On 2 Nov 2000 11:24:03 -0500, Warp wrote:
>  What I don't understand is why people who want to simulate sunlight
>want to locate their light source at 150 millions of kilometers while
>their scene is some meters wide.

It is indeed somewhat unnecessary in this case. But often a scene is
several 100,000 km wide. Just imagine a scene where the moon is visible,
which is 384,000 km (on average) away. Now of course you can just put a
moon-shaped disk somewhere in the air, but to me that's "painting" not
"raytracing". I am much more interested in building accurate models
than in producing pretty pictures.

	hp

-- 


| |   | hjp### [at] wsracat      |    -- Lutz Donnerhacke in dasr.
__/   | http://www.hjp.at/ |

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On Thu, 02 Nov 2000 18:53:35 -0500, Chris Huff wrote:
>The patch apparently does this already, my question was about why Ron 
>thought it was wrong, since doing it the "right" way would make it 
>useless except as a way to slow down renders.

I think having a special case to make "parallel" area lights do something
nonphysical (especially without documenting it, though that's mostly my
fault) is wrong.  The right thing to do, IMHO, would be to throw a warning
if a parallel area light is used.

I don't even know exactly what it is that "parallel" area lights do, but 
since it has no physical basis, it would be preferable to use a properly-sized
area light at a proper distance instead.


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

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Warp wrote:

>   Btw, it's a common misconception that light rays coming from the Sun are
> parallel.
>
>   Actually the Sun is an area light. Its size is significant and thus the
> rays coming from it are not parallel. Light rays coming from opposite sides
> of the Sun disk hit a point in the Earth at different angles.
>
>   Thinking about it, there should not be big difference between a circular
> light source with the radius of the Sun at a distance of the Sun from the
> Earth, and a circular light source at 1km of distance and respectively
> smaller radius (so that its apparent size is the same), given that our
> scene is some meters wide.

If the sun was a point light, the rays coming at Earth would form such a tight
(practically degenerate) cone that you might as well call them parallel, rather
than put a POV light at -1000000000000*x or whatever and get float problems.
This is only good for non-area-lighting though.

Parallel rays with area lights makes no sense, because it is the
non-parallelity(?) of the rays itself that gives soft shadows. So you are
perfectly right about that. However, the size of the shadow relative to the
object casting it (in soft shadows we will define the outline as the area
that's 50% light and 50% shadow) for all practical purposes remains constant, a
phenomenon really only possible with parallel rays, because the sun is at such
a large distance from the Earth. So when using hard lighting, you might as well
make it parallel.

--
David Fontaine  <dav### [at] faricynet>  ICQ 55354965
My raytracing gallery:  http://davidf.faricy.net/

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"David Fontaine" <dav### [at] faricynet> wrote in message
news:3A04E82F.2DDCFF3D@faricy.net...
>
> If the sun was a point light, the rays coming at Earth would form such a
tight
> (practically degenerate) cone that you might as well call them parallel

I'm going off on a tangent here but you made me think of a possible new
feature: converging ray spotlights.  Easily described as a spotlight which
shines in a direction opposite a regular spotlight, starting from a radius and
going to a point.  Wouldn't that be like previous suggestions for a arealight
in some way?  Light coming from an area and the "soft shadows" caused by
wrap-around light.  I'm almost certain this has been discussed before.
Well, this topic caught my attention.  Sorry if I interrupted.

Bob

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I don't think you can get an accurate model by putting the moon at
384000 km. At least it will not be any more accurate as the moon being
at 1000 km.
  If you want an accurate result, you'll have to simulate the refraction and
diffraction of the light rays coming from the moon when they travel through
the atmosphere.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):_;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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: 	Uh? isn't it more or less what the parallel keyword does?

  Horribly wrong keyword.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):_;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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David Fontaine <dav### [at] faricynet> wrote:
: If the sun was a point light

... then the rays would be parallel. But since the Sun is not a point light,
the rays are not parallel.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):_;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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