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1 Dec 2022 00:53:32 EST (-0500)
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From: Thomas de Groot
Subject: Re: Transparent Objects Become Black
Date: 18 Nov 2021 07:27:27
Message: <619646af@news.povray.org>
Op 17-11-2021 om 22:06 schreef Kenneth:
> Thomas de Groot <tho### [at] degrootorg> wrote:
>> Kenneth, I have not tested it here, but are the 'voids' really
>> necessary? Is it not enough to just add the bubbles to the gel, as both
>> have different ior's and do not interact?
>>
> 
> Apparently, the visual result is exactly the same either way.
> 
> Here is my own "poor man's crystal", ha-- just an intersection of three rotated
> boxes. The inclusions are simple shapes. I rendered two versions, with the
> inclusions as a UNION with the crystal vs. being DIFFERENCED from it as voids. I
> see absolutely no visual change, although I was expecting them to look
> completely different. It seems that the ior 'effect' depends only on the
> *surfaces* that the camera ray sees, not really on any imagined interior
> substance of the inclusions.
> 
Yes, that confirms my guess. You didn't say, but I suppose that the 
'union' renders faster than the 'difference'. Additionally, using blobs 
as inclusions, especially if there are many of them, will speed up 
render time.

> I have not yet tested my scene to discover how the different iors affect each
> other-- i.e., simple addition vs. multiplication of the iors. That's difficult
> to tell, visually. I suspect that it's just addition.
> 
I am not sure. It looks like that - independent from ior - the used 
media inside container and inclusions are treated separately. In my 
experiments I do no see any cross-influence, addition or multiplication, 
of colors.

> -----
> BTW, this 'multiple-ior' effect gave me an idea-- which I am experimenting
> with-- about creating what looks like *variable* ior within an object, that
> changes along its length or depth, etc. The basic idea is to pre-slice the
> object into *many* very thin box sections, each with a slightly increasing ior
> value, then re-combine the slices to look like the original solid object. Of
> course, I am running into coincident-surface problems with all of those slice
> recombinations... but I think that's a temporary problem, until I figure out how
> best to re-position them. (Should they be separated by tiny 'air' gaps-- or
> should they be slightly embedded into each other?) Anyway, my experiments look
> promising so far, if a bit...odd ;-)
> 
I am interested to see the result.


@bubble_person: sorry if this discussion 'seems' to ignore you. This is 
not the intention nor the reality; Your original question generated a 
number of issues which we are not sure about, and I think everybody will 
benefit in the end.

-- 
Thomas


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From: Alain Martel
Subject: Re: Transparent Objects Become Black
Date: 18 Nov 2021 12:28:20
Message: <61968d34$1@news.povray.org>
Le 2021-11-17 à 16:06, Kenneth a écrit :

> -----
> BTW, this 'multiple-ior' effect gave me an idea-- which I am experimenting
> with-- about creating what looks like *variable* ior within an object, that
> changes along its length or depth, etc. The basic idea is to pre-slice the
> object into *many* very thin box sections, each with a slightly increasing ior
> value, then re-combine the slices to look like the original solid object. Of
> course, I am running into coincident-surface problems with all of those slice
> recombinations... but I think that's a temporary problem, until I figure out how
> best to re-position them. (Should they be separated by tiny 'air' gaps-- or
> should they be slightly embedded into each other?) Anyway, my experiments look
> promising so far, if a bit...odd ;-)
> 
> 

Try clipped_by to make your slices. That way, there will be no internal 
boundaries.


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From: Kenneth
Subject: Re: Transparent Objects Become Black
Date: 18 Nov 2021 17:30:00
Message: <web.6196d33228696f324cef624e6e066e29@news.povray.org>
Alain Martel <kua### [at] videotronca> wrote:

> >
> > .. Of course, I am running into coincident-surface problems with all of
> > those slice recombinations... but I think that's a temporary problem, until
> > I figure out how best to re-position them.
>
> Try clipped_by to make your slices. That way, there will be no internal
> boundaries.

I didn't even think of clipped_by. Thanks, I'll give that a try.


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From: Kenneth
Subject: Re: Transparent Objects Become Black
Date: 18 Nov 2021 17:50:00
Message: <web.6196d6cc28696f324cef624e6e066e29@news.povray.org>
Thomas de Groot <tho### [at] degrootorg> wrote:
>
> You didn't say, but I suppose that the
> 'union' renders faster than the 'difference'. Additionally, using blobs
> as inclusions, especially if there are many of them, will speed up
> render time.

My current machine is so fast that I didn't even notice a difference in render
times! But I went back and noted the times: 2.067 sec for union, 2.138 sec for
difference. So yes, the union is faster. BTW, I have almost never experimented
with blobs(!); I need to do that. I still find it amazing that they render
faster in a 'difference' than even simple spheres do-- because I assume that
they are far more complex as math constructs.

BTW, I forgot to mention that my previous image post did not include media,
although I plan to experiment with that addition as well.

There is an interesting difference in how POV-ray treats media vs. ior regarding
embedded objects, which might be unexpected to a new user: Embedded objects with
different iors show the ior changes, whereas embedded media objects do NOT show
their different
                    media method...
                    intervals...
                    samples...
For media, the only values recognized for those settings are from the first
object 'surface' that the camera ray 'sees'-- which is the larger container
object. The inner-object values, if different, are ignored, and take on the
outer object values. It might be assumed that the ior and media 'mechanisms' --
both being interior{...} attributes-- would work the same way, but they don't.

> > [Kenneth wrote:]
> > I have not yet tested my scene to discover how the different iors
> > affect each other-- i.e., simple addition vs. multiplication of the
> > iors. That's difficult to tell, visually. I suspect that it's
> > just addition.

This situation is not quite what I originally thought, although it *is* simple
addition, in a different form:  From further tests, I see that the different
iors interact like this example:
              crystal ior (the container): 1.2
              inclusion ior: 1.2
When they are the *same*, the inclusions do not show any 'additional' ior effect
-- like an additional 0.2 for themselves, above the container's 1.2-- but just
the overall ior of 1.2... as if the container and the inclusions are all "made
from the same material" (whether as a union or a difference).  I guess that I
should have expected this-- it seems obvious now-- although I originally thought
that the inclusions would show 1.2 + 0.2 = 1.4.

BTW, I also discovered something interesting along the way:
An ior of 0.0 is the same as ior 1.0. This has actually confused me in the
past-- because 'negative' iors ( < 1.0 and even < 0.0) can be used for
interesting effects, but there is this unexpected  'glitch' at exactly 0.0.


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Attachments:
Download 'identical_iors_jpeg_file.jpg' (75 KB)

Preview of image 'identical_iors_jpeg_file.jpg'
identical_iors_jpeg_file.jpg


 

From: Thomas de Groot
Subject: Re: Transparent Objects Become Black
Date: 19 Nov 2021 04:21:03
Message: <61976c7f$1@news.povray.org>
I adapted the script to what I think is a comprehensive scene. I attach 
it here for everybody interested in fairly complex nested interiors. I 
am sure further improvements can be made, which I leave to you.

In p.b.i I post an image of this scene file.

-- 
Thomas


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Attachments:
Download 'test-script.pov.txt' (10 KB)

From: Alain Martel
Subject: Re: Transparent Objects Become Black
Date: 19 Nov 2021 13:30:15
Message: <6197ed37$1@news.povray.org>
Le 2021-11-18 à 17:48, Kenneth a écrit :
               inclusion ior: 1.2
> When they are the *same*, the inclusions do not show any 'additional' ior effect
> -- like an additional 0.2 for themselves, above the container's 1.2-- but just
> the overall ior of 1.2... as if the container and the inclusions are all "made
> from the same material" (whether as a union or a difference).  I guess that I
> should have expected this-- it seems obvious now-- although I originally thought
> that the inclusions would show 1.2 + 0.2 = 1.4.
> 
> BTW, I also discovered something interesting along the way:
> An ior of 0.0 is the same as ior 1.0. This has actually confused me in the
> past-- because 'negative' iors ( < 1.0 and even < 0.0) can be used for
> interesting effects, but there is this unexpected  'glitch' at exactly 0.0.
> 
> 

The refraction depends on the ratio of the iors on each sides of the 
surface. What you see is what is expected and observed in nature.

I've experienced with negative ior. That makes the surface behave as if 
it had
finish{ reflection 1 }

When you think about it, it makes sense. The ior is the ratio of the 
speed of light in a vacuum divided by the speed of light in a medium.
A negative ior demand a negative speed of light in that medium.


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From: Kenneth
Subject: Re: Transparent Objects Become Black
Date: 21 Nov 2021 16:15:00
Message: <web.619ab60d28696f324cef624e6e066e29@news.povray.org>
Alain Martel <kua### [at] videotronca> wrote:
>
> I've experienced with negative ior. That makes the surface behave as if
> it had
> finish{ reflection 1 }
>

That's an interesting observation. With my own tests here, I was not sure a
'negative' ior was having any effect when lower than 0.5-- until I re-positioned
the crystal and camera 'just so'. Then I could see at least my sphere inclusion
subtly changing its appearance as the ior decreased. But as a practical matter,
the more negative it goes, the less of a visual change there is.


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From: m@b
Subject: Re: Transparent Objects Become Black
Date: 21 Nov 2021 20:16:02
Message: <619aef52$1@news.povray.org>
On 22/11/2021 5:11 am, Kenneth wrote:
> Alain Martel <kua### [at] videotronca> wrote:
>>
>> I've experienced with negative ior. That makes the surface behave as if
>> it had
>> finish{ reflection 1 }
>>
> 
> That's an interesting observation. With my own tests here, I was not sure a
> 'negative' ior was having any effect when lower than 0.5-- until I re-positioned
> the crystal and camera 'just so'. Then I could see at least my sphere inclusion
> subtly changing its appearance as the ior decreased. But as a practical matter,
> the more negative it goes, the less of a visual change there is.
> 


In this animation I cycle the ior of a glass sphere from 1.6 to zero and 
back:

<https://youtu.be/gUzp0Bc6u1I>

Probably not relevant :-)


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From: William F Pokorny
Subject: Re: Transparent Objects Become Black
Date: 22 Nov 2021 05:38:49
Message: <619b7339$1@news.povray.org>
On 11/21/21 4:11 PM, Kenneth wrote:
> Alain Martel <kua### [at] videotronca> wrote:
>>
>> I've experienced with negative ior. That makes the surface behave as if
>> it had
>> finish{ reflection 1 }
>>
> 
> That's an interesting observation. With my own tests here, I was not sure a
> 'negative' ior was having any effect when lower than 0.5-- until I re-positioned
> the crystal and camera 'just so'. Then I could see at least my sphere inclusion
> subtly changing its appearance as the ior decreased. But as a practical matter,
> the more negative it goes, the less of a visual change there is.
> 

FWIW. The ior value ripples into many finish features and how it is used 
in any given feature depend upon additional finish settings and the 
version of POV-Ray - v3.7, v3.8 releases and the (v4.0 master branch) 
all differ somewhat. For example, fresnel being on in a many v3.8 finish 
features will cause the negative negative ior values to be treated as 
positive ones. In other words, only the ior magnitude then matters.

Also FWIW. In the most recent povr branch tarball there is a file called 
albedoFresnelStory.txt in the doc directory with a great deal of finish 
related detail - should anyone be interested. It was started as an 
attempt at albedo documentation I could understand and intended to be a 
wiki subject for the v3.8 release. As I dug, it grew and grew - and got 
more and more complicated. I changed the aim to be a newsgroup post at 
some point - but I've never posted it.

The file does represent my current understanding of the state of many 
finish features in v3.7/v3.8/v4.0. To a significant degree it explains 
why I'm playing with certain changes in my povr branch.

Anyhow...

Bill P.


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From: Kenneth
Subject: Re: Transparent Objects Become Black
Date: 22 Nov 2021 14:50:00
Message: <web.619bf36428696f324cef624e6e066e29@news.povray.org>
"m@b" <sai### [at] googlemailcom> wrote:
>
> In this animation I cycle the ior of a glass sphere from 1.6 to zero and
> back:
>
> <https://youtu.be/gUzp0Bc6u1I>
>

Nicely done!

Are you sure that your sphere's ior goes to exactly 0.0? I am not seeing the
'glitch' that I mentioned earlier, where ior 0.0 = ior 1.0. I suspect that your
animation code gets the ior *very close* to zero, but then cycles back.


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