>   1) As you see, the light beam coming directly from the light source
seems
> to be more dense than the reflected light beams. It looks like the density
> is calculated twice: Once for the light itself and then for the photons.
> Is this so?

Kind of, but not really.  The photons are treated as an extra light source.
For each step through the media, MegaPov will sum up the light contribution
for all light sources plus the light gathered from photons.

>  The mirrors have reflection 1 so they should not diminish the
> reflected light (as you can see in the second mirror: the reflected
> beam is equally dense as the incoming beam).

Yes. I think there is a bug in there.  :-(  When I get time I'll take
another look at that code.

>   2) You can also see a sharp line at the beginning of the first reflected
> ray. Is this caused by the double density effect? Is it a bug? Note that
> this kind of sharp line does not appear at the beginning of the second
> reflection.

When your ray passes through a spotlight, POV will create multiple
intervals.  In this scene, you have two "unlit" intervals and one "lit"
interval.  POV will then only check light sources for the lit intervals,
which can save a lot of computation time.  If you don't hit the spotlight's
cone, POV only creates a single interval.  Unfortuantely, this is causing
problems.  This may be a bug with media method 3 (try using method 2 instead
and see if it makes a difference).

>   3) In order to get this image I had to confine the media inside a box
and
> I had to scale the box so that the light source got inside it. If the box
> is smaller (so that the light source is outside the box) or there's no
> box at all (ie. using only global media), no reflection beams is seen.
> Why? Is it also a bug? Or a deliberate limitation?

For the smaller box (with the light source outside the box), just put
"photons{pass_through}" in the media container object.  (I think that
"no_shadow" might work, too).

The global media is an unintentional limitation (bug).  I currently don't
know of an elegant solution, though.... but I will work on it.

>   4) I have a second light source illuminating the scene. I wanted to
> turn of photon mapping for it by specifying
> photons { reflection off refraction off } in it. However, when I did it,
> no photons are shot at all. I had to remove this line in order to get
photons.
> I suppose it's also a bug.

This bug has been fixed.

-Nathan

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Eric Freeman wrote in message <38df713b@news.povray.org>...
>"Warp" <war### [at] tagpovrayorg> wrote in message
>news:38df68d5@news.povray.org...
>>   I tried the photon mapping in media feature
>> and got this image. The light reflecting from
>> the lens looks very cool.
>
>Very nice.  Coincidentally, I am working on the same theme.  Here's my
>latest.  Still much work to do.


See "Now Possible..." in p.b.a for the full animation.

Mark

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Attachments:
Download 'LASER.JPG' (33 KB)

Preview of image 'LASER.JPG'

Why does the light beam disappear at the glass lens?  Not hollow?

Bob

"Mark Wagner" <mar### [at] gtenet> wrote in message
news:38e05504@news.povray.org...
|
| See "Now Possible..." in p.b.a for the full animation.

Post a reply to this message

Ok, the pass_through helped.
  It's just not very intuitive. If the light passes through transparent
objects, why not the photons? I think many people will be confused by this
feature.
  Of course adding pass_through to the box made the second light source to
send photons to the media too. I, however, found a temporary fix: Adding
looks_like { sphere { 0,.1 pigment { rgbt 1 } } }
to the light source has the same effect as
photons { reflection off refraction off }
should have.

  I made the container box a lot smaller and it looks a lot better now.
The reflected beams are denser and the sharp line is much less obvious now.
  The second image uses method 2. There's a strange non-lit line where there
was a sharp edge before (at the beginning of the first reflected beam). Also
the light reflected from the lens is almost gone.

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Attachments:
Download 'photontest.jpg' (26 KB) Download 'photontest2.jpg' (14 KB)

Preview of image 'photontest.jpg'

Preview of image 'photontest2.jpg'

Bob Hughes <omn### [at] hotmailcom?subject=PoV-News:> wrote...
> Why does the light beam disappear at the glass lens?  Not hollow?
> 

My guess is that there's no media inside the glass.  (not hollow)

-Nathan

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Warp <war### [at] tagpovrayorg> wrote...
>   Ok, the pass_through helped.
>   It's just not very intuitive. If the light passes through transparent
> objects, why not the photons? I think many people will be confused by this
> feature.

Yes, I agree that it is kind of confusing.  Unfortunately, there's not an
easy way around it.  The problem is that, for a photon to be 'valid', it has
to hit the 'target' object.  If it doesn't hit the target object, it is
discarded (for energy conservation reasons).  Sometimes, though, you will
put the target behind a clear object, such as a media container.  When this
happens, you have to put pass_through in the object that blocks the path
from the light to the target.  Then, MegaPov will ignore the pass_through
object and let the photon hit the target.

>   I made the container box a lot smaller and it looks a lot better now.
> The reflected beams are denser and the sharp line is much less obvious
now.

Looks great!  The errors were probably estimation errors caused by too few
samples.  The smaller container box means that the samples would be packed
closer together and therefore would produce a better estimation.

>   The second image uses method 2. There's a strange non-lit line where
there
> was a sharp edge before (at the beginning of the first reflected beam).
Also
> the light reflected from the lens is almost gone.

When you use method 2, you'll have to increase the number of samples.
Otherwise, some of your samples will miss important parts of the scene.  For
example, you probably had samples hit before and after the reflected caustic
from the lens, but no samples hit directly on it.  Adding some jitter might
help a little bit, but nothing works as well as additional samples.

-Nathan

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didn't see it in p.b.a ...

--
Mike

wk: mik### [at] pyxiscom  www.pyxis.com
hm: mwe### [at] sciticom  www.geocities.com/mikepweber


"Mark Wagner" <mar### [at] gtenet> wrote in message
news:38e05504@news.povray.org...
>
> Eric Freeman wrote in message <38df713b@news.povray.org>...
> >"Warp" <war### [at] tagpovrayorg> wrote in message
> >news:38df68d5@news.povray.org...
> >>   I tried the photon mapping in media feature
> >> and got this image. The light reflecting from
> >> the lens looks very cool.
> >
> >Very nice.  Coincidentally, I am working on the same theme.  Here's my
> >latest.  Still much work to do.
>
>
> See "Now Possible..." in p.b.a for the full animation.
>
> Mark
>
>
>

Post a reply to this message

Which brings up the question, ought there be or not?  I'm probably thinking of
semi-transparent glass, colored or something.  Even then the light would
probably show most only on the surface.  No laser here so I can't check it out.

Bob

"Nathan Kopp" <Nat### [at] Koppcom> wrote in message
news:38e0b415$1@news.povray.org...
|
| My guess is that there's no media inside the glass.  (not hollow)

Post a reply to this message

Nathan Kopp wrote in message <38e0b415$1@news.povray.org>...
>Bob Hughes <omn### [at] hotmailcom?subject=PoV-News:> wrote...
>> Why does the light beam disappear at the glass lens?  Not hollow?
>
>My guess is that there's no media inside the glass.  (not hollow)


Right.

Mark

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Mike Weber wrote in message <38e0cc4c$1@news.povray.org>...
>didn't see it in p.b.a ...


It's from a few months back -- November 17, 1999

Mark

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