MERL BRDF Database

[danylyon]

Hi.

Thanks for the links. I've triied brdflab some time ago and couldn't get it to work (or didn't try hard enough). I found it more convenient to have the BRDF directly inside Houdini, so I could compare to the BSDF's that were built into Houdini. BSDF's are often quite implementation dependent. (I've seen so many different cooks :-)

fityk looks really interesting.. I've been doing a lot of "Curve" fitting manually recently. Will have to try it!

[ndeboar]

Hey Peon,

 

Thanks for this.

 

How do I view the BSDF from a Mantra PBR shader to compare to the meassured merl data? Is there a way to plot F comeing out of the shader?

[danylyon]

Hi Nick

 

Yes, you can do that with "eval_bsdf": http://www.sidefx.com/docs/houdini13.0/vex/functions/eval_bsdf

 

This also works in geometry (SOP) context. So prepare your bsdf, go through eval_bsdf to get a specific value.

[ndeboar]

Ah, I love Houdini. Thanks.

[ndeboar]

What does eval_bsdf return? It says a vector in the docs, what does that represent?

[danylyon]

Create a VEX surface node. Copy paste this code:

Feed in a (poly) sphere.

#pragma range ex_nu 0 1000
#pragma range ex_nv 0 1000
#pragma choice BSDFfunc 0 "Phonglobe"
#pragma choice BSDFfunc 1 "Ashikhmin"
#pragma choice BSDFfunc 2 "Blinn"
#pragma choice BSDFfunc 3 "Diffuse"

#include "voplib.h"
#include "voptype.h"

sop prebuilt_BSDFs(float lightangle = 0, ex_nu = 100, ex_nv = 100, SpecScale = 1, BSDFfunc = 0;)
{

    float lr = radians(lightangle);
    vector L = {0,0,0};
    L.x = -sin(lr);
    L.y = cos(lr);               
    L = normalize(L);                       // vector to light

    vector sP = {0,0,0};                    // Point to be shaded
    vector Nn = {0,1,0};                    // Surface Normal
    vector V = normalize(P);                // vector to eye / view vector

    float NdotV = dot(V,Nn);
    vector R = 2 * NdotV * Nn - V;             // reflection Vector

    bsdf SpecBSDF;

    vector X = {1,0,0};
    vector Y = {0,0,1};  // tangents

    if (BSDFfunc == 0) SpecBSDF = phonglobe(Nn, R, ex_nu, ex_nv, X, Y);
    if (BSDFfunc == 1) SpecBSDF = ashikhmin(Nn, ex_nu, ex_nv, X, Y);
    if (BSDFfunc == 2) SpecBSDF = blinn(Nn, ex_nu);
    if (BSDFfunc == 3) SpecBSDF = diffuse(Nn,1, "label", "");

    vector S = eval_bsdf(SpecBSDF, V, L);

    P = P * length(S) * SpecScale;
}

[ndeboar]

I never use tacky internet speak, but OMG, that's awesome.