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In article <3a0031f5@news.povray.org>, Warp <war### [at] tag povrayorg>
wrote:
> Btw, it's a common misconception that light rays coming from the Sun
> are parallel.
It is? I have never heard of anyone saying that...only of people using
parallel lights to simulate the large distance. Not very accurate, but
still sometimes useful when the true scale gives precision problems.
--
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/
<><
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"Jetlag" <bga### [at] microsoftcom> wrote in message
news:3a008179@news.povray.org...
> > Btw, it's a common misconception that light rays coming from the Sun
are
> > parallel.
>
> What I don't understand is why it's so common, it should be obvious; all
you
> have to do is look up when there's some clouds passing by.
Look at the sky at sunset and wonder why the western clouds throw
shadows that -diminish- in width as they reach the opposite horizon.
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In article <3a008179@news.povray.org>, "Jetlag" <bga### [at] microsoftcom>
wrote:
> What I don't understand is why it's so common, it should be obvious; all
> you have to do is look up when there's some clouds passing by.
Umm, if you are talking about what I think you are(the visible "beams"
of light caused by partial clouds and atmospheric scattering), that is a
completely different illusion, caused by perspective. The beams appear
to come from a point because they appear smaller and closer together the
further away they are...the same effect that shows in railroad tracks,
checkered planes, etc. If the light was parallel, you would still get
that effect. I don't know how much a difference the divergence makes,
but I doubt it would be obvious without side-by-side comparisons of
images. It might be an interesting POV project though...
--
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/
<><
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> > What I don't understand is why it's so common, it should be obvious; all
> > you have to do is look up when there's some clouds passing by.
>
> Umm, if you are talking about what I think you are(the visible "beams"
> of light caused by partial clouds and atmospheric scattering), that is a
> completely different illusion, caused by perspective. The beams appear
> to come from a point because they appear smaller and closer together the
> further away they are...the same effect that shows in railroad tracks,
> checkered planes, etc. If the light was parallel, you would still get
> that effect. I don't know how much a difference the divergence makes,
> but I doubt it would be obvious without side-by-side comparisons of
> images. It might be an interesting POV project though...
Yes, you're right, I wasn't thinking it through. Parallel rays would cause
the same effect.
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okay, here is why i brought this up since everyone doesn't get it.
currently, in povray, really big numbers (like that of a realistic sized
sun at realistic distances from the scene) coupled with regular to small
numbers (any really small objects like bugs or whatever) give povray
problems. plus, creating a circular area light (which is what i meant
in the first place) oriented in the right direction and with enough area
light density (and by the way, i think most direct sunlight can be
treated as parallel in most if not all cases) is computationally
expensive in its current form. lightwave's 'distant light' is fast! it
doesn't have to calculate where the source is exactly, there isn't an
area light problem (ex. more lights = slower), and it's a great
approximation to the way the sun acts. another point of consideration
is in making smaller suns proportionately closer. for still scenes,
this probably doesn't make much difference and is a good way around the
large/small number problem. however, for scenes that fly around many
kilometers, it may be noticable that the sun is more like a big
lightbulb since it is closer than it should be. i know i haven't
explained it much better than before, but is anybody starting to
understand what i'm saying?
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.
>
> --
> 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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In article <3A01120B.A6389F2F@yahoo.com>, ryan constantine
<rco### [at] yahoocom> wrote:
> currently, in povray, really big numbers (like that of a realistic sized
> sun at realistic distances from the scene) coupled with regular to small
> numbers (any really small objects like bugs or whatever) give povray
> problems.
So, use a parallel light instead of a point light. This will simulate a
long distance light source where the light rays are nearly parallel.
> plus, creating a circular area light (which is what i meant in the
> first place) oriented in the right direction and with enough area
> light density...is computationally expensive in its current form.
Well, that is because area lights are computationally expensive...there
isn't much that can be done about that. It has nothing to do with the
position, distance, or orientation. If you want speedier rendering,
don't use an area light.
> lightwave's 'distant light' is fast!
How does Lightwave render these shadows? POV uses raytracing for
everything, I seem to recall Lightwave being a hybrid. It may use
something other than raytracing for this "distant light"...or it may be
a plain old parallel light.
> i know i haven't explained it much better than before, but is anybody
> starting to understand what i'm saying?
Unfortunately, no. If you want soft shadows, use an area light, but
rendering will slow down. If you want a distant light, use MegaPOV's
parallel light feature. If you want both...area lights should also give
soft shadows with parallel light, if they don't it is a bug.
You seem to want this but with faster rendering...which won't be
possible without adding some fairly major features with their own set of
drawbacks(shadow maps take more memory, are less accurate, and can still
take a long time to calculate).
--
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/
<><
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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. Putting the light source at 1 km away will make no
visible difference but it will be a lot more accurate.
There's no point in putting the light source at 150 millions of km away
anyways. It will not simulate the real sunlight even at that distance. To
really simulate it you would have to take into account the refraction of
the light when it enters and travels inside the atmosphere (it varies depending
on the temperature of the air at different altitudes) plus the scattering
of the light when it hits air molecules, clouds, etc.
So putting the light at 1km will not make your scene any worse. In some
cases it might even make it look better.
--
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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On Thu, 02 Nov 2000 07:59:08 -0500, Chris Huff wrote:
>Unfortunately, no. If you want soft shadows, use an area light, but
>rendering will slow down. If you want a distant light, use MegaPOV's
>parallel light feature. If you want both...area lights should also give
>soft shadows with parallel light, if they don't it is a bug.
Yes. A bug in your thinking. The idea of a parallel area light doesn't
make any sense. When MCB wrote the parallel-light code, he put in a
special case for parallel area lights that does *something* that produces
soft shadows, but it doesn't have any physical basis and his arguments at
the time were (to me) less than convincing.
If you want parallel lights, use parallel lights. If you want soft shadows,
use area lights. If you want parallel lights with soft shadows, you should
really stop smoking that stuff because it's bad for you, and the brain damage
might get in the way of implementing physically-based diffraction effects.
--
Ron Parker http://www2.fwi.com/~parkerr/traces.html
My opinions. Mine. Not anyone else's.
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In article <slr### [at] fwicom>, ron### [at] povrayorg
wrote:
> Yes. A bug in your thinking. The idea of a parallel area light doesn't
> make any sense.
An actual parallel area light would give the same results as a point
area light, but with slower rendering. Instead of being truely parallel
in the area light case, the rays should be adjusted so soft shadows
still work. This isn't physically realistic, but the realistic method is
useless.
It isn't a bug in my thinking, just a compromise to make a feature more
useful.
> When MCB wrote the parallel-light code, he put in a special case for
> parallel area lights that does *something* that produces soft
> shadows, but it doesn't have any physical basis and his arguments at
> the time were (to me) less than convincing.
Ah, then it already does what I think it should. Why weren't you
convinced?
> If you want parallel lights, use parallel lights. If you want soft
> shadows, use area lights. If you want parallel lights with soft
> shadows, you should really stop smoking that stuff because it's bad
> for you,
I'm not smoking anything...and what is wrong with wanting "parallel"
lights with soft shadows instead of parallel light that simply renders
slower? The alternative, a large area light positioned far away, could
cause precision problems. Parallel area lights can come close to
simulating that case, though they would never occur in the real world.
Besides, people will expect having area lights to blur the shadows.
> and the brain damage might get in the way of implementing
> physically-based diffraction effects.
That *would* be fun...maybe photon mapping could be twisted and hacked
to simulate it.
--
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/
<><
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Chris Huff <chr### [at] maccom> wrote:
: I'm not smoking anything...and what is wrong with wanting "parallel"
: lights with soft shadows instead of parallel light that simply renders
: slower?
Parallel rays can't create soft shadows. With parallel rays each point
is hit by exactly 0 or 1 light ray. There's no way a single point could
be hit by more than 1 light ray when all the rays are parallel to each
other.
Thus, you can't get soft shadows when only 0 or 1 light rays hits each
point.
: The alternative, a large area light positioned far away, could
: cause precision problems.
It's not an alternative since the light rays are not parallel in that
case. More than 1 ray can hit the same point and thus cause soft shadows.
And besides, there's no difference between an area light located at
150 millions of km and 10 km (with equal apparent sizes) when the scene
is small (some meters wide).
: Parallel area lights can come close to
: simulating that case, though they would never occur in the real world.
"Parallel area light" is a completely wrong term here. If the rays are
parallel, they all go to the same direction, ie. are all parallel to each
other, and thus can't cause soft shadows.
The light rays coming from the Sun are not parallel.
: Besides, people will expect having area lights to blur the shadows.
Thus, it can't use parallel rays.
--
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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