Hi Morten,
It's to do with mental ray. Mental ray uses an optimization for
calling light shaders. It checks whether the normal is facing more
than 90 degrees from the light. If it is then it skips calling the
light shader because it's "not possible" for the light to be lighting
the surface. Unfortunately there is no way to switch this off for the
scene.
Though when writing your material shader you can work around it -
though it's a bit of messing around because you then need to do some
trickiness to take care of making shadows cast properly.
I tried to make an ambient light shader (that lit the diffuse evenly -
planning to add distance falloff to it) but because this feature
couldn't be disabled for a particular light (or even for the whole
scene) I could only get the ambient light to illuminate half an object
at the most.
Hope this helps,
Alan.
On 6/21/05, Morten Bartholdy <xsi(at)colorshopvfx.dk> wrote:
>
> Aloha,
>
>
> Been playing with shaders and combining stuff a lot lately so I thought it
> time to find out how to fix or work around this one - see this image:
>
> http://colorshopvfx.dk/XSI/anisotropic_edge.jpg
>
> Now I saw a technique the other day involving an incidence node to get rid
> of the hard edge in the specular on the border between diffuse and ambient,
> but it should hardly be a matter for techniques, rather something basic in
> the way the material shader handles speculars, no!?
>
> So if someone has a more direct approach to solve it I'd be happy to hear
> it.
>
>
> Best regards
>
> Morten Bartholdy
> 3D & VFX Artist
> cell +45 2169 54676
>
>
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