Hi. 

I have seen some very nice pictures in p.b.i. showing outdoor scenes, that
uses radiosity to simulate the light from the sun/sky.

I have not so much experience with radiosity, and would apprechiate if
someone could give me some hints on how to simlate an outdoor scene using
the radiosity functions in MegaPOV. 

What is the main differences between a cloudless sky compared with a sky
with a lot of clouds (e.g. cumulus), or an overcast sky...? What is the
best way to simulate the sun and the "diffuse light" coming from the sky? 

Concerning the "diffuse light" from the sky, is this light in a shade of
blue since the sky is blue? Is it possible to calculate the "color" of the
sun at different times (i.e. sunset/mid-day/dawn)...?

Regards,
Eric

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Eric Johnsson wrote:
> 
> Hi.
> 
> I have seen some very nice pictures in p.b.i. showing outdoor scenes, that
> uses radiosity to simulate the light from the sun/sky.
> 
> I have not so much experience with radiosity, and would apprechiate if
> someone could give me some hints on how to simlate an outdoor scene using
> the radiosity functions in MegaPOV.

Ordinary default radiosity should suffice for most scenes; if it's
having TOO much of an effect, try cutting the brightness a little, as needed.

Mind you, this represents skylight. I've never heard of anyone using
radiosity to simulate direct sunlight, and you're probably better off
using a light_source of some kind for that.

> What is the main differences between a cloudless sky compared with a sky
> with a lot of clouds (e.g. cumulus), or an overcast sky...?

Assuming you've seen what those look like in real life, a little thought
should make the answer obvious.

I'll note one thing: if you're trying to simulate an overcast sky, you
would do well to do something to artificially brighten your scene a
little unless you intend for your image to be viewed on a black
background in a dark room. In real life, the eye adjusts to the dim
lighting of an overcast sky making everything seem to be acceptably
bright, but when a "realistic" overcast sky scene is viewed in tandem
with ordinary room illumination, the eye adjusts to the room lighting,
causing the image to seem to fade to darkness by comparison.

> Concerning the "diffuse light" from the sky, is this light in a shade of
> blue since the sky is blue?

If you're not sure about this, you need to get out more. Someone left a
big ol' reference sky lying out there for you to use.

> Is it possible to calculate the "color" of the
> sun at different times (i.e. sunset/mid-day/dawn)...?

I suppose, yeah.. but you could probably guess and get something reasonable.

-Xplo

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On Mon, 07 Aug 2000 04:02:15 -0700, Xplo Eristotle wrote:
>> Concerning the "diffuse light" from the sky, is this light in a shade of
>> blue since the sky is blue?
>
>If you're not sure about this, you need to get out more. Someone left a
>big ol' reference sky lying out there for you to use.

Except that it's hard to determine whether the statement is correct on
an average day.  The answer is, believe it or not, yes, but the light from
the sun tends to overpower the subtle blue illumination most of the time.
The only time I've seen it is in large areas of shadow on snowy winter days.

>> Is it possible to calculate the "color" of the
>> sun at different times (i.e. sunset/mid-day/dawn)...?
>
>I suppose, yeah.. but you could probably guess and get something reasonable.

There's a whole paper out there somewhere on approximating the color of the
sky.  It might have been presented at SIGGRAPH within the past few years; 
check http://www2.iro.umontreal.ca/~ratib/code/redirect.cgi?Goto=cg-pub.htm

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

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