Don’t know if this is relevant in Max, but in Maya the outputs and results need to be called “color”. Try this:
/* Amplitude reflection coefficient (s-polarized) */
float rs(float n1, float n2, float cosI, float cosT) {
return (n1 * cosI - n2 * cosT) / (n1 * cosI + n2 * cosT);
}
/* Amplitude reflection coefficient (p-polarized) */
float rp(float n1, float n2, float cosI, float cosT) {
return (n2 * cosI - n1 * cosT) / (n1 * cosT + n2 * cosI);
}
/* Amplitude transmission coefficient (s-polarized) */
float ts(float n1, float n2, float cosI, float cosT) {
return 2 * n1 * cosI / (n1 * cosI + n2 * cosT);
}
/* Amplitude transmission coefficient (p-polarized) */
float tp(float n1, float n2, float cosI, float cosT) {
return 2 * n1 * cosI / (n1 * cosT + n2 * cosI);
}
// cosI is the cosine of the incident angle, that is, cos0 = dot(view angle, normal)
// lambda is the wavelength of the incident light (e.g. lambda = 510 for green)
// From http://www.gamedev.net/page/resources/_/technical/graphics-programming-and-theory/thin-film-interference-for-computer-graphics-r2962
float thinFilmReflectance(float cos0, float lambda, float thickness, float n0, float n1, float n2) {
//float PI=3.1415926535897932384626433832795;
float PI=M_PI;
// compute the phase change term (constant)
float d10 = (n1 > n0) ? 0 : PI;
float d12 = (n1 > n2) ? 0 : PI;
float delta = d10 + d12;
// now, compute cos1, the cosine of the reflected angle
float sin1 = pow(n0 / n1, 2) * (1 - pow(cos0, 2));
if (sin1 > 1) return 1.0; // total internal reflection
float cos1 = sqrt(1 - sin1);
// compute cos2, the cosine of the final transmitted angle, i.e. cos(theta_2)
// we need this angle for the Fresnel terms at the bottom interface
float sin2 = pow(n0 / n2, 2) * (1 - pow(cos0, 2));
if (sin2 > 1) return 1.0; // total internal reflection
float cos2 = sqrt(1 - sin2);
// get the reflection transmission amplitude Fresnel coefficients
float alpha_s = rs(n1, n0, cos1, cos0) * rs(n1, n2, cos1, cos2); // rho_10 * rho_12 (s-polarized)
float alpha_p = rp(n1, n0, cos1, cos0) * rp(n1, n2, cos1, cos2); // rho_10 * rho_12 (p-polarized)
float beta_s = ts(n0, n1, cos0, cos1) * ts(n1, n2, cos1, cos2); // tau_01 * tau_12 (s-polarized)
float beta_p = tp(n0, n1, cos0, cos1) * tp(n1, n2, cos1, cos2); // tau_01 * tau_12 (p-polarized)
// compute the phase term (phi)
float phi = (2 * PI / lambda) * (2 * n1 * thickness * cos1) + delta;
// finally, evaluate the transmitted intensity for the two possible polarizations
float ts = pow(beta_s, 2) / (pow(alpha_s, 2) - 2 * alpha_s * cos(phi) + 1);
float tp = pow(beta_p, 2) / (pow(alpha_p, 2) - 2 * alpha_p * cos(phi) + 1);
// we need to take into account conservation of energy for transmission
float beamRatio = (n2 * cos2) / (n0 * cos0);
// calculate the average transmitted intensity (if you know the polarization distribution of your
// light source, you should specify it here. if you don't, a 50%/50% average is generally used)
float t = beamRatio * (ts + tp) / 2;
// and finally, derive the reflected intensity
return 1 - t;
}
shader irridescence
[[ string description = "Thin film coating shader. Use as reflection color for the material, with Fresnel for the material OFF (this texture computes its own Fresnel)" ]]
(
float thicknessMin = 250 [[ string description = "Minimum thickness of the film, in nm" ]],
float thicknessMax = 400 [[ string description = "Maximum thickness of the film, in nm" ]],
string thickness = "test.png" [[ string description = "Texture map (usually noise) that determines the thickness of the film as fraction between the min and max" ]],
float nmedium = 1 [[ string description = "Refractive index of the outer medium (typically air)" ]], // approximate refractive index of air
float nfilm = 1.5 [[ string description = "Refractive index of the thin film itself" ]] , // approximate refractive index of water
float ninternal = 1 [[ string description = "Refractive index of the material below the film" ]], // approximate refractive index of the lower material
output color result = 0
)
{
float cos0 = abs(dot(I , N));
color thickTex=texture(thickness, u, v);
float t=(thickTex[0]+thickTex[1]+thickTex[2])/3.0;
float thick=thicknessMin*(1.0-t)+thicknessMax*t;
float red=thinFilmReflectance(cos0, 650, thick, nmedium, nfilm, ninternal);
float green=thinFilmReflectance(cos0, 510, thick, nmedium, nfilm, ninternal);
float blue=thinFilmReflectance(cos0, 475, thick, nmedium, nfilm, ninternal);
result=color(red, green, blue);
}