POV-Ray : Newsgroups : povray.newusers : How do I stop background colour colouring everything in my scene Server Time: 17 Jun 2019 12:35:58 GMT
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From: omniverse
Subject: Re: How do I stop background colour colouring everything in my scene
Date: 25 Feb 2019 14:05:00
Message: <web.5c73f60162b6255c9c5d6c810@news.povray.org>
"cloudyroo" <nomail@nomail> wrote:
> If I add a blue background to a scene with a grey floor and white sphere I end
> up with a pretty blue everything (like so: https://imgur.com/a/w9dF3Q1).
> Is there a way to stop this or another way to colour only the sky/background?
> Thanks!

You seem to be using radiosity; if so, try adding the keyword gray_threshold (or
grey_threshold) to that part of your scene and use a value greater than the
default of 0.
It will diminish color transference by that amount as a percentage, 0.5 being
50%.


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From: cloudyroo
Subject: Re: How do I stop background colour colouring everything in my scene
Date: 25 Feb 2019 15:05:00
Message: <web.5c74035162b6255cfdc0473a0@news.povray.org>
"omniverse" <omn### [at] charternet> wrote:
> "cloudyroo" <nomail@nomail> wrote:
> > If I add a blue background to a scene with a grey floor and white sphere I end
> > up with a pretty blue everything (like so: https://imgur.com/a/w9dF3Q1).
> > Is there a way to stop this or another way to colour only the sky/background?
> > Thanks!
>
> You seem to be using radiosity; if so, try adding the keyword gray_threshold (or
> grey_threshold) to that part of your scene and use a value greater than the
> default of 0.
> It will diminish color transference by that amount as a percentage, 0.5 being
> 50%.

Won't that lessen transference of colours from everything?
The description of background says "Any ray that does not hit an object will be
colored with this color". I'd thought that would mean it's solely a background
color and doesn't interact with the scene.
Is there no way to achieve that effect without changing the way objects in the
scene interact with eachother?


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From: jr
Subject: Re: How do I stop background colour colouring everything in my scene
Date: 25 Feb 2019 17:55:01
Message: <web.5c742b2262b6255cbcd610c90@news.povray.org>
hi,

"cloudyroo" <nomail@nomail> wrote:
> If I add a blue background to a scene with a grey floor and white sphere I end
> up with a pretty blue everything (like so: https://imgur.com/a/w9dF3Q1).
> Is there a way to stop this or another way to colour only the sky/background?

would adding a black/dark "sky_sphere" help?
http://wiki.povray.org/content/Documentation:Tutorial_Section_3.5#The_Sky_Sphere


regards, jr.


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From: Alain
Subject: Re: How do I stop background colour colouring everything in my scene
Date: 25 Feb 2019 19:02:39
Message: <5c743bcf$1@news.povray.org>
Le 19-02-25 à 10:01, cloudyroo a écrit :
> "omniverse" <omn### [at] charternet> wrote:
>> "cloudyroo" <nomail@nomail> wrote:
>>> If I add a blue background to a scene with a grey floor and white sphere I end
>>> up with a pretty blue everything (like so: https://imgur.com/a/w9dF3Q1).
>>> Is there a way to stop this or another way to colour only the sky/background?
>>> Thanks!
>>
>> You seem to be using radiosity; if so, try adding the keyword gray_threshold (or
>> grey_threshold) to that part of your scene and use a value greater than the
>> default of 0.
>> It will diminish color transference by that amount as a percentage, 0.5 being
>> 50%.
> 
> Won't that lessen transference of colours from everything?
> The description of background says "Any ray that does not hit an object will be
> colored with this color". I'd thought that would mean it's solely a background
> color and doesn't interact with the scene.
> Is there no way to achieve that effect without changing the way objects in the
> scene interact with eachother?
> 
> 

You may add a large sphere around your scene as follow :

sphere{0 e5 // Very large
    pigment{rgb 0.5} // Tweak to your taste
    finish{diffuse 0 ambient 0 emission 1}
  // Won't be affected by lights nor radiosity
    no_image // So that you don't see it directly
    no_reflection // So that it don't appears in reflections
    no_shadow // Just in case you have any light outside the sphere
    hollow // to suppress a warning and allow the use of fog and medias
  }



Alain


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From: Alain
Subject: Re: How do I stop background colour colouring everything in my scene
Date: 25 Feb 2019 19:04:46
Message: <5c743c4e$1@news.povray.org>
Le 19-02-25 à 12:51, jr a écrit :
> hi,
> 
> "cloudyroo" <nomail@nomail> wrote:
>> If I add a blue background to a scene with a grey floor and white sphere I end
>> up with a pretty blue everything (like so: https://imgur.com/a/w9dF3Q1).
>> Is there a way to stop this or another way to colour only the sky/background?
> 
> would adding a black/dark "sky_sphere" help?
> http://wiki.povray.org/content/Documentation:Tutorial_Section_3.5#The_Sky_Sphere
> 
> 
> regards, jr.
> 
> 

No. The sky_sphere is a background feature the same as background. It 
will replace the background anywhere it's not transparent in a way 
similar to a layered texture.


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From: clipka
Subject: Re: How do I stop background colour colouring everything in my scene
Date: 25 Feb 2019 19:47:29
Message: <5c744651$1@news.povray.org>
Am 25.02.2019 um 12:00 schrieb cloudyroo:
> If I add a blue background to a scene with a grey floor and white sphere I end
> up with a pretty blue everything (like so: https://imgur.com/a/w9dF3Q1).
> Is there a way to stop this or another way to colour only the sky/background?
> Thanks!

Well, it _is_ realistic.

In a scene with a blue sky, you'd usually compensate for this by using a 
yellow light source to simulate the sun.


If you don't want realism, use a huge blue sphere instead of a blue 
background, and use "no_radiosity" on the sphere.


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From: clipka
Subject: Re: How do I stop background colour colouring everything in my scene
Date: 25 Feb 2019 19:53:43
Message: <5c7447c7$1@news.povray.org>
Am 25.02.2019 um 15:04 schrieb omniverse:
> "cloudyroo" <nomail@nomail> wrote:
>> If I add a blue background to a scene with a grey floor and white sphere I end
>> up with a pretty blue everything (like so: https://imgur.com/a/w9dF3Q1).
>> Is there a way to stop this or another way to colour only the sky/background?
>> Thanks!
> 
> You seem to be using radiosity; if so, try adding the keyword gray_threshold (or
> grey_threshold) to that part of your scene and use a value greater than the
> default of 0.
> It will diminish color transference by that amount as a percentage, 0.5 being
> 50%.

I strongly recommend against using `gray_threshold`, as it has 
unrealistic and probably undesired side effects. Most notably, it will 
desaturate nooks and crannies more than open areas.


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From: clipka
Subject: Re: How do I stop background colour colouring everything in my scene
Date: 25 Feb 2019 19:56:41
Message: <5c744879$1@news.povray.org>
Am 25.02.2019 um 16:01 schrieb cloudyroo:

> The description of background says "Any ray that does not hit an object will be
> colored with this color". I'd thought that would mean it's solely a background
> color and doesn't interact with the scene.

"Any ray" also encompasses the radiosity sample secondary rays.


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From: jr
Subject: Re: How do I stop background colour colouring everything in my scene
Date: 25 Feb 2019 21:40:00
Message: <web.5c74603062b6255cbcd610c90@news.povray.org>
hi,

Alain <kua### [at] videotronca> wrote:
> Le 19-02-25 à 12:51, jr a écrit :
> > would adding a black/dark "sky_sphere" help?
>
> No.

ok.

> It will replace the background anywhere it's not transparent in a way
> similar to a layered texture.

I got a better understanding from that one sentence than from the whole
description in the docs.  :-)  thanks.


regards, jr.


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From: omniverse
Subject: Re: How do I stop background colour colouring everything in my scene
Date: 26 Feb 2019 05:35:00
Message: <web.5c74cf2462b6255c9c5d6c810@news.povray.org>
clipka <ano### [at] anonymousorg> wrote:
> Am 25.02.2019 um 15:04 schrieb omniverse:
> > "cloudyroo" <nomail@nomail> wrote:
> >> If I add a blue background to a scene with a grey floor and white sphere I end
> >> up with a pretty blue everything (like so: https://imgur.com/a/w9dF3Q1).
> >> Is there a way to stop this or another way to colour only the sky/background?
> >> Thanks!
> >
> > You seem to be using radiosity; if so, try adding the keyword gray_threshold (or
> > grey_threshold) to that part of your scene and use a value greater than the
> > default of 0.
> > It will diminish color transference by that amount as a percentage, 0.5 being
> > 50%.
>
> I strongly recommend against using `gray_threshold`, as it has
> unrealistic and probably undesired side effects. Most notably, it will
> desaturate nooks and crannies more than open areas.

Good point! I only thought about the blue color cast and not about all other
object colors. My fault for narrowing the question down too much.

Your no_radiosity suggestion makes me think it would be even better if there was
a way to apply a percentage instead of simply on/off. Although... with an object
for background, shouldn't (finish) diffuse and emission do something like that
anyway? Or does zeroing those matter to other scene objects?


I must give this a try just to see for myself what happens.


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