This is the image that pertains to my question on photon usage in
povray.newusers. I know. It's pretty ugly. It's obvious I am using photons
incredibly wrong. I need some suggestions on how to begin. Please excuse my
ignorance. Also excuse the extreme low quality of this render.

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Did you make this white dots apposite or not?



news:web.4158124a2dc2e8fea3fcf12a0@news.povray.org...
> This is the image that pertains to my question on photon usage in
> povray.newusers. I know. It's pretty ugly. It's obvious I am using photons
> incredibly wrong. I need some suggestions on how to begin. Please excuse
my
> ignorance. Also excuse the extreme low quality of this render.
>


----------------------------------------------------------------------------
----

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Unfortunately, this isn't an ideal scene for photons.  You'll probably end
up having to set some of the settings manually, specifically the "radius",
the spacing, and possibly the expand_thresholds.  Normally, POV computes
these autmomatically.  However, some scenes that have very disparate photon
densities can confuse these automated algorithms (which are fairly
primative, I admit).

Some things to try:

1) Start by greatly increasing the number of photons in your scene.  The
white dots that you see are individual photons, and each photon is carrying
a LOT of energy.  For good results, you'll probably need at least ten times
as many photons.

2) Use the "multiplier" option for the "radius" setting.  Here's an example:
photons
{
  ...
  radius /*first parameter is optional*/, 5
  ...
}
This will tell POV to take it's auto-guessed gather radius and multiply it
by five.  If that doesn't smooth out the photons, try multiplying by ten.

3) Try specifying the spacing (as opposed to "count") and the exact gather
radius.  Example:
photons
{
  spacing 0.1
  radius 0.05
}
Note that the proper values for spacing and radius will depend on your scene
size.  Spacing will be close to the average space between photons deposited
on the surface.  Note that usually a good radius:spacing ratio is 5:1.

BTW, the "Photons FAQ" section of the documentation specifically addresses
this problem.

-Nathan



"dan B hentschel" <dan### [at] alumritedu> wrote...
> This is the image that pertains to my question on photon usage in
> povray.newusers. I know. It's pretty ugly. It's obvious I am using photons
> incredibly wrong. I need some suggestions on how to begin. Please excuse
my
> ignorance. Also excuse the extreme low quality of this render.
>


----------------------------------------------------------------------------
----

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Since the thin plastic casing will probably have only a very slight effect
on photons, I would suggest not using them as targets and setting
pass_through on them. At least for your first tests, to reduce the number of
photons necessary.

 - Slime
 [ http://www.slimeland.com/ ]

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"Nathan Kopp" <pov### [at] nkoppmailshellcom> wrote:
> Unfortunately, this isn't an ideal scene for photons.  You'll probably end
> up having to set some of the settings manually, specifically the "radius",
> the spacing, and possibly the expand_thresholds.  Normally, POV computes
> these autmomatically.  However, some scenes that have very disparate photon
> densities can confuse these automated algorithms (which are fairly
> primative, I admit).
>
> Some things to try:
>
> 1) Start by greatly increasing the number of photons in your scene.  The
> white dots that you see are individual photons, and each photon is carrying
> a LOT of energy.  For good results, you'll probably need at least ten times
> as many photons.
>

Ok. That's good information. Thanks. I figured that 1,000,000 was probably a
big enough number to use since all the examples I looked at used between
10,000 and 100,000. Also, I watched the count during pre-rendering, and it
stopped shooting photons after about 30,000ish, so I thought that count was
probably high enough. Based on suggestions, I will likely use spacing
rather than count.

> 2) Use the "multiplier" option for the "radius" setting.  Here's an example:
> photons
> {
>   ...
>   radius /*first parameter is optional*/, 5
>   ...
> }
> This will tell POV to take it's auto-guessed gather radius and multiply it
> by five.  If that doesn't smooth out the photons, try multiplying by ten.
>

Ok. It looks like this parameter goes in the photons {} block of the
global_settings? I was starting to get a bit confused about which
parameters to specify in the global_settings and which ones to put in the
individual target blocks, but after reading the pertinent help file a few
more times, it's now starting to make more sense.

> 3) Try specifying the spacing (as opposed to "count") and the exact gather
> radius.  Example:
> photons
> {
>   spacing 0.1
>   radius 0.05
> }
> Note that the proper values for spacing and radius will depend on your scene
> size.  Spacing will be close to the average space between photons deposited
> on the surface.  Note that usually a good radius:spacing ratio is 5:1.
>

I'm assuming this is a typo. Since the ratio needs to be 5:1, then it needs
to be either spacing 0.1; radius 0.5 OR spacing 0.01; radius 0.05. I'm
guessing the latter. It also seems that with the above (#2) comment, my
resulting block should look something like:

photons
{
  spacing 0.01
  radius 0.05, 5
  autostop 0
}

Look right?

> BTW, the "Photons FAQ" section of the documentation specifically addresses
> this problem.

Yes, I have read, and re-read the FAQ, but comments here have helped to
clarify some of my questions. Most importantly, I was under the impression
that count and spacing were interchangeable, in other words, increasing the
count is pretty much the same thing as decreasing the spacing. This doesn't
seem to be the case. I really appreciate the help, and I will let you know
the results of my experiments.

>
> -Nathan
>
>
>
> "dan B hentschel" <dan### [at] alumritedu> wrote...
> > This is the image that pertains to my question on photon usage in
> > povray.newusers. I know. It's pretty ugly. It's obvious I am using photons
> > incredibly wrong. I need some suggestions on how to begin. Please excuse
> my
> > ignorance. Also excuse the extreme low quality of this render.
> >
>
>
> ----------------------------------------------------------------------------
> ----

Post a reply to this message

"Slime" <fak### [at] emailaddress> wrote:
> Since the thin plastic casing will probably have only a very slight effect
> on photons, I would suggest not using them as targets and setting
> pass_through on them. At least for your first tests, to reduce the number of
> photons necessary.
>
>  - Slime
>  [ http://www.slimeland.com/ ]

Ok. Maybe I will try doing that once my current render is finished, sometime
tomorrow. Thanks. I was a bit confused by this option as well, though.
Should I just use the block:

photons
{
  pass_through
  collect off
}

I assume that I should use "collect off" as I show above. I also am making
the assumption that specifying "target" doesn't make sense in this context.

 - dan B hentschel

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"dan B hentschel" <dan### [at] alumritedu> wrote...
> I'm assuming this is a typo. Since the ratio needs to be 5:1, then it
needs
> to be either spacing 0.1; radius 0.5 OR spacing 0.01; radius 0.05. I'm
> guessing the latter. It also seems that with the above (#2) comment, my
> resulting block should look something like:
> photons
> {
>   spacing 0.01
>   radius 0.05, 5
>   autostop 0
> }

Almost.  I meant that you should use either #1 or #2.

So, if you want to use count, then do something like this
photons
{
  count 100000000  // use more photons, because POV's estimate is off
  radius ,5  // tell POV to multiply it's radius guess by 5
  autostop 0
}

Alternatively, you can remove two of the guesses that POV does:
photons
{
  spacing 0.1
  radius 0.5
  autostop 0
}

In the first example, POV will take a look at your count and try to estimate
the angular density of photons that it sends from the light sources,
attempting to get as close to the final count as possible.  Depending on the
scene, this estimate can be very poor.  Next, POV shoots the photons using
that angular density.  POV then analyzes the spacial density of the photons
that have been deposited on the surfaces and tries to choose a good radius
to use when gathering photons.  Depending on the distribution of the
photons, this is also susciptable to poor results.

In the second example, you specify a target spacial density.  POV computes
an angular density for shooting photons in an attempt to get close to the
desired spacial density.  Generally, this is a relatively good estimate
(though it can be poor sometimes).  Because you specify the gather radius,
POV doesn't have to try to estimate anything there.

The numbers for spacing and radius are given in the same units, so they are
directly related to each other.  For this reason, if you choose to hard-code
the gather radius it is usually best to specify the spacing manually, too.

> Yes, I have read, and re-read the FAQ, but comments here have helped to
> clarify some of my questions. Most importantly, I was under the impression
> that count and spacing were interchangeable, in other words, increasing
the
> count is pretty much the same thing as decreasing the spacing. This
doesn't
> seem to be the case. I really appreciate the help, and I will let you know
> the results of my experiments.

Actually, count and spacing are basically interchangable.  I suggest using
spacing instead of count because it makes it easier to achieve the desired
"5:1" ratio of gather radius to average spacing between photons.

-Nathan

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Well, here is my first attempt at a cleanup. In this iteration, I took Mr.
Praay's recommendation of simply removing "area_light" from the photons {}
block of my area light. This yeilds an immediate improvement. I will start
fiddling with other settings once I can get the light levels exactly where
I want them. Right now, I think the insides of the blocks are a bit too
dark. Thanks a lot for the suggestions.

Image was rendered in 400 tiles (20x20 grid) using SMPOV, distributed across
19 3.2 GHz P4 computers. Using HyperThreading, there were a total of 27
POVRay instances running for 28 hours to generate this image.

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"dan B hentschel" <dan### [at] alumritedu> wrote:
> Well, here is my first attempt at a cleanup. In this iteration, I took Mr.
> Praay's recommendation of simply removing "area_light" from the photons {}
> block of my area light. This yeilds an immediate improvement. I will start
> fiddling with other settings once I can get the light levels exactly where
> I want them. Right now, I think the insides of the blocks are a bit too
> dark. Thanks a lot for the suggestions.
>
> Image was rendered in 400 tiles (20x20 grid) using SMPOV, distributed across
> 19 3.2 GHz P4 computers. Using HyperThreading, there were a total of 27
> POVRay instances running for 28 hours to generate this image.

Ooops... I posted the wrong picture! :-) That was the pciture I made 1 month
ago on 08-28. Here is the picture I made yesterday on 09-28. Sorry for the
mistake.

 - dan B hentschel

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