Hello,

I am new in pov ray, and I want to ask a question...

I want to create a human eye, and I have managed to do the basic things, by
reading material from here and there. However, what I want to do is to have
a precise model of the optics of the eye. For example, the cornea is not one
layer, but two layers with different refraction indexes.

So, my question is:

I want that the light refracts in the interface between the two layers. If I
use merge, will this be done? I know that merge removes the inner surfaces,
but in this case we are talking about different refraction indexes...
If I use union, will it just put (e.g) one sphere in the other, with the
internal sphere keeping all the refraction characteristics?

Thank you for your answer, sorry if it was answered somewhere else also...

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"bergy" <chr### [at] gmailcom> wrote:
> Hello,
>
> I am new in pov ray, and I want to ask a question...
>
> I want to create a human eye, and I have managed to do the basic things, by
> reading material from here and there. However, what I want to do is to have
> a precise model of the optics of the eye. For example, the cornea is not one
> layer, but two layers with different refraction indexes.
>
> So, my question is:
>
> I want that the light refracts in the interface between the two layers. If I
> use merge, will this be done? I know that merge removes the inner surfaces,
> but in this case we are talking about different refraction indexes...
> If I use union, will it just put (e.g) one sphere in the other, with the
> internal sphere keeping all the refraction characteristics?
>
> Thank you for your answer, sorry if it was answered somewhere else also...


Union a sphere slightly smaller than the inner surface of the cornea for the
Vitreous Humour. (To avoid coincidental surfaces)

Eye, Aqueous humour 1.33
Eye, Cornea 1.38
Eye, Lens 1.41
Eye, Vitreous humour 1.34

Stephen

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"Stephen" <mcavoys_AT_aolDOT.com> wrote:
> "bergy" <chr### [at] gmailcom> wrote:
> > Hello,
> >
> > I am new in pov ray, and I want to ask a question...
> >
> > I want to create a human eye, and I have managed to do the basic things, by
> > reading material from here and there. However, what I want to do is to have
> > a precise model of the optics of the eye. For example, the cornea is not one
> > layer, but two layers with different refraction indexes.
> >
> > So, my question is:
> >
> > I want that the light refracts in the interface between the two layers. If I
> > use merge, will this be done? I know that merge removes the inner surfaces,
> > but in this case we are talking about different refraction indexes...
> > If I use union, will it just put (e.g) one sphere in the other, with the
> > internal sphere keeping all the refraction characteristics?
> >
> > Thank you for your answer, sorry if it was answered somewhere else also...
>

> Union a sphere slightly smaller than the inner surface of the cornea for the
> Vitreous Humour. (To avoid coincidental surfaces)

> Eye, Aqueous humour 1.33
> Eye, Cornea 1.38
> Eye, Lens 1.41
> Eye, Vitreous humour 1.34
>
> Stephen

Thanks, Stephen, I'll do that...

I've put the one sphere inside the other with union, it seems ok, but I'll
check your approach for better...

Christos

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Stephen <mcavoys_AT_aolDOT.com> wrote:
> Don?t use Merge, use Difference to subtract the spheres for the cornea. Then
> Union a sphere slightly smaller than the inner surface of the cornea for the
> Vitreous Humour. (To avoid coincidental surfaces)

  Are you sure it makes a difference to use an outer layer consisting of
the difference of two spheres, with a third inner sphere inside it,
compared to just putting one sphere inside the other?

-- 
                                                          - Warp

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
> Stephen <mcavoys_AT_aolDOT.com> wrote:
> > Don?t use Merge, use Difference to subtract the spheres for the cornea. Then
> > Union a sphere slightly smaller than the inner surface of the cornea for the
> > Vitreous Humour. (To avoid coincidental surfaces)
>
>   Are you sure it makes a difference to use an outer layer consisting of
> the difference of two spheres, with a third inner sphere inside it,
> compared to just putting one sphere inside the other?
>
> --
>                                                           - Warp

There seems to be a slight difference in the refraction at the edges of the
inner sphere.  Which one is correct, I don't know for certain, but if you
try with two spheres of the same ior, the one without the difference looks
correct, i.e., looks like a solid sphere of continuous ior, while the
differenced one has refraction that is inconsistent.  When using fresnel
reflection, the differenced one will also give incorrect reflections on the
inner sphere, i.e., too intense.  Based on these observations, I think that
it is more correct to 'embed' the second sphere inside the first without
differencing.  Perhaps someone who knows the inner working of how POV
handles refraction has some better information on this.

-tgq

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
> Stephen <mcavoys_AT_aolDOT.com> wrote:
> > Don?t use Merge, use Difference to subtract the spheres for the cornea. Then
> > Union a sphere slightly smaller than the inner surface of the cornea for the
> > Vitreous Humour. (To avoid coincidental surfaces)
>
>   Are you sure it makes a difference to use an outer layer consisting of
> the difference of two spheres, with a third inner sphere inside it,
> compared to just putting one sphere inside the other?
>
> --
>                                                           - Warp


know, and my assumption was, since bergy wanted to have a precise model of




Stephen

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"Trevor G Quayle" <Tin### [at] hotmailcom> wrote:
> Warp <war### [at] tagpovrayorg> wrote:
> > Stephen <mcavoys_AT_aolDOT.com> wrote:
> > > Don?t use Merge, use Difference to subtract the spheres for the cornea. Then
> > > Union a sphere slightly smaller than the inner surface of the cornea for the
> > > Vitreous Humour. (To avoid coincidental surfaces)
> >
> >   Are you sure it makes a difference to use an outer layer consisting of
> > the difference of two spheres, with a third inner sphere inside it,
> > compared to just putting one sphere inside the other?
> >
> > --
> >                                                           - Warp
>
> There seems to be a slight difference in the refraction at the edges of the
> inner sphere.  Which one is correct, I don't know for certain, but if you
> try with two spheres of the same ior, the one without the difference looks
> correct, i.e., looks like a solid sphere of continuous ior, while the
> differenced one has refraction that is inconsistent.  When using fresnel
> reflection, the differenced one will also give incorrect reflections on the
> inner sphere, i.e., too intense.  Based on these observations, I think that
> it is more correct to 'embed' the second sphere inside the first without
> differencing.  Perhaps someone who knows the inner working of how POV
> handles refraction has some better information on this.
>
> -tgq

I think I have found the answer to why these are different.  In general, the
refraction path is almost identical:
ior: 1.0-1.5-1.5-1.5-1.0 for embedded vs
ior: 1.0-1.5-1.0-1.5-1.0-1.5-1.0 for differenced
at each object interface.  In the differenced one, the extra 1.5-1.0-1.5
transition takes place over a very small distance and can be neglected,
effectively giving it the same properties as the embedded.  However, the
problem arises when the 1.5-1.0 transition is beyond the total internal
refraction angle.  The ray erroneously gets reflected and remains inside
the outer object and it never passes through the tiny 'gap' into the inner
object.

Now, both these are valid situations.  If the objects are such that there
actually is a tiny air gap, then the differenced one is correct, however,
for objects without the gap, where the transition is directly from the
outer to the inner material (i.e., they are in contact), then the embedded
one is correct.
Consider two pieces of glass lying on top of one another in real life.  The
interface is quite noticeable if the glass is dry (i.e., have a gap) vs if
there is a thin layer of water essentially making them in direct contact.

Most glazed or embedded objects with act like the second situation where the
materials are in direct contact.  This is especially true when simulating a
varying ior via layers of objects like the eye model, you definitely do not
want to introduce the air gap.

-tgq

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