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From: Stephen
Subject: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 14:25:04
Message: <52d2a5c0$1@news.povray.org>
I thought that negative light was not allowed but I see that it is.
So If you have not tried it. Here is the Stamford Bunny with a point 
light a normal Spotlight and a negative spotlight.

http://www.youtube.com/watch?v=vOnjFer_JKw&feature=youtu.be


-- 
Regards
     Stephen


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From: MichaelJF
Subject: Re: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 15:20:00
Message: <web.52d2b1fa67f081bbadcca6640@news.povray.org>
Strange but interesting. It seems that the shadow from the positive light is not
affected by the negative light. Must try negative emission some day...

Best regards,
Michael


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From: Stephen
Subject: Re: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 16:55:24
Message: <52d2c8fc$1@news.povray.org>
On 12/01/2014 3:17 PM, MichaelJF wrote:
> Strange but interesting. It seems that the shadow from the positive light is not
> affected by the negative light. Must try negative emission some day...
>

Yes interesting I think that it might be to do with the shadow test.
It does not work in a light group, either.


Here is an image with both spotlights at the same locations.
http://i.imgur.com/CAhef3D.jpg


You can get it to suck different proportions of rgb which gives effects.

A shadow colour of: rgb <0.10000,0.75,0.75> * -13.5

http://i.imgur.com/EcS8hdn.jpg


-- 
Regards
     Stephen


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From: MichaelJF
Subject: Re: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 17:55:00
Message: <web.52d2d6db67f081bbadcca6640@news.povray.org>
Stephen <mca### [at] aolcom> wrote:
> On 12/01/2014 3:17 PM, MichaelJF wrote:
> > Strange but interesting. It seems that the shadow from the positive light is not
> > affected by the negative light. Must try negative emission some day...
> >
>
> Yes interesting I think that it might be to do with the shadow test.
> It does not work in a light group, either.
>
>
> Here is an image with both spotlights at the same locations.
> http://i.imgur.com/CAhef3D.jpg
>
>
> You can get it to suck different proportions of rgb which gives effects.
>
> A shadow colour of: rgb <0.10000,0.75,0.75> * -13.5
>
> http://i.imgur.com/EcS8hdn.jpg
>
>
> --
> Regards
>      Stephen

I'm not quite sure what we have here. Negative light is a feature which can be
used artistically, but then the shadows must be affected too. In your additional
scenes the shadows of the positive light still seems to be not influenced by the
negative light. Most likely we're hunting down a bug here. My opinion is that
negative light gives more artistic freedom but then it must be implemented
correctly. For photorealistic renderings it is not necessary. May be Christoph
Lipka can shed more light onto this issue.

Best regards,
Michael


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From: clipka
Subject: Re: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 18:04:06
Message: <52d2d916$1@news.povray.org>
Am 12.01.2014 17:55, schrieb Stephen:

> You can get it to suck different proportions of rgb which gives effects.

Fun fact to know in this context: The math behind the SSLT feature is 
also derived from some "negative light source" model.


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From: clipka
Subject: Re: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 18:26:00
Message: <52d2de38$1@news.povray.org>
Am 12.01.2014 18:54, schrieb MichaelJF:

> I'm not quite sure what we have here. Negative light is a feature which can be
> used artistically, but then the shadows must be affected too. In your additional
> scenes the shadows of the positive light still seems to be not influenced by the
> negative light. Most likely we're hunting down a bug here. My opinion is that
> negative light gives more artistic freedom but then it must be implemented
> correctly. For photorealistic renderings it is not necessary. May be Christoph
> Lipka can shed more light onto this issue.

(I see what you did there :-))

Note that where there is "negative light", there is also "negative 
shadow" - but also note that shadow is nothing but the absence of any 
(in this case negative) light.

The scenes you have posted so far all have the regular shadow lie 
completely inside the "negative shadow", so how should the regular 
shadow be affected by the "negative light"?


Speaking of necessity for photorealistic renderings, negative light 
would play a role if you wanted to simulate light of highly saturated 
colour, such as from LEDs or mercury vapor lights. Such colours are 
outside of the sRGB gamut (or any other RGB gamut, for that matter), and 
would therefore have to be modeled with one or two colour components 
being negative (unless you have spectral rendering available, that is).

I suspect that there are some places within POV-Ray that can't properly 
handle negative colour components though.


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From: clipka
Subject: Re: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 18:27:43
Message: <52d2de9f@news.povray.org>
Am 12.01.2014 15:25, schrieb Stephen:
> I thought that negative light was not allowed but I see that it is.
> So If you have not tried it. Here is the Stamford Bunny with a point
> light a normal Spotlight and a negative spotlight.
>
> http://www.youtube.com/watch?v=vOnjFer_JKw&feature=youtu.be

BTW, it's "Stanford Bunny" (as in "Stanford University"), not "Stamford 
Bunny".


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From: Stephen
Subject: Re: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 19:19:59
Message: <52d2eadf$1@news.povray.org>
On 12/01/2014 6:27 PM, clipka wrote:
> Am 12.01.2014 15:25, schrieb Stephen:
>> I thought that negative light was not allowed but I see that it is.
>> So If you have not tried it. Here is the Stamford Bunny with a point
>> light a normal Spotlight and a negative spotlight.
>>
>> http://www.youtube.com/watch?v=vOnjFer_JKw&feature=youtu.be
>
> BTW, it's "Stanford Bunny" (as in "Stanford University"), not "Stamford
> Bunny".
>

With my accent, there's not much difference.

-- 
Regards
     Stephen


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From: Stephen
Subject: Re: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 20:12:35
Message: <52d2f733$1@news.povray.org>
On 12/01/2014 6:25 PM, clipka wrote:
> Am 12.01.2014 18:54, schrieb MichaelJF:
>
>> I'm not quite sure what we have here. Negative light is a feature
>> which can be
>> used artistically, but then the shadows must be affected too. In your
>> additional
>> scenes the shadows of the positive light still seems to be not
>> influenced by the
>> negative light. Most likely we're hunting down a bug here. My opinion
>> is that
>> negative light gives more artistic freedom but then it must be
>> implemented
>> correctly. For photorealistic renderings it is not necessary. May be
>> Christoph
>> Lipka can shed more light onto this issue.
>
> (I see what you did there :-))
>
> Note that where there is "negative light", there is also "negative
> shadow" - but also note that shadow is nothing but the absence of any
> (in this case negative) light.
>
> The scenes you have posted so far all have the regular shadow lie
> completely inside the "negative shadow", so how should the regular
> shadow be affected by the "negative light"?
>
>

It wasn't Michael that posted the images.
I think of the "negative light" spotlight as being a Souper Dooper Light 
Sooker.

Two further images with the SDLS falling across the true spotlights shadow.
One is intense dark the other has the red component, zero.


> Speaking of necessity for photorealistic renderings, negative light
> would play a role if you wanted to simulate light of highly saturated
> colour, such as from LEDs or mercury vapor lights. Such colours are
> outside of the sRGB gamut (or any other RGB gamut, for that matter), and
> would therefore have to be modeled with one or two colour components
> being negative (unless you have spectral rendering available, that is).
>

I've never been accused of photorealistic renderings. ;-)

> I suspect that there are some places within POV-Ray that can't properly
> handle negative colour components though.
>

The animation I posted just before this one, Needs a flash of reflection 
(?) running down the ribs. I used two concurrent cylinder lights, with 
one of them using "negative light", to make an annular light. It worked 
okay but when I tried to put it in a light group. It just behaved as it 
wasn't in one. :-(

-- 
Regards
     Stephen


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From: Stephen
Subject: Re: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 20:14:50
Message: <52d2f7ba$1@news.povray.org>
On 12/01/2014 6:04 PM, clipka wrote:
> Am 12.01.2014 17:55, schrieb Stephen:
>
>> You can get it to suck different proportions of rgb which gives effects.
>
> Fun fact to know in this context: The math behind the SSLT feature is
> also derived from some "negative light source" model.
>

On my todo list (longer than Thomas's): Try "negative light" on SSLT.

-- 
Regards
     Stephen


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