Displacement with RenderMan

[hulud]

Hi,

calculatenormal(P) returns a 'flat' normal if you use it on polygons. (For the details, calculatenormal(P) = dPdu^dPdv, which is constant on polygons)

You can do something like that :

normal nN = normalize (N);
point newP = P + nN * height;
normal newN = normalize (calculatenormal(newP)) + (nN - normalize (calculatenormal(P)));

In a nutshell, you compute the new "flat normal" and you apply the delta between the old normal and the old "flat normal".

Cyril

[magost]

Thanks for the answer, it helped me a lot ;)

On the code you write what it's supposed to do with the "newN" variable? Assign it to "N"?
And apart from calculating "newP", must you calculate "P"? I thought a new value of P (newP) was necessary only if you want to use bump instead of displacement. I want to use displacement so, is it necessary to calculate newP or should i assign only the new value to P?

I have written this following your advice (i think it works):
P = P + nN * height;
point newP = P + nN * height;
normal newN = normalize (calculatenormal(newP)) + (nN - normalize (calculatenormal(P)));
N = newN;

It's that code ok? It's the same of that one, isn't it?
P = P + nN * height;
N = normalize (calculatenormal(P)) + (nN - normalize (calculatenormal(P)));


Thanks for the help.

[noizFACTORY]

I thought a new value of P (newP) was necessary only if you want to use bump instead of displacement.

Its the other way around. You need the new position P for displacing the geometry but you only need calculatenormal(newP) for bump where you are not necessarily displacing any geo but only using the new normals to fake the effect through bump.

[hulud]

And apart from calculating "newP", must you calculate "P"?
Yes, as noizFACTORY said, you have to change P if you want displacement.
Sorry I forgot it in my exemple, it should be something like :

normal nN = normalize (N);
point newP = P + nN * height;
N = normalize (calculatenormal(newP)) + (nN - normalize (calculatenormal(P)));
P = newP;

Cyril

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