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From: clipka
Subject: Re: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 13:04:06
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 13:26:00
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 13:27:43
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 14:19:59
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 15:12:35
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 15:14:50
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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From: Stephen
Subject: Re: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 15:21:44
Message: <52d2f958$1@news.povray.org>
On 12/01/2014 5:54 PM, MichaelJF wrote:
> 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.

As Christoph explained. See next two images.

> 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.

I wonder how it would work with caustics and photons?

Other than surreal images. You might use it to reduce the brightness 
locally, in a scene.

> For photorealistic renderings it is not necessary. May be Christoph
> Lipka can shed more light onto this issue.

Nice one! :-)


-- 
Regards
     Stephen


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From: clipka
Subject: Re: Stamford Bunny in negative spotlight.
Date: 12 Jan 2014 19:05:55
Message: <52d32de3@news.povray.org>
Am 12.01.2014 21:12, schrieb Stephen:

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

Where?


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From: Stephen
Subject: Re: Stamford Bunny in negative spotlight.
Date: 13 Jan 2014 02:37:47
Message: <52d397cb$1@news.povray.org>
On 13/01/2014 12:05 AM, clipka wrote:
> Am 12.01.2014 21:12, schrieb Stephen:
>
>> Two further images with the SDLS falling across the true spotlights
>> shadow.
>> One is intense dark the other has the red component, zero.
>
> Where?
>

Oops! I thought that I had posted the links.
N.B. Must take more water with it. :-)

Red
http://i.imgur.com/NNPwJS8.jpg


Black
http://i.imgur.com/dCQiQLl.jpg


-- 
Regards
     Stephen


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From: Warp
Subject: Re: Stamford Bunny in negative spotlight.
Date: 13 Jan 2014 11:19:52
Message: <52d41228@news.povray.org>
MichaelJF <mi-### [at] t-onlinede> wrote:
> 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.

The shadows work in a completely logical manner: Inside the shadow
the rendering engine just says "this light is not visible from here,
ignore it". It doesn't care what color the light might be (even if
it's negative.)

If you want even more artistical effects, make some of the color
components of the light positive and others negative.

-- 
                                                          - Warp


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