Has anyone ever seen that funky transparent plastic that turns rainbow
colours when you try to bend it?

  1) does anyone know how this stuff works?
  2) can it be simulated with POV-Ray?

Andrew.

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In article <3dbbc275@news.povray.org>,
 "Andrew Coppin" <orp### [at] btinternetcom> wrote:

> Has anyone ever seen that funky transparent plastic that turns rainbow
> colours when you try to bend it?
> 
>   1) does anyone know how this stuff works?
>   2) can it be simulated with POV-Ray?

I don't know what you are talking about. Does it look like it gets many 
tiny cracks in it? Or does it just get an iridescent oil-film look?

One possibility is microscopic cracks that form a diffraction grating, 
like a CD. You might be able to simulate it with a normal of some kind 
and dispersion.
Another possibility is a more complex effect having to do with 
polarization...put cellophane or some other plastic between two 
polarizing filters for an example. I've seen this effect on things like 
CD jewel cases as well. You might want to look at the "irid" iridescence 
feature, it is a fast way to simulate the "oil film" or "mother of 
pearl" look.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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Right, well, I suppose I was a bit vague...

I visited the London Science Museum one time... They had this plastic stuff.
Apparently they make scale models of bridges and stuff out of it to analyse
stress. Coz, the thing is, when you view it through this clear plastic sheet
(which probably polarises or something), and when you bend it, the most
stressed parts show up rainbow colours. (The lightsource might be "special"
too - it was in a dark room.)

Andrew.

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In article <3dbc5044@news.povray.org>,
 "Andrew Coppin" <orp### [at] btinternetcom> wrote:

> I visited the London Science Museum one time... They had this plastic stuff.
> Apparently they make scale models of bridges and stuff out of it to analyse
> stress. Coz, the thing is, when you view it through this clear plastic sheet
> (which probably polarises or something), and when you bend it, the most
> stressed parts show up rainbow colours. (The lightsource might be "special"
> too - it was in a dark room.)

There was probably a polarizing filter over the light source. POV can't 
simulate this effect, but the irid feature might come close to looking 
the same.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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Andrew Coppin <orp### [at] btinternetcom> wrote:

> Has anyone ever seen that funky transparent plastic that turns rainbow
> colours when you try to bend it?
> 
>   1) does anyone know how this stuff works?

The effect is called photoelesticity, whereby the optical axis of a
material changes depending on the mechanical stress applied to it. 

It's as if each part of the material is a piece of polaroid, but its
axis of polarization changes according to how much that part is under
stress.

You get the coloured effect when you put the material between crossed
polarizers. Look, for example, at:

http://technology.open.ac.uk/materials/mem/mem-photo.html

>   2) can it be simulated with POV-Ray?

I guess it's the same kind of problem as in the polarization thread. A
lot of work!

-- 
Kaveh

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Yep, this indeed appears to be the effect I saw. But of course, POV-Ray
doesn't do polarisation. (I can imagine why!) Oh well, never mind...

Andrew.

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Andrew Coppin <orp### [at] btinternetcom> wrote:

> Yep, this indeed appears to be the effect I saw. But of course, POV-Ray
> doesn't do polarisation. (I can imagine why!) Oh well, never mind...

I am not an expert in povray, but you know, it shouldn't be difficult to
add in polarization. Not compared to the amazing things it does already!
I would love to see it some time.

-- 
Kaveh

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In article <1fkqkvf.knzuo218fc70cN%kaveh@delete_this.focalimage.com>,
 kav### [at] delete_thisfocalimagecom (Kaveh) wrote:

> I am not an expert in povray, but you know, it shouldn't be difficult to
> add in polarization. Not compared to the amazing things it does already!

It would be very difficult and complex. It would basically require 
changing the entire way colors are handled, storing information on the 
amount of light of each color at each polarization angle, to do it well 
would probably require using frequency distributions to describe light 
color...building a raytracer from scratch with these features would be 
far easier than adding it to POV.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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Andrew Coppin wrote:
>Right, well, I suppose I was a bit vague...
>
>I visited the London Science Museum one time... They had this plastic stuff.
>Apparently they make scale models of bridges and stuff out of it to analyse
>stress. Coz, the thing is, when you view it through this clear plastic sheet
>(which probably polarises or something), and when you bend it, the most
>stressed parts show up rainbow colours. (The lightsource might be "special"
>too - it was in a dark room.)

Funny that you should mention this now.
I posted such an image a few days ago to
a photo forum here in Norway.

Here's that image:
http://foto.no/cgi-bin/bildekritikk/vis_bilde.cgi?id=49527

I shot the image here in my living room
about a year ago (IIRC) with a macro lens,
a halogen reading lamp and two polarizing
camera filters (one in front of the lamp
and one on the lens.)


Tor Olav

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Christopher James Huff <chr### [at] maccom> wrote:

> In article <1fkqkvf.knzuo218fc70cN%kaveh@delete_this.focalimage.com>,
>  kav### [at] delete_thisfocalimagecom (Kaveh) wrote:
> 
> > I am not an expert in povray, but you know, it shouldn't be difficult to
> > add in polarization. Not compared to the amazing things it does already!
> 
> It would be very difficult and complex. It would basically require 
> changing the entire way colors are handled, storing information on the
> amount of light of each color at each polarization angle, to do it well
> would probably require using frequency distributions to describe light
> color...building a raytracer from scratch with these features would be
> far easier than adding it to POV.

Well, I said I wasn't an expert. A little knowledge is a dangerous
thing. ;-)

-- 
Kaveh

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