On Thu, 11 Jan 2001 13:45:07 -0000, Tom Melly wrote:
>The trouble with ambient is that to make any real sense it should be
>influenced by the overall strength of the light-sources available. Perhaps
>one should code like this:
>
>#declare light_strength = 0.75;
>light_source{<0,0,0> color rgb<1,1,1>*light_strength translate <-300,
>300, -300>}
>object{foo finish{ambient 0.3*light_strength}}

But one does code like that, if one knows about the all-important
global_settings { ambient_light COLOR } setting.  Just look at it
as a big shadowless light source that casts light in all directions 
from everywhere.

So, the ambient setting in a finish statement seems to me as though it
should be closely related to diffuse, particularly if brilliance is low.
As brilliance goes up, ambient should go down relative to diffuse.  As
diffuse goes up, ambient should also go up.

If I were to propose a rule of thumb, I would say ambient and diffuse
should be roughly the same number, and that ambient shouldn't be included
in the requirement that filter+transmit+reflection+diffuse+specular <= 1.

The logic behind the first part is that ambient is just diffuse reflection
of light with an unspecified source, so if an object diffusely reflects
60% of any light falling on it, it should diffusely reflect 60% of the
ambient light.  (If this makes your scene too bright, turn down the ambient
light in global_settings or consider using radiosity instead of ambient
light.)

The logic behind the second part is that what an object does with light 
from unspecified sources shouldn't have any effect on what it does with 
light from a specific source.  That is, you wouldn't keep everything
below .5 just because you have two light sources, so there's no reason
to do so because you have ambient light.

Actually, now that I think about it, I'm not so sure both specular and 
reflection should be in the sum either, since the purpose of specular is 
to simulate reflection for light sources.  By the same logic, specular and
reflection should probably be fairly closely related unless you're using
blurred reflection in MegaPOV.

-- 
Ron Parker   http://www2.fwi.com/~parkerr/traces.html
My opinions.  Mine.  Not anyone else's.

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"Ron Parker" <ron### [at] povrayorg> wrote in message
news:slr### [at] fwicom...
>
> But one does code like that, if one knows about the all-important
> global_settings { ambient_light COLOR } setting.  Just look at it
> as a big shadowless light source that casts light in all directions
> from everywhere.
>

Well, yes, but even using global ambient, the ambient of objects won't
change as you adjust the strength of the light-sources in the image.

> So, the ambient setting in a finish statement seems to me as though it
> should be closely related to diffuse, particularly if brilliance is low.

Yes.

> As brilliance goes up, ambient should go down relative to diffuse.  As
> diffuse goes up, ambient should also go up.
>
> If I were to propose a rule of thumb, I would say ambient and diffuse
> should be roughly the same number, and that ambient shouldn't be included
> in the requirement that filter+transmit+reflection+diffuse+specular <= 1.
>

I'm sure you're right - however, understanding this would require me to
think of more than two things at the same time, so I cannot verify ;)

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Ron Parker wrote:
> 
> If I were to propose a rule of thumb, I would say ambient and diffuse
> should be roughly the same number, and that ambient shouldn't be included
> in the requirement that filter+transmit+reflection+diffuse+specular <= 1.

Even though, you hint on it in the next paragraph, you should point out
that this formula is only valid if you are not using radiosity.

> 
> The logic behind the first part is that ambient is just diffuse reflection
> of light with an unspecified source, so if an object diffusely reflects
> 60% of any light falling on it, it should diffusely reflect 60% of the
> ambient light.  (If this makes your scene too bright, turn down the ambient
> light in global_settings or consider using radiosity instead of ambient
> light.)
> 
> The logic behind the second part is that what an object does with light
> from unspecified sources shouldn't have any effect on what it does with
> light from a specific source.  That is, you wouldn't keep everything
> below .5 just because you have two light sources, so there's no reason
> to do so because you have ambient light.
> 
> Actually, now that I think about it, I'm not so sure both specular and
> reflection should be in the sum either, since the purpose of specular is
> to simulate reflection for light sources.  By the same logic, specular and
> reflection should probably be fairly closely related unless you're using
> blurred reflection in MegaPOV.
> 
> --
> Ron Parker   http://www2.fwi.com/~parkerr/traces.html
> My opinions.  Mine.  Not anyone else's.

-- 
Francois Labreque | Unfortunately, there's no such thing as a snooze
    flabreque     | button on a cat who wants breakfast.
        @         |      - Unattributed quote from rec.humor.funny
   videotron.ca

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Tom Melly wrote:
> 
> "Ron Parker" <ron### [at] povrayorg> wrote in message
> news:slr### [at] fwicom...
> >
> > But one does code like that, if one knows about the all-important
> > global_settings { ambient_light COLOR } setting.  Just look at it
> > as a big shadowless light source that casts light in all directions
> > from everywhere.
> >
> 
> Well, yes, but even using global ambient, the ambient of objects won't
> change as you adjust the strength of the light-sources in the image.

If global ambient is linked to the strength of your light sources (or
number thereof) via something similar to the "ambient
0.3*Light_Strength*light_num", then the induvidual ambient of objects
doesn't need to be played with as long as you also take into account the
proximity of the light sources to the object (as someone mentioned
earlier: in space, there is very little ambient) when giving these
objects ambient values in the first place.

I personally prefer setting ambient to 0 and let radiosity take care of
all this stuff.  But then again, I have the horsepower necessary for it,
so ymmv.

-- 
Francois Labreque | Unfortunately, there's no such thing as a snooze
    flabreque     | button on a cat who wants breakfast.
        @         |      - Unattributed quote from rec.humor.funny
   videotron.ca

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"Francois Labreque" <fla### [at] videotronca> wrote in message
news:3A5DCD94.E1942106@videotron.ca...
>
> If global ambient is linked to the strength of your light sources (or
> number thereof) via something similar to the "ambient
> 0.3*Light_Strength*light_num", then the induvidual ambient of objects

Pretty much what I suggested earlier.

>
> I personally prefer setting ambient to 0 and let radiosity take care of
> all this stuff.  But then again, I have the horsepower necessary for it,
> so ymmv.
>

I personally prefer it if people don't rub my nose in the fact that a P90 is
not the ideal rendering platform - particularily if they're french ;)

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On Thu, 11 Jan 2001 14:54:52 -0000, Tom Melly wrote:
>Well, yes, but even using global ambient, the ambient of objects won't
>change as you adjust the strength of the light-sources in the image.

Well, I'm of the opinion that if the amount of ambient light you have in
your scene is that dependent on individual light sources, you should be
using radiosity.

But then, I'm also of the opinion that we shouldn't have to adjust the 
brightness of light sources.  Am I alone in wishing there were an "exposure"
option in the camera specification?

-- 
Ron Parker   http://www2.fwi.com/~parkerr/traces.html
My opinions.  Mine.  Not anyone else's.

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On Thu, 11 Jan 2001 10:06:23 -0500, Francois Labreque wrote:
>
>
>Ron Parker wrote:
>> 
>> If I were to propose a rule of thumb, I would say ambient and diffuse
>> should be roughly the same number, and that ambient shouldn't be included
>> in the requirement that filter+transmit+reflection+diffuse+specular <= 1.
>
>Even though, you hint on it in the next paragraph, you should point out
>that this formula is only valid if you are not using radiosity.

From a purely theoretical standpoint, I fail to see why this should be
the case for objects that don't emit their own light (other than the 
reflection/specular thing.)  Could you elaborate?

-- 
Ron Parker   http://www2.fwi.com/~parkerr/traces.html
My opinions.  Mine.  Not anyone else's.

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On 11 Jan 2001 10:56:04 -0500, Ron Parker wrote:
>On Thu, 11 Jan 2001 14:54:52 -0000, Tom Melly wrote:
>>Well, yes, but even using global ambient, the ambient of objects won't
>>change as you adjust the strength of the light-sources in the image.
>
>Well, I'm of the opinion that if the amount of ambient light you have in
>your scene is that dependent on individual light sources, you should be
>using radiosity.

And believe it or not, I'm still of that opinion despite the fact that
this machine is only a P200.  But then, I don't render much these days.

-- 
Ron Parker   http://www2.fwi.com/~parkerr/traces.html
My opinions.  Mine.  Not anyone else's.

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"Ron Parker" <ron### [at] povrayorg> wrote in message
news:slr### [at] fwicom...
> On 11 Jan 2001 10:56:04 -0500, Ron Parker wrote:
>
>But then, I don't render much these days.

On a P200 with radiosity, no one renders much these days....

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On Thu, 11 Jan 2001 16:13:36 -0000, Tom Melly wrote:
>
>"Ron Parker" <ron### [at] povrayorg> wrote in message
>news:slr### [at] fwicom...
>> On 11 Jan 2001 10:56:04 -0500, Ron Parker wrote:
>>
>>But then, I don't render much these days.
>
>On a P200 with radiosity, no one renders much these days....

Well, sometimes I cheat and use Julie's K6-2/400.  But it runs Windows, so
it doesn't perform much better.  (Just kidding, guys!  Please cut me down
now!)

-- 
Ron Parker   http://www2.fwi.com/~parkerr/traces.html
My opinions.  Mine.  Not anyone else's.

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