POV-Ray : Newsgroups : povray.general : Hemispherical light Server Time: 23 Mar 2019 06:33:47 GMT
  Hemispherical light (Message 1 to 5 of 5)  
From: muyu
Subject: Hemispherical light
Date: 16 Apr 2018 23:20:00
Message: <web.5ad52ecd152fa60ae70c858f0@news.povray.org>
I am simulating outdoor scene using radiosity. Regarding the light source, I
used one parallel light to simulate the beam. However, how could I simulate the
hemispherical diffuse radiation? Thanks in advance.

Shouyang


Post a reply to this message

From: Bald Eagle
Subject: Re: Hemispherical light
Date: 16 Apr 2018 23:30:00
Message: <web.5ad531d35a172e445cafe28e0@news.povray.org>
"muyu" <lsy### [at] gmailcom> wrote:
> I am simulating outdoor scene using radiosity. Regarding the light source, I
> used one parallel light to simulate the beam. However, how could I simulate the
> hemispherical diffuse radiation? Thanks in advance.
>
> Shouyang

Create a hemisphere (perhaps with an opaque flat back) and assign a light source
to it.

From the [F1] documentation

3.4.3.1.7 Looks Like

By default a light source has no visible shape. The light simply radiates from
an invisible point or area, however there are cases where this is not desired.
Using looks_like is as an easy way to override this behavior. There is an
implied no_shadow so that light is not blocked by the object, without it the
light inside a non-transparent object could not escape. The object would, in
effect, cast a shadow over everything.

When using looks_like there are a few important things to consider:
1.the object should be positioned at the origin
2.it's generally easier but not necessary to declare the object beforehand
3.works with point and spot lights not parallel lights
4.use a union instead if you want the object to block light and remember to make
some portion of the object transparent

See the following examples:
#declare My_Lamp_Shape = sphere { <0, 0, 0>, Some_Radius }

// using looks_like
light_source {
  <100, 200, -300> color White
  looks_like { My_Lamp_Shape }
  }

// using union
union {
  light_source { <100, 200, -300> color White }
  object { My_Lamp_Shape translate <100, 200, -300> }
  }


Post a reply to this message

From: muyu
Subject: Re: Hemispherical light
Date: 16 Apr 2018 23:40:00
Message: <web.5ad533f05a172e44e70c858f0@news.povray.org>
"Bald Eagle" <cre### [at] netscapenet> wrote:
> "muyu" <lsy### [at] gmailcom> wrote:
> > I am simulating outdoor scene using radiosity. Regarding the light source, I
> > used one parallel light to simulate the beam. However, how could I simulate the
> > hemispherical diffuse radiation? Thanks in advance.
> >
> > Shouyang
>
> Create a hemisphere (perhaps with an opaque flat back) and assign a light source
> to it.
>
> From the [F1] documentation
>
> 3.4.3.1.7 Looks Like
>
> By default a light source has no visible shape. The light simply radiates from
> an invisible point or area, however there are cases where this is not desired.
> Using looks_like is as an easy way to override this behavior. There is an
> implied no_shadow so that light is not blocked by the object, without it the
> light inside a non-transparent object could not escape. The object would, in
> effect, cast a shadow over everything.
>
> When using looks_like there are a few important things to consider:
> 1.the object should be positioned at the origin
> 2.it's generally easier but not necessary to declare the object beforehand
> 3.works with point and spot lights not parallel lights
> 4.use a union instead if you want the object to block light and remember to make
> some portion of the object transparent
>
> See the following examples:
> #declare My_Lamp_Shape = sphere { <0, 0, 0>, Some_Radius }
>
> // using looks_like
> light_source {
>   <100, 200, -300> color White
>   looks_like { My_Lamp_Shape }
>   }
>
> // using union
> union {
>   light_source { <100, 200, -300> color White }
>   object { My_Lamp_Shape translate <100, 200, -300> }
>   }

I think I did not explain myslef clearly. I would like to simulate diffuse sky
radiation, which distribute isotropically over the hemisphere. Thanks in
advance.


Post a reply to this message

From: Alain
Subject: Re: Hemispherical light
Date: 16 Apr 2018 23:44:27
Message: <5ad5355b$1@news.povray.org>
Le 18-04-16 à 19:16, muyu a écrit :
> I am simulating outdoor scene using radiosity. Regarding the light source, I
> used one parallel light to simulate the beam. However, how could I simulate the
> hemispherical diffuse radiation? Thanks in advance.
> 
> Shouyang
> 
> 

Basic way :
Simply use a sky_sphere and radiosity.

The sky_sphere will take the role of the blue sky and cast a bluish tint 
on your scene.
sky_sphere{
	pigment{
		planar colour_map{[0 rgb<0.01, 0.1, 0.9>][1 rgb 0.7]}
		}
	}

Make your «Sun» slightly yellow to compensate.

Use radiosity. It will enable the sky_sphere to actually emit some light 
onto your scene.
You can use radiosity with the default parameters like this :
global_settings{ radiosity{}}

The defaults can be good enough for a scene using a regular light 
source, such as your Sun.

If you want some clouds, you can use a plane or a large flattened sphere 
with some cloud-like pattern. Make that object hollow to allow you to 
use any media of fog, it also will suppress a warning about the camera 
been inside a non-hollow object. In several cases, it's good enough, 
especially if you don't see the sky directly.

More advanced, and much longer to render :

Keep a black background.
Surround your whole scene with some huge object, like a plane or very 
large box, sphere or cylinder with the hollow option.
Fill that object with scattering media using the Raleigh model. Make 
that somewhat blue.
This will scatter the blue around and make your «Sun» appear yellow.
Take a look at the sample scene mediasky.pov for an example of that.
Optionally, add some media based clouds.
Here also, radiosity can help. You need to, at least, add media on in 
the radiosity block :
radiosity{media on}


Post a reply to this message

From: Alain
Subject: Re: Hemispherical light
Date: 16 Apr 2018 23:53:08
Message: <5ad53764@news.povray.org>
Le 18-04-16 à 19:29, Bald Eagle a écrit :
> "muyu" <lsy### [at] gmailcom> wrote:
>> I am simulating outdoor scene using radiosity. Regarding the light source, I
>> used one parallel light to simulate the beam. However, how could I simulate the
>> hemispherical diffuse radiation? Thanks in advance.
>>
>> Shouyang
> 
> Create a hemisphere (perhaps with an opaque flat back) and assign a light source
> to it.
> 
>  From the [F1] documentation
> 
> 3.4.3.1.7 Looks Like
> 
> By default a light source has no visible shape. The light simply radiates from
> an invisible point or area, however there are cases where this is not desired.
> Using looks_like is as an easy way to override this behavior. There is an
> implied no_shadow so that light is not blocked by the object, without it the
> light inside a non-transparent object could not escape. The object would, in
> effect, cast a shadow over everything.
> 
> When using looks_like there are a few important things to consider:
> 1.the object should be positioned at the origin
> 2.it's generally easier but not necessary to declare the object beforehand
> 3.works with point and spot lights not parallel lights
> 4.use a union instead if you want the object to block light and remember to make
> some portion of the object transparent
> 
> See the following examples:
> #declare My_Lamp_Shape = sphere { <0, 0, 0>, Some_Radius }
> 
> // using looks_like
> light_source {
>    <100, 200, -300> color White
>    looks_like { My_Lamp_Shape }
>    }
> 
> // using union
> union {
>    light_source { <100, 200, -300> color White }
>    object { My_Lamp_Shape translate <100, 200, -300> }
>    }
Beter way in this case :
union {
   light_source { 0 color White }
   object { My_Lamp_Shape }
   translate <100, 200, -300>
   }
Create at the origin, THEN translate.
> 
> 

Did you actually read the question ?
He want to have a sky that contribute to the scene's illumination.

Last time I looked, looks_like do work with parallel. I often use that 
feature with parallel. It's only with shadowless that it /may/ not work.

The object's location is always relative to the light. The reason to 
place it at the origin : It's origin is it's parent light.


Post a reply to this message

Copyright 2003-2008 Persistence of Vision Raytracer Pty. Ltd.