OK. One final example: a pot with albedo added as well as some specular 
value.

I don't know how to interpret or make sense of all this, so some 
feedback is appreciated.

-- 
Thomas

Post a reply to this message

Attachments:
Download 'clipka_diffuse reflection_test.png' (991 KB)

Preview of image 'clipka_diffuse reflection_test.png'

Thomas de Groot <tho### [at] degrootorg> wrote:
> OK. One final example: a pot with albedo added as well as some specular
> value.
>
> I don't know how to interpret or make sense of all this, so some
> feedback is appreciated.
>
> --
> Thomas

The texture is fun. It has some contrast to the before.

Post a reply to this message

On 5-4-2016 15:19, And wrote:
> Thomas de Groot <tho### [at] degrootorg> wrote:
>> OK. One final example: a pot with albedo added as well as some specular
>> value.
>>
>> I don't know how to interpret or make sense of all this, so some
>> feedback is appreciated.
>>
>> --
>> Thomas
>
> The texture is fun. It has some contrast to the before.
>

I confess that I am confused. The textures appear darker and somehow 
with less "contrast". So, my question really is: /when/ to use these 
alternatives instead of pure Lambertian? To me, at this stage and 
totally instinctively, pure Lambertian has my preference.

-- 
Thomas

Post a reply to this message

Thomas de Groot <tho### [at] degrootorg> wrote:
> On 5-4-2016 15:19, And wrote:
> > Thomas de Groot <tho### [at] degrootorg> wrote:
> >> OK. One final example: a pot with albedo added as well as some specular
> >> value.
> >>
> >> I don't know how to interpret or make sense of all this, so some
> >> feedback is appreciated.
> >>
> >> --
> >> Thomas
> >
> > The texture is fun. It has some contrast to the before.
> >
>
> I confess that I am confused. The textures appear darker and somehow
> with less "contrast". So, my question really is: /when/ to use these
> alternatives instead of pure Lambertian? To me, at this stage and
> totally instinctively, pure Lambertian has my preference.
>
> --
> Thomas

Lambertian diffuse is very well truly. I think the others can be used when you
feel the pure diffuse is plodding, or sometimes Oren-Nayar can be used in an
....powdery object like chalk.

Post a reply to this message

Or the carton.

Post a reply to this message

Attachments:
Download 'imag2961.jpg' (909 KB)

Preview of image 'imag2961.jpg'

On 6-4-2016 12:28, And wrote:
> Or the carton.
>

Powdery/chalk, carton, yes that makes sense indeed for oren_nayar.

Got back to the original clipka message:

[quote] The Lommel-Seeliger model has its roots in astronomy, where it 
is used to model "lunar type" (i.e. rocky or dusty) surfaces, while the
Lambertian model is used for "icy" surfaces, and weighted averages of
the two are used to model anything in between. [/quote]

What would you say for the following:

- Lambertian for icy or shiny surfaces;
- oren_nayar for fine, powdery-like surfaces;
- lommel_seeliger for coarser, dusty-like surfaces.

-- 
Thomas

Post a reply to this message

Thomas de Groot <tho### [at] degrootorg> wrote:
> On 6-4-2016 12:28, And wrote:
> > Or the carton.
> >
>
> Powdery/chalk, carton, yes that makes sense indeed for oren_nayar.
>
> Got back to the original clipka message:
>
> [quote] The Lommel-Seeliger model has its roots in astronomy, where it
> is used to model "lunar type" (i.e. rocky or dusty) surfaces, while the
> Lambertian model is used for "icy" surfaces, and weighted averages of
> the two are used to model anything in between. [/quote]
>
> What would you say for the following:
>
> - Lambertian for icy or shiny surfaces;
> - oren_nayar for fine, powdery-like surfaces;
> - lommel_seeliger for coarser, dusty-like surfaces.
>
> --
> Thomas

Well. Oren-Nayar is for such objects I know. But Lambertian should be the
simplest diffuse material. I just know it is suitable for stones,
(unpainted)concrete. Lommel Seeliger I never heard.

Post a reply to this message

Am 06.04.2016 um 11:42 schrieb And:
> Thomas de Groot <tho### [at] degrootorg> wrote:
>> On 5-4-2016 15:19, And wrote:
>>> Thomas de Groot <tho### [at] degrootorg> wrote:
>>>> OK. One final example: a pot with albedo added as well as some specular
>>>> value.
>>>>
>>>> I don't know how to interpret or make sense of all this, so some
>>>> feedback is appreciated.
>>>>
>>>> --
>>>> Thomas
>>>
>>> The texture is fun. It has some contrast to the before.
>>>
>>
>> I confess that I am confused. The textures appear darker and somehow
>> with less "contrast". So, my question really is: /when/ to use these
>> alternatives instead of pure Lambertian? To me, at this stage and
>> totally instinctively, pure Lambertian has my preference.

The darker appearance is inherent in the mathematical models: At the
same nominal brightness, the alternative models result in a lower
effective bihemispherical albedo (ratio of total reflected light vs.
total incoming light); a similar effect happens with the conventional
model when you use a high "brilliance" parameter. I'll need to implement
correction factors for that, just as I did for the "brilliance"
mechanism, when the "albedo" keyword is used.

As for having less "contrast", that is also to be expected, and is
actually the feature making them more realistic for some materials.

> Lambertian diffuse is very well truly. I think the others can be used when you
> feel the pure diffuse is plodding, or sometimes Oren-Nayar can be used in an
> ....powdery object like chalk.

Chalk, plaster, sandstone, unglazed clay, Shapeways' 3d-printed
"unpolished strong & flexible plastic", textiles, eggshells, even paper
-- there's plenty of stuff in our world that is seriously non-lambertian.

Granted, smooth plastic, glossy cardboard, finished wood and painted
surfaces are quite dominant in our world, and Lambertian does quite a
good job for those. But as soon as you have something in your scene that
in real life exhibits non-Lambertian reflection, using one of the
alternative models may give your scene that little extra realism.

Post a reply to this message

On 6-4-2016 13:56, clipka wrote:
> Am 06.04.2016 um 11:42 schrieb And:
>> Thomas de Groot <tho### [at] degrootorg> wrote:
>>> On 5-4-2016 15:19, And wrote:
>>>> Thomas de Groot <tho### [at] degrootorg> wrote:
>>>>> OK. One final example: a pot with albedo added as well as some specular
>>>>> value.
>>>>>
>>>>> I don't know how to interpret or make sense of all this, so some
>>>>> feedback is appreciated.
>>>>>
>>>>> --
>>>>> Thomas
>>>>
>>>> The texture is fun. It has some contrast to the before.
>>>>
>>>
>>> I confess that I am confused. The textures appear darker and somehow
>>> with less "contrast". So, my question really is: /when/ to use these
>>> alternatives instead of pure Lambertian? To me, at this stage and
>>> totally instinctively, pure Lambertian has my preference.
>
> The darker appearance is inherent in the mathematical models: At the
> same nominal brightness, the alternative models result in a lower
> effective bihemispherical albedo (ratio of total reflected light vs.
> total incoming light); a similar effect happens with the conventional
> model when you use a high "brilliance" parameter. I'll need to implement
> correction factors for that, just as I did for the "brilliance"
> mechanism, when the "albedo" keyword is used.
>
> As for having less "contrast", that is also to be expected, and is
> actually the feature making them more realistic for some materials.
>
>> Lambertian diffuse is very well truly. I think the others can be used when you
>> feel the pure diffuse is plodding, or sometimes Oren-Nayar can be used in an
>> ....powdery object like chalk.
>
> Chalk, plaster, sandstone, unglazed clay, Shapeways' 3d-printed
> "unpolished strong & flexible plastic", textiles, eggshells, even paper
> -- there's plenty of stuff in our world that is seriously non-lambertian.
>
> Granted, smooth plastic, glossy cardboard, finished wood and painted
> surfaces are quite dominant in our world, and Lambertian does quite a
> good job for those. But as soon as you have something in your scene that
> in real life exhibits non-Lambertian reflection, using one of the
> alternative models may give your scene that little extra realism.
>

Thanks. I am beginning to see/understand where this may lead my use. I 
am still a bit unsure about the dedicated use of either of the two 
alternatives though for given materials.

-- 
Thomas

Post a reply to this message

Am 06.04.2016 um 13:24 schrieb Thomas de Groot:
> On 6-4-2016 12:28, And wrote:
>> Or the carton.
>>
> 
> Powdery/chalk, carton, yes that makes sense indeed for oren_nayar.
> 
> Got back to the original clipka message:
> 
> [quote] The Lommel-Seeliger model has its roots in astronomy, where it
> is used to model "lunar type" (i.e. rocky or dusty) surfaces, while the
> Lambertian model is used for "icy" surfaces, and weighted averages of
> the two are used to model anything in between. [/quote]
> 
> What would you say for the following:
> 
> - Lambertian for icy or shiny surfaces;
> - oren_nayar for fine, powdery-like surfaces;
> - lommel_seeliger for coarser, dusty-like surfaces.

Actually, having read up a bit on the models' origins and technical
background by now, I'd say the following:

- There is probably no significant practical difference between powdery,
dusty or coarse surfaces.

- Lommel-Seeliger is a niche model, designed to be computationally
"lightweight" while reasonably approximating dark (*) coarse surfaces;
from all I know, its use is almost exclusively restricted to astronomy.

(* Many people don't realize it, but the moon is about as bright as a
lump of anthrazit coal.)

- Pure Lambertian is only reasonably realistic for /some/ (**) bright
coarse surfaces.

- With an additional fresnel-based term to compensate for light lost to
specular reflection (finish-level "fresnel on" in recent POV-Ray
versions), the Lambertian model is also reasonably realistic for
[non-metallic] uncoated shiny surfaces, and /some/ (***) coated shiny
surfaces.

- For most coarse surfaces, including /some/ (**) bright ones,
Oren-Nayar [with noteworthy roughness] is better suited.

- For /some/ (***) coated shiny surfaces, Oren-Nayar with an additional
fresnel-based compensating term (as mentioned above) is better suited.

(**) For bright coarse surfaces, the choice betwen the Lambertian and
Oren-Nayar model depends on the underlying material's optical
properties: If, at the dimensions of the surface's roughness, the
material exhibits some degree of translucency, the Lambertian model is
sufficient (e.g. snow, not-yet-licked ice cream, wood, Spectralon,
possibly Styrofoam), though it may call for SSLT if shown up close. If
on the other hand the material is opaque at those dimensions, Oren-Nayar
is better suited (e.g. Shapeways' unpolished "strong & flexible"
plastic, unglazed ceramics).

(Actually this is also true for dark coarse surfaces, but they are
almost exclusively opaque at the dimensions in question.)

(***) For coated shiny surfaces, the choice between the Lambertian and
Oren-Nayar model depends on which one would be appropriate if the
surface was uncoated. In both cases, use of finish-level "fresnel on" is
recommended.


Also, for the sake of completeness:

- POV-Ray's "brilliance"-based variation of the Lambertian model is an
abomination that has no physical justification whatsoever; my current
guess is that it originated as a botched attempt to implement the
so-called Minnaert model.

Post a reply to this message