Hello,
  When I use double_illuminate in a radiosity scene it does nothing (it 
does get double_illuminated by light_sources).
Here's a scene to illustrate:

default{finish{ambient 0}}
camera{location <3,4,5> look_at 0}
polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate  z}
polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate -z}
background{rgb z}
global_settings{radiosity{}}
sphere{-y/2,0.5 texture{pigment{rgb y}finish{ambient 10 diffuse 0}}}
light_source{y/2,rgb x}

Is it a bug or it is intentional?

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Jan Dvorak <jan### [at] centrumcz> wrote:
> Hello,
>   When I use double_illuminate in a radiosity scene it does nothing (it
> does get double_illuminated by light_sources).
> Here's a scene to illustrate:
>
> Is it a bug or it is intentional?

it's not a bug!  There are no light sources in pure radiosity, so you shouldn't
expect an object to get "doubly illuminated".  Use at least one light_source
together with radiosity.

It's the same for reflection:  no light_sources, no reflection.

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nemesis wrote:
> It's the same for reflection:  no light_sources, no reflection.
> 
	?? Where did you see that? There is absolutely no relation between
light sources and reflections, and you *can* still get reflections
even if there are no light sources. Are you sure you didn't mean
"photons"?

		Jerome
- --
+------------------------- Jerome M. BERGER ---------------------+
|    mailto:jeb### [at] freefr      | ICQ:    238062172            |
|    http://jeberger.free.fr/     | Jabber: jeb### [at] jabberfr   |
+---------------------------------+------------------------------+
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nemesis nous apporta ses lumieres en ce 2007/11/11 12:28:
> Jan Dvorak <jan### [at] centrumcz> wrote:
>> Hello,
>>   When I use double_illuminate in a radiosity scene it does nothing (it
>> does get double_illuminated by light_sources).
>> Here's a scene to illustrate:
>>
>> Is it a bug or it is intentional?
> 
> it's not a bug!  There are no light sources in pure radiosity, so you shouldn't
> expect an object to get "doubly illuminated".  Use at least one light_source
> together with radiosity.
> 
> It's the same for reflection:  no light_sources, no reflection.
> 
> 
> 
You can always see an object with a high ambient value in a reflection without 
any light.
Not the case with photons. No light_source = no photons.

-- 
Alain
-------------------------------------------------
Real Time, adj.:
     Here and now, as opposed to fake time, which only occurs there
and then.

Post a reply to this message

Alain napsal(a):
> nemesis nous apporta ses lumieres en ce 2007/11/11 12:28:
>> Jan Dvorak <jan### [at] centrumcz> wrote:
>>> Hello,
>>>   When I use double_illuminate in a radiosity scene it does nothing (it
>>> does get double_illuminated by light_sources).
>>> Here's a scene to illustrate:
>>>
>>> Is it a bug or it is intentional?
>>
>> it's not a bug!  There are no light sources in pure radiosity, so you 
>> shouldn't
>> expect an object to get "doubly illuminated".  Use at least one 
>> light_source
>> together with radiosity.
>>
>> It's the same for reflection:  no light_sources, no reflection.
>>
>>
>>
> You can always see an object with a high ambient value in a reflection 
> without any light.
> Not the case with photons. No light_source = no photons.
> 
I use a plain white background + radiosity to light the scene. Or I use 
a blue gradient and a yellowish light_source.

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I would expect the front square to be illuminated by the green sphere 
(ambient 10) in the example scene.

Post a reply to this message

Alain <ele### [at] netscapenet> wrote:
> You can always see an object with a high ambient value in a reflection without
> any light.

oh!  but that *is* a bug! :P

or presumibly the high ambient object is considered a light source?

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=?ISO-8859-1?Q?=22J=E9r=F4me_M=2E_Berger=22?= <jeb### [at] freefr> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> nemesis wrote:
> > It's the same for reflection:  no light_sources, no reflection.
> >
>  ?? Where did you see that? There is absolutely no relation between
> light sources and reflections, and you *can* still get reflections
> even if there are no light sources. Are you sure you didn't mean
> "photons"?

no, I meant reflections:  if there are no light sources, reflections *shouldn't*
be seen anywhere.  If there are no light rays, what is tracing the reflective
images?

I remember lighting some test scene with pure radiosity and reflective materials
and getting no reflections at all.  And it sounds ok to me.

Post a reply to this message

nemesis napsal(a):
> Alain <ele### [at] netscapenet> wrote:
>> You can always see an object with a high ambient value in a reflection without
>> any light.
> 
> oh!  but that *is* a bug! :P
> 
> or presumibly the high ambient object is considered a light source?
> 
Yes. And that's the only way to make a solid object shine, AFAIK.

Post a reply to this message

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nemesis wrote:
> =?ISO-8859-1?Q?=22J=E9r=F4me_M=2E_Berger=22?= <jeb### [at] freefr> wrote:
>> nemesis wrote:
>>> It's the same for reflection:  no light_sources, no reflection.
>>>
>>  ?? Where did you see that? There is absolutely no relation between
>> light sources and reflections, and you *can* still get reflections
>> even if there are no light sources. Are you sure you didn't mean
>> "photons"?
> 
> no, I meant reflections:  if there are no light sources, reflections *shouldn't*
> be seen anywhere.  If there are no light rays, what is tracing the reflective
> images?
> 
	In that case, if there are no light rays, what is tracing the
image? You should see a pure black image, which obviously you don't.

> I remember lighting some test scene with pure radiosity and reflective materials
> and getting no reflections at all.  And it sounds ok to me.
> 
	I'd be really interested to see such a scene if you could make one,
but you can't (unless you use the "no_reflection" keyword of
course). I have made some test scenes with pure ambient lighting
(neither light sources nor radiosity) and reflective materials and
the reflections showed just fine.

		Jerome
- --
+------------------------- Jerome M. BERGER ---------------------+
|    mailto:jeb### [at] freefr      | ICQ:    238062172            |
|    http://jeberger.free.fr/     | Jabber: jeb### [at] jabberfr   |
+---------------------------------+------------------------------+
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=?ISO-8859-1?Q?=22J=E9r=F4me_M=2E_Berger=22?= <jeb### [at] freefr> wrote:
>  I'd be really interested to see such a scene if you could make one,
> but you can't (unless you use the "no_reflection" keyword of
> course).

global_settings {

        ambient_light 0
 radiosity { brightness 1.2 }

}



union {

 plane { y, -1 pigment { rgb 1 } finish { ambient 6 } }

 sphere { -x, 1 pigment { rgb x } }

 sphere { x, 1 pigment { rgb y } finish { reflection 1 }}



 translate z*5

}


I can't see a thing.  Reflections or otherwise.

> I have made some test scenes with pure ambient lighting
> (neither light sources nor radiosity) and reflective materials and
> the reflections showed just fine.

yes, but you know ambient light is by itself faking a constant ambient lighting,
don't you?  That's the hidden light by which povray is tracing the scene.  No
lights, no reflection, nor image. nothing...

Let there be light! :)

Post a reply to this message

> I can't see a thing.  Reflections or otherwise.
> 
>> I have made some test scenes with pure ambient lighting
>> (neither light sources nor radiosity) and reflective materials and
>> the reflections showed just fine.
> 
> yes, but you know ambient light is by itself faking a constant ambient lighting,
> don't you?  That's the hidden light by which povray is tracing the scene.  No
> lights, no reflection, nor image. nothing...
> 
> Let there be light! :)
> 
> 
> 

You were talkang about light sources. Ambient light has no source.

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nemesis <nam### [at] gmailcom> wrote:
> Alain <ele### [at] netscapenet> wrote:
> > You can always see an object with a high ambient value in a reflection without
> > any light.

> oh!  but that *is* a bug! :P

  Why?

-- 
                                                          - Warp

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Jan Dvorak <jan### [at] centrumcz> wrote:
> You were talkang about light sources. Ambient light has no source.

true.  But the point is:  a raytracer works by tracing light rays.  If there's
no light, there's no image.

I'm guessing that when povray sees at least ambient light enabled, it traces the
image by shooting light rays to get the basic image with the shadow areas
illuminated by constant color.  I may be wrong.  In any case, I don't think we
can get an image out of povray without tracing rays around.  Radiosity and
photons are addons to the basic raytracing rendering framework...

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
> nemesis <nam### [at] gmailcom> wrote:
> > Alain <ele### [at] netscapenet> wrote:
> > > You can always see an object with a high ambient value in a reflection without
> > > any light.
>
> > oh!  but that *is* a bug! :P
>
>   Why?
>
> --
>                                                           - Warp

because reflections without lights don't quite sound right...

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nemesis <nam### [at] gmailcom> wrote:
> because reflections without lights don't quite sound right...

  A "light source" in POV-Ray terms is a "light_source" definition.
Surfaces (and media) can have non-black coloring even without any
light_source definitions.

-- 
                                                          - Warp

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
> nemesis <nam### [at] gmailcom> wrote:
> > because reflections without lights don't quite sound right...
>
>   A "light source" in POV-Ray terms is a "light_source" definition.
> Surfaces (and media) can have non-black coloring even without any
> light_source definitions.

ok, and how do you explain the reflections?

Post a reply to this message

nemesis <nam### [at] gmailcom> wrote:
> > Surfaces (and media) can have non-black coloring even without any
> > light_source definitions.

> ok, and how do you explain the reflections?

  Well, if a surface has a non-black coloring even without light sources,
it's obviously going to reflect from reflective surfaces.

-- 
                                                          - Warp

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
> nemesis <nam### [at] gmailcom> wrote:
> > > Surfaces (and media) can have non-black coloring even without any
> > > light_source definitions.
>
> > ok, and how do you explain the reflections?
>
>   Well, if a surface has a non-black coloring even without light sources,
> it's obviously going to reflect from reflective surfaces.

come on, warp, you know that's not the answer I want.

How is povray rendering those reflections?

Post a reply to this message

nemesis escribió:
> Warp <war### [at] tagpovrayorg> wrote:
>> nemesis <nam### [at] gmailcom> wrote:
>>>> Surfaces (and media) can have non-black coloring even without any
>>>> light_source definitions.
>>> ok, and how do you explain the reflections?
>>   Well, if a surface has a non-black coloring even without light sources,
>> it's obviously going to reflect from reflective surfaces.
> 
> come on, warp, you know that's not the answer I want.
> 
> How is povray rendering those reflections?
> 

If an object has a non-zero ambient, it doesn't need a lightsource to be 
visible.

Post a reply to this message

Nicolas Alvarez <nic### [at] gmailisthebestcom> wrote:
> nemesis escribió:
> > Warp <war### [at] tagpovrayorg> wrote:
> >> nemesis <nam### [at] gmailcom> wrote:
> >>>> Surfaces (and media) can have non-black coloring even without any
> >>>> light_source definitions.
> >>> ok, and how do you explain the reflections?
> >>   Well, if a surface has a non-black coloring even without light sources,
> >> it's obviously going to reflect from reflective surfaces.
> >
> > come on, warp, you know that's not the answer I want.
> >
> > How is povray rendering those reflections?
> >
>
> If an object has a non-zero ambient, it doesn't need a lightsource to be
> visible.

that's all fine and dandy.  What I'm asking for is how povray renders
reflections in the absence of traceable light rays.

Post a reply to this message

nemesis napsal(a):
> Nicolas Alvarez <nic### [at] gmailisthebestcom> wrote:
>> nemesis escribió:
>>> Warp <war### [at] tagpovrayorg> wrote:
>>>> nemesis <nam### [at] gmailcom> wrote:
>>>>>> Surfaces (and media) can have non-black coloring even without any
>>>>>> light_source definitions.
>>>>> ok, and how do you explain the reflections?
>>>>   Well, if a surface has a non-black coloring even without light sources,
>>>> it's obviously going to reflect from reflective surfaces.
>>> come on, warp, you know that's not the answer I want.
>>>
>>> How is povray rendering those reflections?
>>>
>> If an object has a non-zero ambient, it doesn't need a lightsource to be
>> visible.
> 
> that's all fine and dandy.  What I'm asking for is how povray renders
> reflections in the absence of traceable light rays.
> 
> 
> 
raytracers raytrace from the eye and when they hit a surface they split 
or stop (or just turn around). This doesn't need any light_sources to work.

Post a reply to this message

nemesis <nam### [at] gmailcom> wrote:
> that's all fine and dandy.  What I'm asking for is how povray renders
> reflections in the absence of traceable light rays.

  You should perhaps acquaint yourself with the basics of raytracing.
There's an intro for example here: http://en.wikipedia.org/wiki/Ray_tracing

  Backwards raytracing works by sending rays from the camera towards the
scene. When a ray hits a surface, its color is calculated (from various
parameters, such as the pigment, light sources, etc). If the surface is
reflective, a new ray is spawned from that point, which can hit other
surfaces.
  A surface doesn't need a light source in order to contribute color to
the ray.

-- 
                                                          - Warp

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nemesis escribió:
> Nicolas Alvarez <nic### [at] gmailisthebestcom> wrote:
>> nemesis escribió:
>>> How is povray rendering those reflections?
>>>
>> If an object has a non-zero ambient, it doesn't need a lightsource to be
>> visible.
> 
> that's all fine and dandy.  What I'm asking for is how povray renders
> reflections in the absence of traceable light rays.
>

POV-Ray traces rays from the camera to the objects. Then from the 
intersection point on the object to the lightsources (if any!), to see 
if there is any object on the way (for shadows), and with some 
calculation based on the light angle, you get the basic object color. If 
the object has ambient, it would have a non-black color even without lights.

If the object is reflective, more rays are traced from the intersection 
point on the object to the other objects. That's why if one of those 
rays hits an object that has ambient, it would show on the reflection 
even without any light.

Post a reply to this message

Nicolas Alvarez <nic### [at] gmailisthebestcom> wrote:
> POV-Ray traces rays from the camera to the objects.

I should get some sleep... :P

I know raytracing works backwards, I don't know what I was thinking... sorry!

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nemesis wrote:
> =?ISO-8859-1?Q?=22J=E9r=F4me_M=2E_Berger=22?= <jeb### [at] freefr> wrote:
>>  I'd be really interested to see such a scene if you could make one,
>> but you can't (unless you use the "no_reflection" keyword of
>> course).
> 
> ...
> I can't see a thing.  Reflections or otherwise.
> 
	You've just proved my point (if you go back to what I said but you
didn't quote). If you have neither light sources nor ambient then
you don't get an image, only pure black. This has nothing to do with
reflections. The bare minimum to get an image is to have some
ambient light, and then you will get reflections too.

		Jerome
- --
+------------------------- Jerome M. BERGER ---------------------+
|    mailto:jeb### [at] freefr      | ICQ:    238062172            |
|    http://jeberger.free.fr/     | Jabber: jeb### [at] jabberfr   |
+---------------------------------+------------------------------+
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nemesis nous apporta ses lumieres en ce 2007/11/11 16:25:
> =?ISO-8859-1?Q?=22J=E9r=F4me_M=2E_Berger=22?= <jeb### [at] freefr> wrote:
>>  I'd be really interested to see such a scene if you could make one,
>> but you can't (unless you use the "no_reflection" keyword of
>> course).
> 
> global_settings {
> 
>         ambient_light 0
>  radiosity { brightness 1.2 }
> 
> }
> 
> 
> 
> union {
> 
>  plane { y, -1 pigment { rgb 1 } finish { ambient 6 } }
> 
>  sphere { -x, 1 pigment { rgb x } }
> 
>  sphere { x, 1 pigment { rgb y } finish { reflection 1 }}
> 
> 
> 
>  translate z*5
> 
> }
> 
> 
> I can't see a thing.  Reflections or otherwise.
> 
>> I have made some test scenes with pure ambient lighting
>> (neither light sources nor radiosity) and reflective materials and
>> the reflections showed just fine.
> 
> yes, but you know ambient light is by itself faking a constant ambient lighting,
> don't you?  That's the hidden light by which povray is tracing the scene.  No
> lights, no reflection, nor image. nothing...
> 
> Let there be light! :)
> 
> 
> 
It's obvious why you don't see anything: you set "ambient_light 0" in your 
global_settings, whitch effectively negate your high ambient object.
The ambient_light should ONLY be used if you want to alter the coloration of the 
ambient components for the whole scene.

ambient_light is multiplied with the ambient part of every finish. Anything 
multiplied by zero equals zero.
Realy, in any radiosity scene lighted by ambient object, you MUST NEVER set 
ambient_light to 0.
Rerun your sample scene as follow:

#default{finish{ambient 0 diffuse 1}}
global_settings{radiosity{}}
union {

  plane { y, -1 pigment { rgb 1 } finish { ambient 1 } }

  sphere { -x, 1 pigment { rgb x } }

  sphere { x, 1 pigment { rgb y } finish { reflection 1 }}



  translate z*5

}

Now, you can see everything. I've reduced your ambient 6 to 1 to prevent 
oversaturation.

-- 
Alain
-------------------------------------------------
Real Time, adj.:
     Here and now, as opposed to fake time, which only occurs there
and then.

Post a reply to this message

Jan Dvorak nous apporta ses lumieres en ce 2007/11/11 05:48:
> Hello,
>  When I use double_illuminate in a radiosity scene it does nothing (it 
> does get double_illuminated by light_sources).
> Here's a scene to illustrate:
> 
> default{finish{ambient 0}}
> camera{location <3,4,5> look_at 0}
> polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate  z}
> polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate -z}
> background{rgb z}
> global_settings{radiosity{}}
> sphere{-y/2,0.5 texture{pigment{rgb y}finish{ambient 10 diffuse 0}}}
> light_source{y/2,rgb x}
> 
> Is it a bug or it is intentional?
Working as expected.
The polygons are double_illuminate be the light_source (red), whitch contribute 
on both faces.
They are lighted in blue from the background, giving that magenta tint.
The faces near the high ambient grees sphere are illuminated in green. They are 
also affected by the other polygon, lighted in red, and, weekly, the blue 
background.

-- 
Alain
-------------------------------------------------
You know you've been raytracing too long when you have ever brought your 
computer to its knees by mistakenly launching 64 simultaneous frames to be 
traced, while trying to maximizing the benefits of parallelizing them.
Carsten Whimster

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Alain <ele### [at] netscapenet> wrote:
> It's obvious why you don't see anything: you set "ambient_light 0" in your
> global_settings

and you realize it was intentional, don't you?  The point was to prove that
without any ray sources at all, there's no *raytraced* images.

Anyways, I forgot raytracing works backwards, by shooting rays into the scene
and then, possibly, to light sources.  Indeed, it can still work without light
rays, by just tracing rays to objects with high ambient values.

Post a reply to this message

=?ISO-8859-1?Q?=22J=E9r=F4me_M=2E_Berger=22?= <jeb### [at] freefr> wrote:
> If you have neither light sources nor ambient then
> you don't get an image, only pure black. This has nothing to do with
> reflections. The bare minimum to get an image is to have some
> ambient light, and then you will get reflections too.

I forgot raytracing works backwards and was assuming the rays needed to trace
the reflections were rays coming from light sources.  From there on, I assumed
when ambient lighting was present, povray automatically created a no_shadow
light_source to trace the scene.  silly me...

Post a reply to this message

Alain napsal(a):
> Jan Dvorak nous apporta ses lumieres en ce 2007/11/11 05:48:
>> Hello,
>>  When I use double_illuminate in a radiosity scene it does nothing (it 
>> does get double_illuminated by light_sources).
>> Here's a scene to illustrate:
>>
>> default{finish{ambient 0}}
>> camera{location <3,4,5> look_at 0}
>> polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate  z}
>> polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate -z}
>> background{rgb z}
>> global_settings{radiosity{}}
>> sphere{-y/2,0.5 texture{pigment{rgb y}finish{ambient 10 diffuse 0}}}
>> light_source{y/2,rgb x}
>>
>> Is it a bug or it is intentional?
> Working as expected.
> The polygons are double_illuminate be the light_source (red), whitch 
> contribute on both faces.
> They are lighted in blue from the background, giving that magenta tint.
> The faces near the high ambient grees sphere are illuminated in green. 
> They are also affected by the other polygon, lighted in red, and, 
> weekly, the blue background.
> 

As for MegaPOV 1.2.1 @ WinXP the front square doesn't get doubly 
illuminated by the green sphere. Can you post your configuration, please?

Post a reply to this message

Jan Dvorak nous apporta ses lumieres en ce 2007/11/13 00:25:
> Alain napsal(a):
>> Jan Dvorak nous apporta ses lumieres en ce 2007/11/11 05:48:
>>> Hello,
>>>  When I use double_illuminate in a radiosity scene it does nothing 
>>> (it does get double_illuminated by light_sources).
>>> Here's a scene to illustrate:
>>>
>>> default{finish{ambient 0}}
>>> camera{location <3,4,5> look_at 0}
>>> polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate  z}
>>> polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate -z}
>>> background{rgb z}
>>> global_settings{radiosity{}}
>>> sphere{-y/2,0.5 texture{pigment{rgb y}finish{ambient 10 diffuse 0}}}
>>> light_source{y/2,rgb x}
>>>
>>> Is it a bug or it is intentional?
>> Working as expected.
>> The polygons are double_illuminate be the light_source (red), whitch 
>> contribute on both faces.
>> They are lighted in blue from the background, giving that magenta tint.
>> The faces near the high ambient grees sphere are illuminated in green. 
>> They are also affected by the other polygon, lighted in red, and, 
>> weekly, the blue background.
>>
> 
> As for MegaPOV 1.2.1 @ WinXP the front square doesn't get doubly 
> illuminated by the green sphere. Can you post your configuration, please?
It's not double_illuminated by radiosity, and sould'nt be.

-- 
Alain
-------------------------------------------------
You know you've been raytracing too long when you've convinced your boss that 
raytracing is really an integral part of your job title (and you really don't 
work in a position that requires it).
Quietly Watching

Post a reply to this message

Alain napsal(a):
> Jan Dvorak nous apporta ses lumieres en ce 2007/11/13 00:25:
>> Alain napsal(a):
>>> Jan Dvorak nous apporta ses lumieres en ce 2007/11/11 05:48:
>>>> Hello,
>>>>  When I use double_illuminate in a radiosity scene it does nothing 
>>>> (it does get double_illuminated by light_sources).
>>>> Here's a scene to illustrate:
>>>>
>>>> default{finish{ambient 0}}
>>>> camera{location <3,4,5> look_at 0}
>>>> polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate  z}
>>>> polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate -z}
>>>> background{rgb z}
>>>> global_settings{radiosity{}}
>>>> sphere{-y/2,0.5 texture{pigment{rgb y}finish{ambient 10 diffuse 0}}}
>>>> light_source{y/2,rgb x}
>>>>
>>>> Is it a bug or it is intentional?
>>> Working as expected.
>>> The polygons are double_illuminate be the light_source (red), whitch 
>>> contribute on both faces.
>>> They are lighted in blue from the background, giving that magenta tint.
>>> The faces near the high ambient grees sphere are illuminated in 
>>> green. They are also affected by the other polygon, lighted in red, 
>>> and, weekly, the blue background.
>>>
>>
>> As for MegaPOV 1.2.1 @ WinXP the front square doesn't get doubly 
>> illuminated by the green sphere. Can you post your configuration, please?
> It's not double_illuminated by radiosity, and sould'nt be.
> 
How come it shouldn't?

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Jan Dvorak nous apporta ses lumieres en ce 2007/11/13 10:09:
> Alain napsal(a):
>> Jan Dvorak nous apporta ses lumieres en ce 2007/11/13 00:25:
>>> Alain napsal(a):
>>>> Jan Dvorak nous apporta ses lumieres en ce 2007/11/11 05:48:
>>>>> Hello,
>>>>>  When I use double_illuminate in a radiosity scene it does nothing 
>>>>> (it does get double_illuminated by light_sources).
>>>>> Here's a scene to illustrate:
>>>>>
>>>>> default{finish{ambient 0}}
>>>>> camera{location <3,4,5> look_at 0}
>>>>> polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate  z}
>>>>> polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate -z}
>>>>> background{rgb z}
>>>>> global_settings{radiosity{}}
>>>>> sphere{-y/2,0.5 texture{pigment{rgb y}finish{ambient 10 diffuse 0}}}
>>>>> light_source{y/2,rgb x}
>>>>>
>>>>> Is it a bug or it is intentional?
>>>> Working as expected.
>>>> The polygons are double_illuminate be the light_source (red), whitch 
>>>> contribute on both faces.
>>>> They are lighted in blue from the background, giving that magenta tint.
>>>> The faces near the high ambient grees sphere are illuminated in 
>>>> green. They are also affected by the other polygon, lighted in red, 
>>>> and, weekly, the blue background.
>>>>
>>>
>>> As for MegaPOV 1.2.1 @ WinXP the front square doesn't get doubly 
>>> illuminated by the green sphere. Can you post your configuration, 
>>> please?
>> It's not double_illuminated by radiosity, and sould'nt be.
>>
> How come it shouldn't?
Radiosity ignore double_illuminate, it never use samples comming from behind the 
surface.
You can fake it as follow:
default{finish{ambient 0}}
camera{location <3,4,5> look_at 0}
polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate  z no_shadow}
polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate -z no_shadow}
background{rgb z}
global_settings{radiosity{count 1000 error_bound 0.9}}
sphere{-y/2,0.5 texture{pigment{rgb y}finish{ambient 10 diffuse 0}}}
light_source{y/2,rgb x}
#declare Offset = 1.05;
polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate Offset*z
	finish{reflection 1} no_image no_shadow no_reflection}
polygon{5,x,y,-x,-y,x pigment{rgb 1}double_illuminate translate -Offset*z
	finish{reflection 1} no_image no_shadow no_reflection}

This is you original scene with the following changes:
Added no_shadow on the original polygons.
Added 2 reflective polygons just outside the original ones, with no_everything. 
That way, they reflect but don't show, don't have shadow and don't reflect on 
each other.
Increased count, less that 1000 and you get artefacts.
Had also to reduce error_bound.

-- 
Alain
-------------------------------------------------
Bend the facts to fit the conclusion. It's easier that way.

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Jan Dvorak <jan### [at] centrumcz> wrote:
> >> As for MegaPOV 1.2.1 @ WinXP the front square doesn't get doubly
> >> illuminated by the green sphere. Can you post your configuration, please?
> > It's not double_illuminated by radiosity, and sould'nt be.
> >
> How come it shouldn't?

This is meant to say that the POV code is written such that radiosity does not
use the double_illuminate flag, so it shouldn't be expected to act as such, not
that theoretically or conceptually it shouldn't (couldn't) be considered.

However, that brings up the question on whether it should or could be
implemented in future code (beta if it's very simple, patched versions for
those coding enthusiasts, POV4 beyond that)

Seeing as it's a feature that is present for traditional light sources, I don't
see why it shoulnd't be allowed to affect radiosity as well, but I don't know
how easy it would be to add to the source code, perhaps it's not a trivial
matter.  This would be for the coders out there to decide.

-tgq

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Trevor G Quayle <Tin### [at] hotmailcom> wrote:
> Seeing as it's a feature that is present for traditional light sources, I don't
> see why it shoulnd't be allowed to affect radiosity as well

  A double_illuminate feature in radiosity would make it slower because
then each radiosity ray would need to be doubled (for each regular radiosity
ray a second ray would also need to be spawned on the other side of the
surface).

-- 
                                                          - Warp

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Warp napsal(a):
> Trevor G Quayle <Tin### [at] hotmailcom> wrote:
>> Seeing as it's a feature that is present for traditional light sources, I don't
>> see why it shoulnd't be allowed to affect radiosity as well
> 
>   A double_illuminate feature in radiosity would make it slower because
> then each radiosity ray would need to be doubled (for each regular radiosity
> ray a second ray would also need to be spawned on the other side of the
> surface).
> 
do you really mind? :-)

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Warp a écrit :
> Trevor G Quayle <Tin### [at] hotmailcom> wrote:
>> Seeing as it's a feature that is present for traditional light sources, I don't
>> see why it shoulnd't be allowed to affect radiosity as well
> 
>   A double_illuminate feature in radiosity would make it slower because
> then each radiosity ray would need to be doubled (for each regular radiosity
> ray a second ray would also need to be spawned on the other side of the
> surface).

True, but it also happens when you specify 100 samples instead of 50.

If being able to take double_illuminate into account in radiosity (it
seems that it makes sense, anyway) helps getting more realistic
pictures, it should be envisioned.

Fabien.

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Fa3ien <fab### [at] yourshoesskynetbe> wrote:
> If being able to take double_illuminate into account in radiosity (it
> seems that it makes sense, anyway) helps getting more realistic
> pictures, it should be envisioned.

  double_illuminate is not realistic. It doesn't correspond to any
physical phenomenon.

-- 
                                                          - Warp

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Warp napsal(a):
> Fa3ien <fab### [at] yourshoesskynetbe> wrote:
>> If being able to take double_illuminate into account in radiosity (it
>> seems that it makes sense, anyway) helps getting more realistic
>> pictures, it should be envisioned.
> 
>   double_illuminate is not realistic. It doesn't correspond to any
> physical phenomenon.
> 
It does correspond to any surface that is thin enough to be translucent 
but thick enough not to be transparent. A good example is a paper 
(although the light path translute-diffuse reflect-translute is much 
weaker than the direct reflection the double_illumination effect is very 
strong if the paper is backlit).
Try taking two pieces of paper, the bottom one having been written on. 
You only see the top one. Then backlight them - you can see clearly what 
is written on the bottom side, which means paper is either a bit 
transparent or a bit double_illuminated. Now lift the top one. You will 
see the bottom paper's projection blur srongly, which means that paper 
is translucent, not transparent. And that's what double_illuminate is for.
However, I do see a bit of unrealism:
- It's an object property. It should belong to a texture.
- It should be possible to regulare the amount of double_illumination. 
Right now it is 1:1.
   My idea is a back_diffuse keyword in the finish statement. 
[diffuse]*[direct lighting*]+[back_diffuse]*[backlight*]=total 
illumination. back_diffuse defaults to diffuse if double_illuminate is 
set and 0 if unset. It causes a warning if both appear.
*including radiosity
- Povray should consider every texture layer, not just the top one.
-- Also I think that the default inside texture shoud be the outside one 
*in the reverse order* (imagine gluing a few stickers onto a glass. 
Which one do you see from the back side, the top one?). Or even forbid 
interior_texture completely (it is more realistic but it reduces 
artistic possibilities). But that's another chapter.

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Jan Dvorak <jan### [at] centrumcz> wrote:
> >   double_illuminate is not realistic. It doesn't correspond to any
> > physical phenomenon.
> > 
> It does correspond to any surface that is thin enough to be translucent 
> but thick enough not to be transparent.

  No, it doesn't. Just take any material you want and compare its
illumination on the side facing the light and on the opposite side.
The illumination is totally different. With double_illuminate the
illumination on both sides will be *identical*. This corresponds to
nothing in nature.

  The only reason why double_illuminate exists in povray is because it's
so laughably easy to implement (it's a 1-liner: 1 single 'if' statement;
or more precisely, one additional condition in an existing 'if' statement),
and it can be used in certain situations to eg. alleviate some rendering
artifacts, as well as other purposes. It's not even intended to be a
physically accurate feature.

> A good example is a paper 

  Emulating the illumination on the backside of a paper in a physically
accurate way would be quite laborious and slow.

  double_illuminate might be a somewhat acceptable approximation in some
situations, assuming you are not seeing both sides at the same time (in
which case it would be very unrealistic).

-- 
                                                          - Warp

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Warp napsal(a):
> Jan Dvorak <jan### [at] centrumcz> wrote:
>>>   double_illuminate is not realistic. It doesn't correspond to any
>>> physical phenomenon.
>>>
>> It does correspond to any surface that is thin enough to be translucent 
>> but thick enough not to be transparent.
> 
>   No, it doesn't. Just take any material you want and compare its
> illumination on the side facing the light and on the opposite side.
> The illumination is totally different. With double_illuminate the
> illumination on both sides will be *identical*. This corresponds to
> nothing in nature.
> 
>   The only reason why double_illuminate exists in povray is because it's
> so laughably easy to implement (it's a 1-liner: 1 single 'if' statement;
> or more precisely, one additional condition in an existing 'if' statement),
> and it can be used in certain situations to eg. alleviate some rendering
> artifacts, as well as other purposes. It's not even intended to be a
> physically accurate feature.
> 
>> A good example is a paper 
> 
>   Emulating the illumination on the backside of a paper in a physically
> accurate way would be quite laborious and slow.
> 
>   double_illuminate might be a somewhat acceptable approximation in some
> situations, assuming you are not seeing both sides at the same time (in
> which case it would be very unrealistic).
> 
Read the "However, I do see a bit of unrealism" part.

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