Re: mR shading network utility node descriptions?

["Tim Leydecker" <BauerOink(at)gmx.de>]

Hi Kim and Daniel,

thanks for your time, I didn´t find the time to reply earlier
and wrap my head around your tips and test them today.

I´ll do so tommorrow,

thanks again,

Cheers,

tim



----- Original Message ----- 
From: "Daniel Rind" <daniel.rind(at)chello.at>
To: <XSI(at)Softimage.COM>
Sent: Friday, January 19, 2007 4:12 PM
Subject: Re: mR shading network utility node descriptions?


Hi!

Maybe I can help you a bit.

The easy one first:
Here is what the mental ray documentation has to say about the U and V sampling 
parameters for area light sources:

--- 8< ---
The u_samples and v_samples parameters subdivide the area light source primitive. For 
discs and spheres, u_samples subdivides the radius and v_samples subdivides the 
angle. For a cylinder, u_samples subdivides the height and v_samples subdivides the 
angle. When sampling the area light source, mental ray samples one point in each 
subdivision at a location precisely determined by the sample parameters and a 
predefined lighting distribution, and then combines the results. The default is 3 for 
each sample parameter, so an area light source without explicitly given samples 
parameters is sampled 9 times.

--- 8< ---

So, that's that.

If you want to get full control over the illumination model of a material, I'm afraid 
you'll have to write your own shader. There's only so much you can do with incidence, 
gradients and a Flat_light node, and it usually starts to break apart when you have 
lighting situations that consist of more than a single light source.

Generally, for getting scene information into the render tree, you can just put 
expressions on shader parameters. For example, to get the position of a light into 
the render tree, just drop a Scalars2vector node into your rendertree, and link the 
light's global X, Y and Z translation to the three inputs. If you now want the 
distance of a shaded surface point to your light, use a Vector_state node set to 
"Intersection point", and stuff the two nodes into a Vector_math_scalar set to 
"Distance between V1 and V2".

I'm not sure what you mean with language-barrier.

Ciao, Daniel!
 ----- Original Message ----- 
 From: Tim Leydecker
 To: XSI(at)Softimage.COM
 Sent: Friday, January 19, 2007 3:05 PM
 Subject: Re: mR shading network utility node descriptions?


 Hi Bernard,

 thanks for pointing me into the general direction.

 What I´d like to achieve is mostly two things,
 angle based material attributes with custom
 response curves per RGB channel (e.g. an
 angle based texture look-up or gradientmapping)
 and more control over lighting, especially the shape,
 intensity and colorattribute linked to position and
 distance to the source.

 I run into the most basic problems due to the
 language barrier and GUI, like for arealights,
 not understanding the orientation of the U/V
 sampling, is it U>Yscale and V>Xscale or vice versa?

 I know the incidence node, checked the architectural
 material´s BRDF curve input attribute and toy with
 ways to map intensities, values and depths in comp.

 I´d guess those nodes would bring me closer to
 getting the (fast-rendering) utility passes that
 could prove handy for setting and finetuning
 a lightingsituation in post, containing DOF,
 godrays and lights in custom shapes and shadow...

 Cheers

 tim



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