I have some questions about light groups.

In this scene I have a white sphere iluminated by red light, both forming a
light group. I also have a white light iluminating the global scene. The
plane is correctly unnaffected by the red light: the sphere casts no shadow
resulting from this red light. Now here is what seems odd to me. If the
white light is, in this case, the global light, and I have the command
global_lights set to off, how come the white light still interacts with the
sphere, causing it to cast a shadow on the plane? Is this really the
intended behaviour? If so, is there a reason for it being so?

Ruy

========================

Here is the code:

#declare RedLight = light_source {
  <-5,5,-5>
  rgb <1,0,0>
}

camera{
  location <0, 5, -10>
  look_at 0
}

light_source { y*100 rgb 1 }

light_group {
  light_source {RedLight}
  sphere {0,1 pigment {rgb 1}}
  global_lights on
}

plane { y, -2 pigment { rgb 1 } }

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Attachments:
Download 'light group test.jpg' (8 KB)

Preview of image 'light group test.jpg'

> How come the white light still interacts with the
> sphere, causing it to cast a shadow on the plane? Is this really the
> intended behaviour? If so, is there a reason for it being so?

I'm not sure why it does it, but using no_shadow in your sphere {} block
fixes it.

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> I'm not sure why it does it, but using no_shadow in your sphere {} block
> fixes it.

Yes, I realize this, but what I'm trying to achieve here is control over the
shadows. For my real scene, I have set three different light sources, but I
only wanted some of them to cast shadows. I thought I could achieve this
with light groups, but it doesn't seem so. If I use no_shadow, all my
shadows are gone, and I need some of them.

On the example I gave, is there a way to iluminate the ground plane and
still avoid the shadow that the white light currently forces the sphere to
cast over it?

Ruy

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In article <3bee69bf@news.povray.org>, ruy### [at] hipernetcombr says...

> global_lights set to off, how come the white light still interacts with the
> sphere, causing it to cast a shadow on the plane? 

Even worse: a global light will cast a shadow from an object A in 
lightgroup X on an object B in lightgroup Y, but another object C in 
lightgroup X  wich is directly behind B (from the view of A), will not be 
shadowed by A, neither by the global light, nor by a light_source in 
light_group X, wich is in the same location as the global light. 
"global_lights off" is specified for X. 

Irritating, isn't it ;-)

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> On the example I gave, is there a way to iluminate the ground plane and
> still avoid the shadow that the white light currently forces the sphere to
> cast over it?

Make the light shadowless?

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> Make the light shadowless?

Yup, it'll have to do. Truth is I wanted some of the objects to cast a
shadow for that particular light, while some I would like to remain
shadowless for the same light, but not for others. *sigh*

Gracias, anyway :o)

Ruy

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You might have to have several lights repeated in the same spot, but in
different light groups and with shadowless on and off. If you could
illustrate the effect, I could try to figure something out for you.

PS: De nada. :)

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