Trevor G Quayle <Tin### [at] hotmailcom> wrote:
> Rather than using photon mapping, which can add to render time

  In such a simple case as this one photon mapping doesn't add almost
any render time at all. And it's far easier to use than trying some
odd kludge to try to save some 0.1 seconds from the render time.

-- 
                                                          - Warp

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"Philipp" <pov### [at] phipracom> wrote:
....
> I then would like to determine when (date and time of the day) the reflected
> sunlight falls on a certain area of the floor (or objects). It need not be a
> realistic rendering - I just want to determine the days in the year when the
> reflected sunlight hits an object that sits on the floor.
>
> To my understanding, this should be possible by using photon mapping, and
> then trying to make a fast-motion video from it. Is that correct, and is it
> feasable for, say, a hobby programmer to do this within two working weeks?
>
> (Is there some module or code-snippet for automatically calculating the
> sun's position, given geographical position and date/time?)
>
To answer your questions: yes, yes, and yes.  POV-Ray comes with sunpos.inc,
a macro to do just what you would like (2.7.20.6 sunpos.inc).   Two weeks
should be sufficient.  Pay particular attention to coordinate handedness.
You'll need a separate piece of software to make the animation.  Here's a
link to the sunpos.inc doc page:

http://www.povray.org/documentation/view/3.6.1/498/

I like to use SunPos() to show the seasonal solar exposure and penetration
of structures so as to minimize heat gain in the summer and maximize it in
the winter.  I throw a bunch of intensity- and color-coded 'suns' in the
sky (red for summer 'cooling days', and blue for winter 'heating days').
The resulting red-magenta-blue hues illuminating the surfaces of the
structure indicate where wanted and unwanted solar radiation is
intercepted.  Passive solar performance should be good when exterior
surfaces appear red (and are well-insulated there) and the surfaces of
internal thermal masses are blue.

Hope this helps.

-David

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in news:web.45a82fb9aa573b97442464bf0@news.povray.org waggy wrote:

> I like to use SunPos() to show the seasonal solar exposure and
> penetration of structures so as to minimize heat gain in the summer
> and maximize it in the winter.

That is exactly what I wrote it for :)

>  I throw a bunch of intensity- and
> color-coded 'suns' in the sky (red for summer 'cooling days', and
> blue for winter 'heating days'). The resulting red-magenta-blue hues
> illuminating the surfaces of the structure indicate where wanted and
> unwanted solar radiation is intercepted.

Ah, great trick, never thought of it.

Thanks,

Ingo

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"Philipp" <pov### [at] phipracom> wrote:
> Hi,
>
> I am considering to dig into PovRay, if it can do what I need - so I hope
> that someone of you can tell me whether it will:
>
> A light source (the sun) shines on a reflecting surface (say, a
> square-shaped mirror that is hanging on a wall). As the sunlight comes from
> above and shines onto the mirror, part of the sunlight is reflected
> downwards to the floor. Thus, a basically parallelogram-shaped lighter area
> should be visible on the floor.
>
> I then would like to determine when (date and time of the day) the reflected
> sunlight falls on a certain area of the floor (or objects). It need not be a
> realistic rendering - I just want to determine the days in the year when the
> reflected sunlight hits an object that sits on the floor.
>
> To my understanding, this should be possible by using photon mapping, and
> then trying to make a fast-motion video from it. Is that correct, and is it
> feasable for, say, a hobby programmer to do this within two working weeks?
>
> (Is there some module or code-snippet for automatically calculating the
> sun's position, given geographical position and date/time?)
>
>
> Many thanks!
> Philipp

I hate to say so, since one more Povver is obviously good in the greater
scheme of things, ;-) but you don't need to use POV at all to do that.
Simply calculate the angle from the object to the mirror, then determine
where the sun should be in the sky. You may want to convert that to Right
Ascension and Declension (or whatever they're called in English), just
google it. Then look up ephemerides (for example
http://cfa-www.harvard.edu/iau/MPEph/MPEph.html)

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"Trevor G Quayle" <Tin### [at] hotmailcom> wrote:
> "Philipp" <pov### [at] phipracom> wrote:
[...]
> Assuming you know the position of the sun and the plane of the mirror, you
> would just mirror the sun's position relative to the mirror surface,
> creating a virtual sun.  Then you can use the projected_through option for
> the virtual sun using the mirror surface as the projected_through object.
>
> This may take a little bit more mathematics, but would save considerably on
> render time.
>

Hi, thanks Warp and Trevor for your responses;

Well, I had simplified the problem a bit.
Actually there are some other objects in the scene, which will be partly
between the sun and the mirror, and also between the mirror and the
"detector object" (I call it like that for now, it is the object that sits
on the floor, waiting to to get hit by reflected sunlight). So this makes
Trevor's "virtual sun" approach more complicated, though this would be a
good idea. The other objects in the scene are quite simple (flat surfaces,
no transparencies).

In fact, I would rather prefer simple programming over little rendering
time, because I have little experience with povray, this is probably a
one-time job, and I want to reduce the risk of program bugs. (So no matter
if rendering takes 3 days in total. :-)

Basically, I want a means to determine when the "detector object" is hit by
the sunlight. I thought about only calculating / drawing it in CAD, but
obviously it is nice to have some visual plausibility check, and a few good
pictures to show.
So I thought I could make a video of one (half?) simulated year, say, at one
frame per hour, and then watch the video (say, 3 secondes per frame) to see
what happens; ...and to find the date/time of critical situations (assuming
a visible date-timeline in the video, hmmmm...).

Regards,
Philipp

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ingo <ing### [at] tagpovrayorg> wrote:
> in news:web.45a82fb9aa573b97442464bf0@news.povray.org waggy wrote:
>
> > I like to use SunPos() to show the seasonal solar exposure and
> > penetration of structures so as to minimize heat gain in the summer
> > and maximize it in the winter.
>
> That is exactly what I wrote it for :)

David,

great, I will look into it - many thanks for pointing me there!
(You multi-sun method also looks quite clever...)

And of course, big thanks to ingo for writing SunPos() :-)

Regards,
Philipp

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"Grassblade" <nomail@nomail> wrote:
> "Philipp" <pov### [at] phipracom> wrote:
> > Hi,

[...]

> I hate to say so, since one more Povver is obviously good in the greater
> scheme of things, ;-) but you don't need to use POV at all to do that.
> Simply calculate the angle from the object to the mirror, then determine
> where the sun should be in the sky. You may want to convert that to Right
> Ascension and Declension (or whatever they're called in English), just
> google it. Then look up ephemerides (for example
> http://cfa-www.harvard.edu/iau/MPEph/MPEph.html)

Thanks Grassblade, true, yes its basically a geometric problem.

But, the mirror has some dimensions, and also the "detector object" (the one
that waits for catching reflected sunlight) is not a point, but a larger
volume. All the objects are quite close to each other.
So, I thought, I would have to do a separate calculation at least for each
of the four extreme corner points of the mirror, each of these calculations
variated by a range of points within the detector object. (Or, think
thoroughly about which ones to leave away...)
All of that for all different sun positions.

So, I thought, I let POV do all these variations for me.

Does this make sence that way?

Regards,
Philipp

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If you are interested in just the answer and not the
rendered animation you could place the camera at
your target object location, then point it at the mirror.
Then just place something like a torus in the sky
along the path of the sun with hash marks in the texture
so you can see where the sun will pass at what time.

Place the camera at both sides of your object, you
should only have to render this twice to get the
variation in when the shadow will start passing over
the object.

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Hello,

What i understand is that you want to know if the sun will hilight something
after  a reflection in a miror.

I suggest to use the property of light which say that a beam shoot in the
opposite direction will follow the same path (in reverse).

So you can evalute the direction of the sun with two angles (may sunpos can
help) and then place an orthograhic camera in that direction, which will
look at the mirror. All the objects that will be hit by the sun will be
visible in the mirror.

Pascal

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"chaps" <cha### [at] yahoocom> wrote:
> Hello,
>
> What i understand is that you want to know if the sun will hilight something
> after  a reflection in a miror.

Yes.

> I suggest to use the property of light which say that a beam shoot in the
> opposite direction will follow the same path (in reverse).

[...]

Hi Pascal,

that is certainly a good idea!

I am working now together with a friend, we finally decided to go and use
photon mapping, and it seems to work out great. It is the more intuitive
way somehow. The mirror reflects colored light, so we see very clearly
where the mirrored light goes.
Though, we did not simulate days and hours, but rather sun positions, which
we later translate into date/time ranges using a graph - that way we need
not do so many frames that would be very similar anyway.

Thank you all for your very helpful support!

Philipp

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