hughes, b. <omn### [at] charternet> wrote:
> In the context I meant it would just be anti-light for POV scenes, light
> subtraction.

  Well, regular light sources cause 0 lighting to shadows. What is the
negative of 0?
  That's right: 0. :)

-- 
#macro N(D)#if(D>99)cylinder{M()#local D=div(D,104);M().5,2pigment{rgb M()}}
N(D)#end#end#macro M()<mod(D,13)-6mod(div(D,13)8)-3,10>#end blob{
N(11117333955)N(4254934330)N(3900569407)N(7382340)N(3358)N(970)}//  - Warp -

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hughes, b. <omn### [at] charternet> wrote:
> I'd say a logical anti-light would illuminate the shadow regions, behaving
> exactly opposite of a regular light.

  A logical anti-light would not work that way. A logical anti-light
substracts light from its surroundings (light which is casted by regular
light sources). That is, they act as negated light sources.
  Regular light sources have no effect whatsoever on shadows. Negating
the effect of doing nothing is still doing nothing (it's the same in
math and in logic: -0 = 0; if you haven't done anything, there's nothing to
undo, so you still do nothing).

  Thus, a POV-Ray light source with negative color works very well as
a logical anti-light.

-- 
#macro N(D)#if(D>99)cylinder{M()#local D=div(D,104);M().5,2pigment{rgb M()}}
N(D)#end#end#macro M()<mod(D,13)-6mod(div(D,13)8)-3,10>#end blob{
N(11117333955)N(4254934330)N(3900569407)N(7382340)N(3358)N(970)}//  - Warp -

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"Warp" <war### [at] tagpovrayorg> wrote in message
news:3dac428c$1@news.povray.org...
> hughes, b. <omn### [at] charternet> wrote:
> > I'd say a logical anti-light would illuminate the shadow regions,
behaving
> > exactly opposite of a regular light.
>
>   Regular light sources have no effect whatsoever on shadows. Negating
> the effect of doing nothing is still doing nothing

I can agree with that, yes. But I have to look differently at the way it
works to get your point about that. I was thinking of a light source as
being the controlling factor of shadows, i.e. have light, have shadow. If
ignoring the existence of radiosity anyhow.

So in that way I was thinking of it as shadows being areas non-illuminated
by regular (positive color) light and illuminated by negative color light.
Although that idea is actually different than what I really expect POV
should do anyhow, I was only wondering what such a thing might be like were
negative lights worked into the program source code.  Affecting shadows in
that way could be interesting, and I'd guess potentially problematic to work
with transparency, to name one of many things.

Anyway, I can see now that negative values for lights do what they should.
That being, subtracting positive light and not affecting shadows (where no
light appears).

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Christopher James Huff wrote:

> In article <3dac17d4$1@news.povray.org>,
>  "hughes, b." <omn### [at] charternet> wrote:
> 
>> I guess if negative lights were to do as expected of such a thing,
>> though, then they'd actually illuminate shadows, or be a anti-shadow.
> 
> Why? Ordinary lights don't darken shadows, they just fail to illuminate
> them.

This brings Terry Pratchett to my mind. Normal darkness is just the absence 
of light, but Discworld Darkness is negated light :D

OK, there are at least two ways of thinking about negative lights. If normal 
lights lighten illuminated zones and don't affect shadows, negative lights 
could:

1.- Darken "illuminated" zones, don't touch shadows. (This is the most 
logical approach)
2.- Lighten shadows, don't touch illuminated zones. (It has its logic too, 
don't you think so?)

Is there a way to achieve the second way in POV-Ray? If there isn't (and if 
there's any chance of implementing it someday), I suggest calling it 
"inverted" light.

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> 2.- Lighten shadows, don't touch illuminated zones. (It has its logic too,
> don't you think so?)
>
> Is there a way to achieve the second way in POV-Ray?

Perhaps a shadowless light projected_through the union of the entire scene,
but then it'd have the side effects of shadowless lights (no specular
reflection, a few other things).

 - Slime
[ http://www.slimeland.com/ ]

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Jellby wrote:

> Christopher James Huff wrote:
> 
> 
>>In article <3dac17d4$1@news.povray.org>,
>> "hughes, b." <omn### [at] charternet> wrote:
>>
>>
>>>I guess if negative lights were to do as expected of such a thing,
>>>though, then they'd actually illuminate shadows, or be a anti-shadow.
>>>
>>Why? Ordinary lights don't darken shadows, they just fail to illuminate
>>them.
>>
> 
> This brings Terry Pratchett to my mind. Normal darkness is just the absence 
> of light, but Discworld Darkness is negated light :D
> 
> OK, there are at least two ways of thinking about negative lights. If normal 
> lights lighten illuminated zones and don't affect shadows, negative lights 
> could:
> 
> 1.- Darken "illuminated" zones, don't touch shadows. (This is the most 
> logical approach)
> 2.- Lighten shadows, don't touch illuminated zones. (It has its logic too, 
> don't you think so?)
> 
> Is there a way to achieve the second way in POV-Ray? If there isn't (and if 
> there's any chance of implementing it someday), I suggest calling it 
> "inverted" light.
> 


You are thinking about 'positive' and 'negative' light source as the 
only way to touch the image.
You are forgetting 'Ambient' settings.

For:
  1. Do not touch ambient, add negative light
  2. add negative light and compensate the negative light with a raise 
of ambient light. It might be tricky if many textures have different 
ambient ratios.

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> 2.- Lighten shadows, don't touch illuminated zones. (It has its logic too,
> don't you think so?)

You can already do this with the 'projected_through' option for lights.

-tgq

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Le Forgeron wrote:

> [...]
>
>> OK, there are at least two ways of thinking about negative lights. If
>> normal lights lighten illuminated zones and don't affect shadows,
>> negative lights could:
>> 
>> 1.- Darken "illuminated" zones, don't touch shadows. (This is the most
>> logical approach)
>> 2.- Lighten shadows, don't touch illuminated zones. (It has its logic
>> too, don't you think so?)
>>
>> [...]
> 
> You are thinking about 'positive' and 'negative' light source as the
> only way to touch the image.
> You are forgetting 'Ambient' settings.
> 
> For:
>   1. Do not touch ambient, add negative light
>   2. add negative light and compensate the negative light with a raise
> of ambient light. It might be tricky if many textures have different
> ambient ratios.

The problem with ambient is that it's object-based, and not 
light-source-based.
I like the 'projected_through' option better (I had forgotten about it). I 
don't know if it would be really practical, but that's the idea, I think.

Anyway, I was just brainstorming, I'm not planing on using these 'inverted' 
lights or anything, it's just that the idea came to my mind.

Jellby

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I've just tried using light groups and negative rgb values for the lights,
and it does work. The problem with what I was trying, was that I didn't
realise at the time was that the dark spots (from the texture used) would
obscure parts of the lower sphere.

A better soloution would be "transparency lighting". By using the
appropriate keyword in the pigment{} code, the texture becomes more
transparent with more light. Now this would be best for creating nightside
lights.

sphere{<0,0,0> 2
  pigment{
    image_map{
      png "nightlights.png"
      map_type 1 // I think that's spherical mapping
      }
    transparent_light transmit 0,1
    // or use this for filter transparency
    // transparent_light filter 0,1
    }
  }

"transparent_light" - keyword for using transparent light

transmit/filter - transparency method to use

0,1 - transparency for no light, full brighness (e.g. 0.5,0.9 gives 50%
transparency in full darkness, and 90% transparency in full light)

This could be handy for creating special effects. In the case of the
nightside lights, they would fade in over the terminator, and be full
brighness on the dark side of the Earth. I think this is what you would see
looking at the terminator of Earth at night, the lights getting brighter as
it gets darker, as more people start turning on their lights.

Rohan _e_ii

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"Rohan Bernett" <rox### [at] yahoocom> wrote in message
news:web.3db4b7d5ee9636d518ccf4f70@news.povray.org...
> I've just tried using light groups and negative rgb values for the lights,
> and it does work. The problem with what I was trying, was that I didn't
> realise at the time was that the dark spots (from the texture used) would
> obscure parts of the lower sphere.

Got to use a widespread transparency if the image doesn't have sharp
boundaries of color.

> "transparent_light" - keyword for using transparent light
>
> transmit/filter - transparency method to use
>
> 0,1 - transparency for no light, full brighness (e.g. 0.5,0.9 gives 50%
> transparency in full darkness, and 90% transparency in full light)

Interesting idea, yeah. Same thing for shadows might also be useful. At
least I can't think of any reasons for not having such a feature, at the
moment anyway.

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