Hi all

I have an application for POVRAY that seems a little different from what
most users are doing, and it doesn't seem to be something that's directly
addressed in the documentation, so I wonder if anyone could offer any
suggestions.

I'm trying to assess the imaging properties of a number of reflectors. This
means that I want to create some geometry for a set of mirrors including
scattering,  geometric surface imperfections, etc, then I want to create a
light source in the form of a 'sun shape', then I want to see what the
resulting light intensity at my target plane becomes.

The normal use of POVRAY seems to revolve around the idea of a 'camera'
which is a single point 'observer'; Raytracing in this case means tracing
*backwards* from the eye of the beholder to the different spots in the
scene.

My use seems a bit different: I want to have an imaging plane and I want to
'catch' rays over that plane. The overall result should be data about the
intensity of light striking various points on my imaging (target) plane.

Obviously I can approximate this analysis by creating an imaging plane, and
then creating a camera that looks at that imaging plane. This won't always
be practical however, as sometimes the act of creating a visible barrier at
the imaging plane will cause the optics of the problem to change, eg by
creating repeated reflections when none are desired.

So, can anyone offer any suggestions on how one my start out with using
POVRAY in this case?

Finally, is there any way that POVRAY can generate output that shows the
location of rays in 3-D space? For example if I create a ray-trace any only
wish to trace a couple of hundred rays, is there any way I can view the
paths taken by those rays through my scene?

Cheers
JP
joh### [at] anuremovethisrubbisheduau

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"John Pye" <nomail@nomail> wrote:
> Hi all
>
> I have an application for POVRAY that seems a little different from what
> most users are doing, and it doesn't seem to be something that's directly
> addressed in the documentation, so I wonder if anyone could offer any
> suggestions.
>
> I'm trying to assess the imaging properties of a number of reflectors. This
> means that I want to create some geometry for a set of mirrors including
> scattering,  geometric surface imperfections, etc, then I want to create a
> light source in the form of a 'sun shape', then I want to see what the
> resulting light intensity at my target plane becomes.
>
> The normal use of POVRAY seems to revolve around the idea of a 'camera'
> which is a single point 'observer'; Raytracing in this case means tracing
> *backwards* from the eye of the beholder to the different spots in the
> scene.
>
> My use seems a bit different: I want to have an imaging plane and I want to
> 'catch' rays over that plane. The overall result should be data about the
> intensity of light striking various points on my imaging (target) plane.
>
> Obviously I can approximate this analysis by creating an imaging plane, and
> then creating a camera that looks at that imaging plane. This won't always
> be practical however, as sometimes the act of creating a visible barrier at
> the imaging plane will cause the optics of the problem to change, eg by
> creating repeated reflections when none are desired.
>
> So, can anyone offer any suggestions on how one my start out with using
> POVRAY in this case?
>
> Finally, is there any way that POVRAY can generate output that shows the
> location of rays in 3-D space? For example if I create a ray-trace any only
> wish to trace a couple of hundred rays, is there any way I can view the
> paths taken by those rays through my scene?


Hi John, I don't have a complete answer, but one idea came to mind.
POV-Ray's 'radiosity' feature traces rays from an object out into the scene
after the camera ray hits it. POV-Ray 'Radiosity' applies to diffuse
objects, and specular reflection is already handled by reflective objects.
This seems to me related to what you're looking for.

Render on,
George

---
MegaPOV XRS: render faster than light!
http://www.gammaburst.net/xrs

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Wasn't it John Pye who wrote:
>Hi all
>
>I have an application for POVRAY that seems a little different from what
>most users are doing, and it doesn't seem to be something that's directly
>addressed in the documentation, so I wonder if anyone could offer any
>suggestions.
>
>I'm trying to assess the imaging properties of a number of reflectors. This
>means that I want to create some geometry for a set of mirrors including
>scattering,  geometric surface imperfections, etc, then I want to create a
>light source in the form of a 'sun shape', then I want to see what the
>resulting light intensity at my target plane becomes.
>
>The normal use of POVRAY seems to revolve around the idea of a 'camera'
>which is a single point 'observer'; Raytracing in this case means tracing
>*backwards* from the eye of the beholder to the different spots in the
>scene.
>
>My use seems a bit different: I want to have an imaging plane and I want to
>'catch' rays over that plane. The overall result should be data about the
>intensity of light striking various points on my imaging (target) plane.

That sounds very much like casting photons, which POV-Ray can do. It's
typically used for getting correct shadows cast by curved refractive
objects.


>
>Obviously I can approximate this analysis by creating an imaging plane, and
>then creating a camera that looks at that imaging plane. This won't always
>be practical however, as sometimes the act of creating a visible barrier at
>the imaging plane will cause the optics of the problem to change, eg by
>creating repeated reflections when none are desired.

You can switch off those reflections by specifying no_reflection for
your image plane object. The plane will be visible to the camera but it
won't be reflected.

You might also want to also specify no_shadow if there's any possibility
of the image plane interfering with the optics by casting shadows onto
the geometry.

>Finally, is there any way that POVRAY can generate output that shows the
>location of rays in 3-D space? For example if I create a ray-trace any only
>wish to trace a couple of hundred rays, is there any way I can view the
>paths taken by those rays through my scene?

There's no built in facility for doing that, but some of the concepts
described in the SDL Raytracer tutorial in the documentation. [In the
3.6 documentation it's "2.3.10  SDL tutorial: A raytracer"]. Some of the
SDL ray tracing code might be what you need to calculate the paths of
some rays, then you could indicate the paths with thin cylinders.

-- 
Mike Williams
Gentleman of Leisure

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John Pye nous apporta ses lumieres en ce 07/12/2006 06:45:
> Hi all

> I have an application for POVRAY that seems a little different from what
> most users are doing, and it doesn't seem to be something that's directly
> addressed in the documentation, so I wonder if anyone could offer any
> suggestions.

> I'm trying to assess the imaging properties of a number of reflectors. This
> means that I want to create some geometry for a set of mirrors including
> scattering,  geometric surface imperfections, etc, then I want to create a
> light source in the form of a 'sun shape', then I want to see what the
> resulting light intensity at my target plane becomes.
If you use the various primitives, you have mathematicaly ideal shapes. You can 
simulate imperfections using normals. You can simulate imperfectly polished 
mirrors, or lences, by averaging some normals.

> The normal use of POVRAY seems to revolve around the idea of a 'camera'
> which is a single point 'observer'; Raytracing in this case means tracing
> *backwards* from the eye of the beholder to the different spots in the
> scene.

> My use seems a bit different: I want to have an imaging plane and I want to
> 'catch' rays over that plane. The overall result should be data about the
> intensity of light striking various points on my imaging (target) plane.

> Obviously I can approximate this analysis by creating an imaging plane, and
> then creating a camera that looks at that imaging plane. This won't always
> be practical however, as sometimes the act of creating a visible barrier at
> the imaging plane will cause the optics of the problem to change, eg by
> creating repeated reflections when none are desired.
Add "no_reflection" and "no_shadow". This will effectively create a virtual 
object than will receive any "image" but wont affect light and be invisible in 
any reflection.

> So, can anyone offer any suggestions on how one my start out with using
> POVRAY in this case?

> Finally, is there any way that POVRAY can generate output that shows the
> location of rays in 3-D space? For example if I create a ray-trace any only
> wish to trace a couple of hundred rays, is there any way I can view the
> paths taken by those rays through my scene?
This can be done using some parallel, cylindrical light sources that you can 
colour to enable you to differienciate them. You can also use a mask pierced by 
a collection of holes. It's beter to use a composite mask if you want a large 
number of holes: small boxes pierced by a single hole stacked in an array.
You will also need to add some media so you can see the paths.

> Cheers
> JP
> joh### [at] anuremovethisrubbisheduau

The feature you want is called "photons". It's used for forward tracing of light 
and it's effects. Mike William made an error: photons are NOT usefull for the 
casting of realistic shadows, but for the realistic rendering of caustics 
effects as well as realistic opticaly active objects.

-- 
Alain
-------------------------------------------------
For THIS I bought a computer?

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"John Pye" <nomail@nomail> wrote:
>
> Finally, is there any way that POVRAY can generate output that shows the
> location of rays in 3-D space? For example if I create a ray-trace any only
> wish to trace a couple of hundred rays, is there any way I can view the
> paths taken by those rays through my scene?

For this you might try scattering media, although it can *really* slow
things
down.

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> The normal use of POVRAY seems to revolve around the idea of a 'camera'
> which is a single point 'observer'; Raytracing in this case means tracing
> *backwards* from the eye of the beholder to the different spots in the
> scene.
>
> My use seems a bit different: I want to have an imaging plane and I want
to
> 'catch' rays over that plane. The overall result should be data about the
> intensity of light striking various points on my imaging (target) plane.

The best way to see what light is hitting a plane is to actually put a plane
there with a white pigment, and set up a camera to look at it. In order to
get the best approximation of the light hitting it, you will likely want to
use photons on refracting or reflecting objects, and possibly radiosity to
gather any other ambient light. The only difficulty you may have is that
light incoming at a shallow angle will have less of an effect than light
approaching perpendicularly.

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

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