Ambient()

[B.Walters]

As a wonderful Christmas present, I received the Advanced Renderman book and I've been plowing through it full-speed. Now that I'm back at home/work, I've been trying to actually implement a lot of the shaders in VEX (mostly from memory to see if I really understand them). But I hit a problem-spot early on with the ambient function.

Here's what I've tried:

vector my_ambient()

{

vector C = 0;

illuminance(P, N, PI*2, LIGHT_AMBIENT)

{

C += Cl;

}

return C;

}

The problem is my_ambient always returns 0 - no lights are getting passed through the luminance loop. If I change the argument LIGHT_AMBIENT to LIGHT_AMBIENT|LIGHT_DIFFUSE|LIGHT_SPECULAR - then it will loop through any other normal lights I have in the scene (and sum up their values), but still not the ambient light.

So I know it's something small/simple, but I can't figure it out. By the way, is there anyway to see what Houdini's built-in ambient() looks like?

[Mario Marengo]

An ambient lights is a different object type than a normal light and, as in PRMan, the only way to access its contribution is through the built-in function "ambient()". So your my_ambient() function would simply forward the call to the built-in one (not very useful except as an exercise):

vector my_ambient() {
   return ambient();
}

IOW: ambient lights are not iterated over in an illuminance loop. Instead, their contribution is fetched using the ambient() function.

However, this doesn't mean that you can't have a normal light that behaves like an ambient light and that will show up in normal illuminance loops. The following *light* shader is an example:

light my_ambient(
	  vector lightcolor = 1;
	  float  intensity  = 1;
   ) 
{
   Cl = lightcolor*intensity;
   L  = 0;
}

If you assign this last shader to a normal light, it will show up in an illuminance loop (due to the fact that it is a light object as opposed to an ambient light object), and it will emulate the behaviour of a standard ambient light (in terms of shading).

HTH.

P.S: Happy New Year Everyone!

[B.Walters]

Thanks for the quick reply Mario

[crunch]

As a wonderful Christmas present, I received the Advanced Renderman book and I've been plowing through it full-speed. Now that I'm back at home/work, I've been trying to actually implement a lot of the shaders in VEX (mostly from memory to see if I really understand them). But I hit a problem-spot early on with the ambient function.

Here's what I've tried:

The problem is my_ambient always returns 0 - no lights are getting passed through the luminance loop. If I change the argument LIGHT_AMBIENT to LIGHT_AMBIENT|LIGHT_DIFFUSE|LIGHT_SPECULAR - then it will loop through any other normal lights I have in the scene (and sum up their values), but still not the ambient light.

So I know it's something small/simple, but I can't figure it out. By the way, is there anyway to see what Houdini's built-in ambient() looks like?

Actually, I think this was a bug in H8. I think it works in H9 (from what a little bird tells me).

[Mario Marengo]

Actually, I think this was a bug in H8. I think it works in H9 (from what a little bird tells me).

Hehe... well... I'd go ahead and trust that little bird, B.Walters...