Hi.

This is probably a FAQ, but I can not manage to find its answer.

I am looking for light settings for a sunny outdoor scene: deep blue sky,
sharp shadows, but the parts in the shadow must still be visible.

My base scene has this:

light_source { <100000, 100000, -100000> rgb 2 }
sky_sphere {
  pigment { 
    gradient y
    color_map {
      [ 0.0 rgb <0.9, 0.9, 1> ]
      [ 0.5 rgb <0.3, 0.3, 1> ]
    }
  }
}

The best I could do was to set the global_settings ambient_light to 3 and
default radiosity, but I am not entirely satisfied, because the blue tint of
the sky is distinctly visible.

Can someone give me some advice, please?

Post a reply to this message

Nicolas George <nicolas$george@salle-s.org> wrote:
> The best I could do was to set the global_settings ambient_light to 3 and
> default radiosity, but I am not entirely satisfied, because the blue tint of
> the sky is distinctly visible.

  Well, if there isn't anything else to reflect than the blue sky, then of
course objects will get a blue tint. This is realistic. For example,
consider this image (rendered with povray):

http://warp.povusers.org/sponza.jpg

  Observe how the upper parts of the walls have a blue tint because of the
sky. I wouldn't call it unrealistic.

  If you want some other tint, you have to put some other objects around
the scene.

-- 
                                                          - Warp

Post a reply to this message

Warp  wrote in message <471c74f2@news.povray.org>:
>   Well, if there isn't anything else to reflect than the blue sky, then of
> course objects will get a blue tint.

I never noticed a blue tint in the sand of a beach, for example. At least,
not as much as I see on my scene.

Maybe the brain is compensating, but in that case, I would like advice on
how to emulate this compensation.

I wrote a minimal example to show the amount of difference I get. It is at
the end.

>					This is realistic. For example,
> consider this image (rendered with povray):
> 
> http://warp.povusers.org/sponza.jpg

This is really impressing. Before I noticed "rendered with povray", I
thought it was a photo illustrating the blue tint in nature-rendered images.

Is the source code available somewhere. I would really like to study it and
learn from it.

>   Observe how the upper parts of the walls have a blue tint because of the
> sky. I wouldn't call it unrealistic.

That is really realistic, but the blue tint you get is very faint, nothing
compared to what I am trying to get rid of.


Minimal example:

plane {
  y, 0
  pigment { rgb <0.804, 0.569, 0.494> }
  normal { bumps 0.5 }
}

camera {
  location <220, 50, 0> look_at <0, 0, 0>
}

sky_sphere {
  pigment {
    gradient y
    color_map {
      [ 0.0 rgb <0.9, 0.9, 1> ]
      [ 0.5 rgb <0.3, 0.3, 1> ]
    }
  }
}

light_source { <100000, 100000, -100000> rgb 3 }

global_settings {
  #if(1)
    ambient_light 3.4
  #else
    radiosity { }
  #end
}

Post a reply to this message

As Warp pointed out, a sky-blue tinge to shadows is perfectly normal, and
exactly what you see in real life on a sunny day with a clear sky.

However, you can adjust gray_threshold in the radiosity settings. This
specifies how much colour to include in the radiosity calculations (1 = no
colours, 0 = full colour radiosity). Obviously, if you get rid of the sky
colour you will also lose colours emitted from nearby objects, so you can't
have it both ways I'm afraid!

One more point - it's best to set all objects' ambient to 0, otherwise they
actively emit light in radiosity calculations. If you have the global
ambient=3 then your scene must be really, really bright :)

Bill

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"Bill Pragnell"  wrote in message
<web.471c85bad8eb8586731f01d10@news.povray.org>:
> However, you can adjust gray_threshold in the radiosity settings. This
> specifies how much colour to include in the radiosity calculations (1 = no
> colours, 0 = full colour radiosity).

Thanks, that does exactly what I was hoping.

>					Obviously, if you get rid of the sky
> colour you will also lose colours emitted from nearby objects, so you can't
> have it both ways I'm afraid!

That is ok for me in that particular scene.

As a more theoretical question, would it be possible, with the radiosity
algorithm, to have different gray_threshold for each object?

> One more point - it's best to set all objects' ambient to 0, otherwise they
> actively emit light in radiosity calculations.

Is it enough to set the global_settings ambient_light to 0, or is it
necessary to add a finish { ambient 0 } to each object in the scene.

>						 If you have the global
> ambient=3 then your scene must be really, really bright :)

Indeed. I use radiosity OR global ambient, not both.

Thanks for your help.

Post a reply to this message

Nicolas George <nicolas$george@salle-s.org> wrote:
> As a more theoretical question, would it be possible, with the radiosity
> algorithm, to have different gray_threshold for each object?

I'm not sure. It's certainly not possible with the current implementation; I
don't know how easy (or worthwhile) a patch it would be.

> Is it enough to set the global_settings ambient_light to 0, or is it
> necessary to add a finish { ambient 0 } to each object in the scene.

From the documentation:

"The actual ambient used is: Ambient = Finish_Ambient * Global_Ambient."

So if you set global ambient = 0 then it will work fine. If you want nonzero
ambient light then it's better to use the #default finish { } and only
specify the ambient values you want to override.

Have fun. I like bright daylight scenes with radiosity, you can get some
really realistic results!

Post a reply to this message

>>   Well, if there isn't anything else to reflect than the blue sky, then 
>> of
>> course objects will get a blue tint.
>
> I never noticed a blue tint in the sand of a beach, for example. At least,
> not as much as I see on my scene.

The problem is, when you are at a beach, everything else around you has the 
same blue tint so you don't notice it.

If you could suddenly switch the sky from blue to grey overcast then you'd 
surely notice.  Beware that cameras automatically adjust the colour tint 
too, so comparing photos is no use unless you know they were taken with the 
same colour-balance settings.

> Maybe the brain is compensating, but in that case, I would like advice on
> how to emulate this compensation.

Yes, the brain compensates for the illumination colour up to a certain 
extent.  What it thinks should be a certain colour (eg white) it will see as 
a certain colour and use the difference to determine the global lighting 
colour and then the colour of unknown objects.  There are many optical 
illusions that exploit the way the brain works in this situation.

> I wrote a minimal example to show the amount of difference I get. It is at
> the end.

How are you choosing the RGB values to use for the sky, sunlight and sand? 
Those are probably not physically accurate...  Unless you have some 
sophisticated optical measurement equipment, I suggest you just tweak with 
these values until your scene looks the way you want it.  Also check your 
gamma settings...

Post a reply to this message

Nicolas George nous apporta ses lumieres en ce 2007/10/22 05:37:
> Hi.
> 
> This is probably a FAQ, but I can not manage to find its answer.
> 
> I am looking for light settings for a sunny outdoor scene: deep blue sky,
> sharp shadows, but the parts in the shadow must still be visible.
> 
> My base scene has this:
> 
> light_source { <100000, 100000, -100000> rgb 2 }
> sky_sphere {
>   pigment { 
>     gradient y
>     color_map {
>       [ 0.0 rgb <0.9, 0.9, 1> ]
>       [ 0.5 rgb <0.3, 0.3, 1> ]
>     }
>   }
> }
> 
> The best I could do was to set the global_settings ambient_light to 3 and
> default radiosity, but I am not entirely satisfied, because the blue tint of
> the sky is distinctly visible.
> 
> Can someone give me some advice, please?
Normaly, you should refrain from using ambient_light in the global_settings. 
It's purpose is, in non-radiosity scenes, to give some overall colouration to 
the ambient part of the finishes.
If you set is to 3, as you did, you effectively multiply the ambient value of 
any finish by 3, whitch is not recomended i when using radiosity.

Have some non-blue objects outside the visible part of the scene masking part of 
the sky.
Set gray-threshold to something like 0.5 to 0.8, it will desaturate the 
radiosity samples, reducing the blueness somewhat.

-- 
Alain
-------------------------------------------------
Important letters that contain no errors will develop errors in the mail.

Post a reply to this message

Nicolas George nous apporta ses lumieres en ce 2007/10/22 07:34:
> "Bill Pragnell"  wrote in message
> <web.471c85bad8eb8586731f01d10@news.povray.org>:
>> However, you can adjust gray_threshold in the radiosity settings. This
>> specifies how much colour to include in the radiosity calculations (1 = no
>> colours, 0 = full colour radiosity).
> 
> Thanks, that does exactly what I was hoping.
> 
>> 					Obviously, if you get rid of the sky
>> colour you will also lose colours emitted from nearby objects, so you can't
>> have it both ways I'm afraid!
> 
> That is ok for me in that particular scene.
> 
> As a more theoretical question, would it be possible, with the radiosity
> algorithm, to have different gray_threshold for each object?
> 
>> One more point - it's best to set all objects' ambient to 0, otherwise they
>> actively emit light in radiosity calculations.
> 
> Is it enough to set the global_settings ambient_light to 0, or is it
> necessary to add a finish { ambient 0 } to each object in the scene.
> 
>> 
Beter to use:
#default{finish{ambient 0}} and to override the provided ambient of ready-made 
finishes to ambient 0.

Seting ambient_lights 0 will also turn off any lighting by high ambient object 
you may have.

-- 
Alain
-------------------------------------------------
  "Having a smoking area in a restaurant is like having a peeing area in a pool."
   --Thomas Pfeffer, American Heart Association

Post a reply to this message

scott wrote:
>> I wrote a minimal example to show the amount of difference I get. It 
>> is at
>> the end.
> 
> How are you choosing the RGB values to use for the sky, sunlight and 
> sand? Those are probably not physically accurate...  Unless you have 
> some sophisticated optical measurement equipment, I suggest you just 
> tweak with these values until your scene looks the way you want it.  
> Also check your gamma settings...
> 

Maybe increasing the brightness of the "sun" could also help. Or 
diminishing that of the sky_sphere. I would have thought that on a sunny 
day you get a lot more light from the Sun than from the rest of the 
sky... Not a mere 3 times more.

-- 
Vincent

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