = twist * TWOPI * rsq;
sine = sin (angle);
cosine = cos (angle);
PP = point (xcomp(Ptexture)*cosine - ycomp(Ptexture)*sine,
xcomp(Ptexture)*sine + ycomp(Ptexture)*cosine,
zcomp(Ptexture));
/* Compute VLfBm */
l = 1; o = 1; a = 0;
for (i = 0; i < octaves; i += 1) {
a += o * snoise (PP * l);
l *= 2;
o *= omega;
}
value = abs (offset + scale * a);
/* Shade like matte, but with color scaled by cloud color */
Oi = 1;
Ci = Os * (value * Cs) * (Ka * ambient() +
Kd * diffuse(faceforward(normalize(N),I)));
}