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Le_Forgeron schrieb:
>> The 4th component, called filter, specifies the amount of filtered
>> transparency of a substance. Some real-world examples of filtered
>> transparency are stained glass windows or tinted cellophane.
>> ...
>> The 5th component, called transmit, specifies the amount of non-filtered
>> light that is transmitted through a surface. Some real-world examples of
>> non-filtered transparency are thin see-through cloth, fine mesh netting
>> and dust on a surface.
>> ...
>
> I would expect transmit to not suffer ior, whereas filter to do. [see
> later **]
Not really: The see-through cloth and similar stuff mental picture of
what transmit actually does only work well when dealing with sufaces
only - in that case the question whether transmit suffers ior is moot.
Actually, filter doesn't suffer ior either: It is the light attenuated
by them that suffers refraction.
> For filter handling, look at source/backend/render/trace.cpp, circa line
> 928 :
Hold it! --
Don't look at the code if you want to find out how things *should* be
modeled :-P
You know why both filter and transmit are there?
Because a single coefficient is not enough to describe transparency.
Now Gues why that is?
Because strictly speaking it would require *three* coefficients: Red,
blue and green transparency :-P
With those three, you could do everything filter and transmit do right
now - plus some.
And when going for spectral math, it would require as many coefficients
as there are wavelengths: Each could be attenuated differently.
> The fishy thing here seems to be that the same colour for reflection is
> used for refraction/filter.
Not really: Normally, when transparency is involved, you get
straightforward "white" reflection. This is different when metallic
materials are involved, but those are not transparent - unless there are
small holes in it, in which case the transparent color is "white" (as
modelled by transmit).
(There are a few exceptions to this, but as far as I know those are
rare. iridescence is such an effect.)
> I might wish for a second Transmit, straight line for incoming rays, for
> at least semi-refracting objects (real see-thru when ior come into play)
What real-world effect would that model?
>> But do there exist any (practically relevant) effects where
>> a photon loses or gains energy without being scattered quite randomly?
>
> The notion of diffuse refraction (sort of) could be a perturbation of
> the normal, or some integration of a sampled cone...
That has nothing to do with my question.
There's also no such thing as diffuse refraction - there is only diffuse
transparency, caused by either (a) as you say pertubations of the
normal, or (b) volumetric scattering in the material.
> For straight refraction, I know at least of laser-protection filter.
>
> And whether a laser-beam is obtained from excitation of peripheral
> (reflection) or of internal (refraction)
I have no idea what you're saying here.
> For the SDL, at least it would allow a refraction on a black reflecting
> glass.
??
> Now, the real question might be to modelise the refraction at the
> surface, or in the media only ?
Refraction only occurs at surfaces.
Do you perhaps mean attenuation of refracted light? For that, POV-Ray
provides both the "filter" color, as well as the interior fading. The
one would be suitable to model coatings made of semi-transparent
material, while the other would model large volumes of semi-transparent
material.
> And should we bother for pleochroism ? what about birefringence ?
A bit of peochroism would be neat. Some 6 instead of 3 color channels
would be a start. Plus a UV channel of course.
> (from wikipedia, handle with care: Cotton (Gossypium hirsutum) fiber is
> birefringent because of high levels of cellulosic material in the
> fiber's secondary cell wall.
>
> If Cotton is not everywhere... )
As it is typically used in fibrous form, the birefringence effect is of
no importance whatsoever at the scales of usual scenes.
> Maybe that's a part for media, not pigment.
Birefringence is definitively neither a media nor pigment thing, but a
straightforward interior thing, and would have to join ranks with ior
and dispersion (as a matter of fact it's a difference in ior depending
on polarization with respect to the "optical axis" of the (AFAIK
necessarily) crystalline material).
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clipka <ano### [at] anonymous org> wrote:
> You know why both filter and transmit are there?
>
> Because a single coefficient is not enough to describe transparency.
>
> Now Gues why that is?
>
> Because strictly speaking it would require *three* coefficients: Red,
> blue and green transparency :-P
>
> With those three, you could do everything filter and transmit do right
> now - plus some.
>
> And when going for spectral math, it would require as many coefficients
> as there are wavelengths: Each could be attenuated differently.
This makes a good case for actually stripping filter and transmit out of the SDL
entirely (or at least deprecating them) and throwing in a separate
"transparency" keyword under pigments.
....especially if we _are_ going to have spectral coloring/lighting. (For the
time being, I'm leaning towards an implementation that's strictly peak/width
pairs. It'd be extended later, but it's at least a start towards a "good"
system for this.)
> > For the SDL, at least it would allow a refraction on a black reflecting
> > glass.
>
> ??
Obsidian, at present, is one of the more pain-in-the-neck RL substances to
model. I assume this is what he's trying to get at, but I could be wrong.
> > And should we bother for pleochroism ? what about birefringence ?
>
> A bit of p[l]eochroism would be neat. Some 6 instead of 3 color channels
> would be a start. Plus a UV channel of course.
Pleochroism pretty well falls under the requests for an AOI pigment pattern,
doesn't it? :-D
> > Maybe that's a part for media, not pigment.
>
> Birefringence is definitively neither a media nor pigment thing, but a
> straightforward interior thing, and would have to join ranks with ior
> and dispersion (as a matter of fact it's a difference in ior depending
> on polarization with respect to the "optical axis" of the (AFAIK
> necessarily) crystalline material).
Dispersion is an ugly hack at present, just like iridescence. :-D (Technically
it should be implemented as the spectral variant of ior... yet another thing on
The List (tm) for this.) Birefringence seems like it'd require an extension to
ior as well (similar to the current two-value reflection form - specify an axis
of anisotropy and a second ior), though maybe I'm loading a bit much on the ior
keyword now...
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MDenham schrieb:
> This makes a good case for actually stripping filter and transmit out of the SDL
> entirely (or at least deprecating them) and throwing in a separate
> "transparency" keyword under pigments.
Indeed - though it might be more correct to re-use the "transmit"
keyword for that purpose.
> ....especially if we _are_ going to have spectral coloring/lighting. (For the
> time being, I'm leaning towards an implementation that's strictly peak/width
> pairs. It'd be extended later, but it's at least a start towards a "good"
> system for this.)
For the time being, I guess we'll have to do with RGB for quite a while
still :-)
> Obsidian, at present, is one of the more pain-in-the-neck RL substances to
> model. I assume this is what he's trying to get at, but I could be wrong.
It appears to me that this would be due to the blackness not being a
surface effect at all, and would need subsurface scattering or (probably
more efficient in this case) media to be modelled properly.
>>> And should we bother for pleochroism ? what about birefringence ?
>> A bit of p[l]eochroism would be neat. Some 6 instead of 3 color channels
>> would be a start. Plus a UV channel of course.
>
> Pleochroism pretty well falls under the requests for an AOI pigment pattern,
> doesn't it? :-D
Yes, that would probably be an alternative in some semi-rare cases.
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clipka wrote:
>> Maybe that's a part for media, not pigment.
>
> Birefringence is definitively neither a media nor pigment thing, but a
> straightforward interior thing, and would have to join ranks with ior
> and dispersion (as a matter of fact it's a difference in ior depending
> on polarization with respect to the "optical axis" of the (AFAIK
> necessarily) crystalline material).
How hard would it be to implement birefringence ...? Seems like it could
easily be faked
Syntax would be trivial:
ior 1.5, 1.7 //(birefringent material)
You don't really need to care about the polarization of light, since
light in POVRay doesn't have polarity anyway.
I would imagine tracing a second ray at a different IOR would be rather
simple to add.
--
~Mike
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Mike Raiford <"m[raiford]!at"@gmail.com> wrote:
> clipka wrote:
>
> >> Maybe that's a part for media, not pigment.
> >
> > Birefringence is definitively neither a media nor pigment thing, but a
> > straightforward interior thing, and would have to join ranks with ior
> > and dispersion (as a matter of fact it's a difference in ior depending
> > on polarization with respect to the "optical axis" of the (AFAIK
> > necessarily) crystalline material).
>
> How hard would it be to implement birefringence ...? Seems like it could
> easily be faked
>
> Syntax would be trivial:
>
> ior 1.5, 1.7 //(birefringent material)
>
> You don't really need to care about the polarization of light, since
> light in POVRay doesn't have polarity anyway.
>
> I would imagine tracing a second ray at a different IOR would be rather
> simple to add.
The problem is that you still need to know two other pieces of information:
1) the axis of anisotropy
2) the "ordinary" axis
It turns into a useful (though limited) form of the otherwise not-gonna-happen
"variable ior" request thanks to this. (Generalized variable ior isn't going
to happen for reasons that are mentioned in the documentation, such as
"pockets" of high IOR - or even not-so-high IOR - having potentially
pathological behavior. You'd need to switch to a solely
rays-start-at-light-sources approach - photons, only slower - to make this even
slightly viable, and that kind of rewrite probably isn't worth it. [If it turns
out that it's necessary for a "good" spectral system, then we may have issues.
:-D])
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MDenham wrote:
>
> 1) the axis of anisotropy
> 2) the "ordinary" axis
>
> It turns into a useful (though limited) form of the otherwise not-gonna-happen
> "variable ior" request thanks to this. (Generalized variable ior isn't going
> to happen for reasons that are mentioned in the documentation, such as
> "pockets" of high IOR - or even not-so-high IOR - having potentially
> pathological behavior. You'd need to switch to a solely
> rays-start-at-light-sources approach - photons, only slower - to make this even
> slightly viable, and that kind of rewrite probably isn't worth it. [If it turns
> out that it's necessary for a "good" spectral system, then we may have issues.
> :-D])
>
Ah, and here I was thinking it was just as simple as tracing a second
refraction ray.
Bummer.
--
~Mike
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> clipka wrote:
>> Hey, get real: You need to press the "shift" key for quite a host of
>> things, I guess. As a German, for instance, I need to press "shift" to
>> get a multiplication asterisk, dividing slash, opening or closing
>> parentheses, quotes etc,
>
> And for all nouns too, right? :)
>
On my side, all the followings need two keys:
\!@#$%?&*()_ (Shift +...)
My Alt Char key is worn smooth...
Alain
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>
> Wondering, is ambient light (global setting) in rgb or just gray ?
>
It's a full rgb value. Whenever you provide only a single float
(ambient_lights 1), it get's promoted to a color vector (ambient_lights
<1, 1, 1>)
Setting "ambient_lights <1, 0.2, 5>" is perfectly valid.
Alain
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> clipka wrote:
>
>>> Maybe that's a part for media, not pigment.
>>
>> Birefringence is definitively neither a media nor pigment thing, but a
>> straightforward interior thing, and would have to join ranks with ior
>> and dispersion (as a matter of fact it's a difference in ior depending
>> on polarization with respect to the "optical axis" of the (AFAIK
>> necessarily) crystalline material).
>
> How hard would it be to implement birefringence ...? Seems like it could
> easily be faked
>
> Syntax would be trivial:
>
> ior 1.5, 1.7 //(birefringent material)
>
> You don't really need to care about the polarization of light, since
> light in POVRay doesn't have polarity anyway.
>
> I would imagine tracing a second ray at a different IOR would be rather
> simple to add.
>
In addition of the second ior, you also need an axis or "direction".
If you take a sphere of birefringent material and make it turn, the
difference in refractions changes. It also changes depending on the
distance from the view axis.
There is a plane where both iors are the same and an axis where they are
the most different, usualy. They are frequently perpendicular, but I
think that there are cases where the axis is not normal to the plane...
I think that you "may" attempt to simulate it. Some experimentation in
view. Some layering and/or averaging would be required.
Alain
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Alain <aze### [at] qwertyorg> wrote:
> > clipka wrote:
> >
> >>> Maybe that's a part for media, not pigment.
> >>
> >> Birefringence is definitively neither a media nor pigment thing, but a
> >> straightforward interior thing, and would have to join ranks with ior
> >> and dispersion (as a matter of fact it's a difference in ior depending
> >> on polarization with respect to the "optical axis" of the (AFAIK
> >> necessarily) crystalline material).
> >
> > How hard would it be to implement birefringence ...? Seems like it could
> > easily be faked
> >
> > Syntax would be trivial:
> >
> > ior 1.5, 1.7 //(birefringent material)
> >
> > You don't really need to care about the polarization of light, since
> > light in POVRay doesn't have polarity anyway.
> >
> > I would imagine tracing a second ray at a different IOR would be rather
> > simple to add.
> >
> In addition of the second ior, you also need an axis or "direction".
> If you take a sphere of birefringent material and make it turn, the
> difference in refractions changes. It also changes depending on the
> distance from the view axis.
> There is a plane where both iors are the same and an axis where they are
> the most different, usualy. They are frequently perpendicular, but I
> think that there are cases where the axis is not normal to the plane...
If the "axis" isn't normal to the plane, you're working with something with at
least two axes of anisotropy. (This would be a _much_ harder case to work with.
Uniaxial birefringence is probably doable; trying to write it to handle two or
more axes will result in programmers chasing you with hatchets. :-D)
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