Le 14/09/2009 20:38, clipka nous fit lire :
> Le_Forgeron schrieb:
>> In fact the reflection's colour is a matrix, and the refraction's colour
>> is another. (Today, refraction = coef*reflection ... known as filter)
>> Transmit should remains a single number, right ?
> 
> Are you sure about this?

Today, reflection is just a 1D-vector (3 components: R,G,B), and docs says:

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 light
passing through such objects is tinted by the appropriate color as the
material selectively absorbs some frequencies of light while allowing
others to pass through. The color of the object is subtracted from the
light passing through so this is called subtractive transparency.

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. In these examples, all frequencies of light are
allowed to pass through tiny holes in the surface. Although the amount
of light passing through is diminished, the color of the light passing
through is unchanged.

### end of citation

I would expect transmit to not suffer ior, whereas filter to do. [see
later **]

For filter handling, look at source/backend/render/trace.cpp, circa line
928 : the filtering accumulator get added with the product of single
filter by surface colour (vector rgb). If it was independent, it would
use a different information instead of multiplying for that.
(and it might be more memory consuming, but saving 3 "*" each
ray/intersection, but that would not matter i guess)

at least, it looks like ambient finish can be colored, sort of a good
news for me.

** : it seems that whatever, filter or transmit, both get ior in play,
as a single ray is shot (line 963, trace.cpp again). Which somehow is
not what I would expect from my reading of the docs.

Transmit is refracted, unfiltered;
Filter is refracted, filtered by layer colour (!?)

Reflected colour is special (according to metallic reflection and such
fancy), but basically has far more control (ambient, diffuse, reflection...)

The fishy thing here seems to be that the same colour for reflection is
used for refraction/filter.

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)




> 
> For diffuse reflection you're right: That one would need a matrix, to
> account for fluorescence effects. The material absorbs light, and
> re-emits the energy of it at a different wavelength in random
> directions. 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...

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)

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 ? Should the surface_color*filter be
dropped for an additional refracted transmit (bis, sort of) ?
Should we continue to fake a "filtering" (the actual refraction) ?




And should we bother for pleochroism ? what about birefringence ?

(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... )

Maybe that's a part for media, not pigment.

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