I've just released an experimental version of POV-Ray implementing two
alternative models for diffuse reflection, besides the classic so-called
Lambertian model:

https://github.com/POV-Ray/povray/releases/tag/v3.7.1-alpha.8545805%2Bav122

I'd like to invite you to toy around with them as you deem fit.


To explain what these alternative models do, let me first bore you to
death with some background information:


In a totally strict sense, diffuse reflection is the effect that light
hitting a surface bounces off in a direction that is entirely
independent of the angle of incidence. There is essentially just one
manner in which this can happen, and it is perfectly described by the
so-called Lambertian model, which is what POV-Ray uses.

Surprisingly enough, for this type of reflection to happen, the object's
surface must be _perfectly smooth_; all of what we might call
"diffuseness" must be from scattering within the material itself -- the
same process we now know as "subsurface scattering". So the Lambertian
model can be thought of as a subsurface scattering model, suitable for
materials in which the light doesn't travel far before exiting again (in
technical terms, materials with a very low "mean free path" between
scattering events).

As soon as the surface exhibits any kind of roughness, other effects
kick in that mess with the neat non-directionality of the Lambertian
model; for instance, in a bumpy surface, portions of the surface which
are angled towards the light source will receive more incoming light;
since each portion of the surface will emit light preferentially in the
direction of its surface normal, this will bias the outgoing light
towards the direction of incoming light.


The alternative models try to account for such effects in different ways:


- The Oren-Nayar model is based on the presumption that the surface is
comprised of microscopic facets of somewhat random orientation that form
V-shaped pits, with each individual facet exhibiting Lambertian
reflection. The model takes into account not only microfacet
orientation, but also shadowing and masking of light by the opposite
side of each pit, as well as interreflection between the sides. For the
sake of computational simplicity, instead of the full model I've
implemented the more commonly used simplified version, known as the
"qualitative model", which eliminates one term that contributes only
little to the result, and uses a very rough approximation for
interreflection.

To use the (qualitative) Oren-Nayar model, specify:

    finish {
      diffuse FLOAT
      oren_nayar ROUGHNESS
    }

where the ROUGHNESS parameter is a metric for the roughness of the
material. (Currently this is the _sigma_ parameter in the Oren-Nayar
equations; however, please be aware that this may still be subject to
change without notice.)

The Lambertian model can be considered a special case of the Oren-Nayar
model, with ROUGHNESS = 0.0.


- 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.

To use the Lommel-Seeliger model, specify:

    finish {
      diffuse FLOAT
      lommel_seeliger K
    }

where K specifies the weight of the Lommel-Seeliger model, ranging from
0.0 (pure Lambertian) to 1.0 (pure Lommel-Seeliger)


Have fun, and feel free to go crazy testing!

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Le 31/03/2016 19:27, clipka a écrit :
> oren_nayar ROUGHNESS

ok... what is a good value, excepted for 0.0 which disable it ?

No link to equations, no idea about what is sigma.

For lommel_seeliger, it's clear: from 0 to 1.

It also seems the parser allow both oren_nayar & lommel_seeliger at the same time.
Is it intended ? Is there a fusion of models (when neither is 0) ?

Post a reply to this message

Am 31.03.2016 um 21:08 schrieb Le_Forgeron:
> Le 31/03/2016 19:27, clipka a écrit :
>> oren_nayar ROUGHNESS
> 
> ok... what is a good value, excepted for 0.0 which disable it ?
> 
> No link to equations, no idea about what is sigma.

It's the standard deviation of the facet slopes, and can range anywhere
from 0 to just short of infinity. Something around 0.3 might be a good
place to start.

> It also seems the parser allow both oren_nayar & lommel_seeliger at the same time.
> Is it intended ? Is there a fusion of models (when neither is 0) ?

Strictly speaking, with the current implementation a non-zero
lommel_seeliger setting gives you an average not of the Lommel-Seeliger
and Lambertian models, but of the Lommel-Seeliger and Oren-Nayar models,
with the Lambertian model just being the sigma = 0.0 special case thereof.

Post a reply to this message

Am 31.03.2016 um 19:27 schrieb clipka:

> In a totally strict sense, diffuse reflection is the effect that light
> hitting a surface bounces off in a direction that is entirely
> independent of the angle of incidence. There is essentially just one
> manner in which this can happen, and it is perfectly described by the
> so-called Lambertian model, which is what POV-Ray uses.

I think I wrote nonsense there.

Also, a few more notes:

- My implementation of the Oren-Nayar model turns out to be bugged.

- The mix of Lommel-Seeliger and Lambert is also known as "Lunar-Lambert".

Post a reply to this message

clipka <ano### [at] anonymousorg> wrote:
> Am 31.03.2016 um 19:27 schrieb clipka:
>
>
> - My implementation of the Oren-Nayar model turns out to be bugged.
>

I like the feature, could I help test it? I have read this model on wikipedia
and thought about it for a while.

Post a reply to this message

"And" <49341109@ntnu.edu.tw> wrote:
> clipka <ano### [at] anonymousorg> wrote:
> > Am 31.03.2016 um 19:27 schrieb clipka:
> >
> >
> > - My implementation of the Oren-Nayar model turns out to be bugged.
> >
>
> I like the feature, could I help test it? I have read this model on wikipedia
> and thought about it for a while.

I just tried your new texture. The bigger the sigma the surface appears darker
when applying the same diffuse value. It is from the Oren-Nayar model itself I
have confirmed.

Is this the bug you said?

Post a reply to this message

Am 05.04.2016 um 14:50 schrieb And:
> "And" <49341109@ntnu.edu.tw> wrote:
>> clipka <ano### [at] anonymousorg> wrote:
>>> Am 31.03.2016 um 19:27 schrieb clipka:
>>>
>>>
>>> - My implementation of the Oren-Nayar model turns out to be bugged.
>>>
>>
>> I like the feature, could I help test it? I have read this model on wikipedia
>> and thought about it for a while.
> 
> I just tried your new texture. The bigger the sigma the surface appears darker
> when applying the same diffuse value. It is from the Oren-Nayar model itself I
> have confirmed.
> 
> Is this the bug you said?

No; since it's an intrinsic property of the model, I don't consider it a
bug.

What I got wrong is the dependence on "sideways" illumination, if you
know what I mean.

In the model there's a term depending on the angle between the light
source and eye directions /projected/ onto the surface (the azimuth
angle, if you like). Normally I should compute the projected vectors,
normalize them, and compute the dot product, to get the cosine of the
angle. But I forgot the normalization step.

Post a reply to this message

clipka <ano### [at] anonymousorg> wrote:
> Am 05.04.2016 um 14:50 schrieb And:
> > "And" <49341109@ntnu.edu.tw> wrote:
> >> clipka <ano### [at] anonymousorg> wrote:
> >>> Am 31.03.2016 um 19:27 schrieb clipka:
> >>>
> >>>
> >>> - My implementation of the Oren-Nayar model turns out to be bugged.
> >>>
> >>
> >> I like the feature, could I help test it? I have read this model on wikipedia
> >> and thought about it for a while.
> >
> > I just tried your new texture. The bigger the sigma the surface appears darker
> > when applying the same diffuse value. It is from the Oren-Nayar model itself I
> > have confirmed.
> >
> > Is this the bug you said?
>
> No; since it's an intrinsic property of the model, I don't consider it a
> bug.
>
> What I got wrong is the dependence on "sideways" illumination, if you
> know what I mean.
>
> In the model there's a term depending on the angle between the light
> source and eye directions /projected/ onto the surface (the azimuth
> angle, if you like). Normally I should compute the projected vectors,
> normalize them, and compute the dot product, to get the cosine of the
> angle. But I forgot the normalization step.


OkOK. That sounds very easy to fix for you.

I derived a solution on what I said the albedo value getting dark when sigma
increase.
I study the formula from the wikipedia. It said A=1-0.5*..., B=0.45*...(Both
rely on the sigma)

And the result albedo seems rho*A + rho*B*(2/3-64/45/pi) instead rho
itself. So maybe you can divide it when apply the diffuse albedo feature.

Post a reply to this message

Am 06.04.2016 um 09:52 schrieb And:

> I derived a solution on what I said the albedo value getting dark when sigma
> increase.
> I study the formula from the wikipedia. It said A=1-0.5*..., B=0.45*...(Both
> rely on the sigma)
> 
> And the result albedo seems rho*A + rho*B*(2/3-64/45/pi) instead rho
> itself. So maybe you can divide it when apply the diffuse albedo feature.

Thanks! That correction factor appears to make a lot more sense than the
hack I had come up with :)

Not too surprisingly, experiments indicate that it does indeed fit like
a glove.

Post a reply to this message

"And" <49341109@ntnu.edu.tw> wrote:
>
> I derived a solution on what I said the albedo value getting dark when sigma
> increase.
> I study the formula from the wikipedia. It said A=1-0.5*..., B=0.45*...(Both
> rely on the sigma)
>
> And the result albedo seems rho*A + rho*B*(2/3-64/45/pi) instead rho
> itself. So maybe you can divide it when apply the diffuse albedo feature.

Should say:

For a small area 'da', incident light irradiance 'E0', and if the incident angle
'theta_i' is fixed, it will receive E0*da*cos(theta_i) energy per second. then
emit
rho*E0*da*cos(theta_i)*A
+rho*E0*da*B/pi*(
                 sin(2*theta_i)*(theta_i/2 - sin(2*theta_i)/4)
                +2*sin(theta_i)*(1/3 - pow(sin(theta_i),3)/3)
)
energy per second.


Then I apply a condition that a homogeneous hemisphere lighting, equivalent to a
fixed incident radiance 'Li', then the small area 'da' receives Li*pi*da energy
per second, emit rho*Li*pi*da*A + rho*Li*da*B*(2*pi/3-64/45) energy per second.
So get the actual albedo rho*A + rho*B*(2/3-64/45/pi).

Post a reply to this message

clipka <ano### [at] anonymousorg> wrote:
> Am 06.04.2016 um 09:52 schrieb And:
>
> > I derived a solution on what I said the albedo value getting dark when sigma
> > increase.
> > I study the formula from the wikipedia. It said A=1-0.5*..., B=0.45*...(Both
> > rely on the sigma)
> >
> > And the result albedo seems rho*A + rho*B*(2/3-64/45/pi) instead rho
> > itself. So maybe you can divide it when apply the diffuse albedo feature.
>
> Thanks! That correction factor appears to make a lot more sense than the
> hack I had come up with :)
>
> Not too surprisingly, experiments indicate that it does indeed fit like
> a glove.

Ok ok! cheers.

If I don't make mistake. but it should correct because I calculate formula
carefully.

Post a reply to this message

Sometimes I read these threads and wonder when "POV-Ray version QED" will be
available:

https://www.youtube.com/watch?v=eLQ2atfqk2c&list=PL8590A6E18255B3F4

I must confess, I watched the whole thing, and I found it fascinating, because
he says things about the behaviour of light that I've always wondered about, and
that they Never Ever mentioned in my entire academic experience, which was very
long and very painful   ;)

I think it's excellent that a new diffuse reflection model is being experimented
with - I'm sure there ought to be some interesting technical and artistic works
appearing in short order.

Just curious if the thin film refraction model that POV-Ray uses is based on
what Feynman discusses in his lectures...

Post a reply to this message

Am 06.04.2016 um 14:06 schrieb And:
> clipka <ano### [at] anonymousorg> wrote:
>> Am 06.04.2016 um 09:52 schrieb And:
>>
>>> I derived a solution on what I said the albedo value getting dark when sigma
>>> increase.
>>> I study the formula from the wikipedia. It said A=1-0.5*..., B=0.45*...(Both
>>> rely on the sigma)
>>>
>>> And the result albedo seems rho*A + rho*B*(2/3-64/45/pi) instead rho
>>> itself. So maybe you can divide it when apply the diffuse albedo feature.
>>
>> Thanks! That correction factor appears to make a lot more sense than the
>> hack I had come up with :)
>>
>> Not too surprisingly, experiments indicate that it does indeed fit like
>> a glove.
> 
> Ok ok! cheers.
> 
> If I don't make mistake. but it should correct because I calculate formula
> carefully.

Actually, once I thought about your result, I realized that the
bihemispherical albedo /must/ be

    rho*( A*a + B*b )

with the terms A and B depending on sigma as specified in the Oren-Nayar
description, and a,b being constant.

From the fact that Oren-Nayar includes the Lambertian model as a special
case with A=1,B=0 it follows that a /must/ be 1.

The value 2/3+(64/45)*(1/pi) for b looks a bit complicated, but
experiments with high sigma clearly indicate that it indeed at least
approximates the proper value very closely, and it looks plausible
enough as an exact value, so I'm pretty sure you did indeed get the
hemispherical integral right.

Post a reply to this message

Am 06.04.2016 um 14:54 schrieb Bald Eagle:

> Just curious if the thin film refraction model that POV-Ray uses is based on
> what Feynman discusses in his lectures...

POV-Ray uses a thin film refraction model?

I don't think so. I'm pretty sure it uses a standard single-interface
model for refraction.

Post a reply to this message

clipka <ano### [at] anonymousorg> wrote:
> Am 06.04.2016 um 14:06 schrieb And:
> > clipka <ano### [at] anonymousorg> wrote:
> >> Am 06.04.2016 um 09:52 schrieb And:
> >>
> >>> I derived a solution on what I said the albedo value getting dark when sigma
> >>> increase.
> >>> I study the formula from the wikipedia. It said A=1-0.5*..., B=0.45*...(Both
> >>> rely on the sigma)
> >>>
> >>> And the result albedo seems rho*A + rho*B*(2/3-64/45/pi) instead rho
> >>> itself. So maybe you can divide it when apply the diffuse albedo feature.
> >>
> >> Thanks! That correction factor appears to make a lot more sense than the
> >> hack I had come up with :)
> >>
> >> Not too surprisingly, experiments indicate that it does indeed fit like
> >> a glove.
> >
> > Ok ok! cheers.
> >
> > If I don't make mistake. but it should correct because I calculate formula
> > carefully.
>
> Actually, once I thought about your result, I realized that the
> bihemispherical albedo /must/ be
>
>     rho*( A*a + B*b )
>
> with the terms A and B depending on sigma as specified in the Oren-Nayar
> description, and a,b being constant.
>
> From the fact that Oren-Nayar includes the Lambertian model as a special
> case with A=1,B=0 it follows that a /must/ be 1.
>
> The value 2/3+(64/45)*(1/pi) for b looks a bit complicated, but
> experiments with high sigma clearly indicate that it indeed at least
> approximates the proper value very closely, and it looks plausible
> enough as an exact value, so I'm pretty sure you did indeed get the
> hemispherical integral right.

Well. :)




"Bald Eagle" <cre### [at] netscapenet> wrote:
> Sometimes I read these threads and wonder when "POV-Ray version QED" will be
available:

QED... I think that is far from the rendering application.

>
> https://www.youtube.com/watch?v=eLQ2atfqk2c&list=PL8590A6E18255B3F4
>
> I must confess, I watched the whole thing, and I found it fascinating, because
> he says things about the behaviour of light that I've always wondered about, and
> that they Never Ever mentioned in my entire academic experience, which was very
> long and very painful   ;)
Well.

>
> I think it's excellent that a new diffuse reflection model is being experimented
> with - I'm sure there ought to be some interesting technical and artistic works
> appearing in short order.

Oren-Nayar reflection model is a very useful feature for me.

Post a reply to this message

Am 31.03.2016 um 19:27 schrieb clipka:
> I've just released an experimental version of POV-Ray implementing two
> alternative models for diffuse reflection, besides the classic so-called
> Lambertian model:
> 
> https://github.com/POV-Ray/povray/releases/tag/v3.7.1-alpha.8545805%2Bav122

A new build is currently in the making:

- The bug in the Oren-Nayar implementation has been fixed.

- I got the interaction between Oren-Nayar and Lommel-Seeliger the wrong
way round; this has also been fixed, so that it now works as described
previously in this thread.

- The difference in effective brightness (bihemispherical albedo for the
technically inclined) between the models can now be compensated for by
using the `diffuse albedo FLOAT` syntax (which had already been doing
the same job with respect to the `brilliance` mechanism). The `diffuse
FLOAT` syntax still gives a canonical brightness parameterization of the
models. (*)

- Interaction with radiosity has been improved. As the radiosity
algorithm cannot make use of the new models, and therefore always falls
back to the Lambertian model, it also suffered from the difference in
effective brightness between the models, with materials using Oren-Nayar
or Lommel-Seeliger appearing brighter under radiosity illumination than
under classic illumination. This has also been fixed along the way.


(* It might be worth noting that the `diffuse albedo FLOAT` syntax does
/not/ change the Lommel-Seeliger parameter's original meaning: Using
`lommel_seeliger K` /always/ computes a K:(1-K) weighted average of
_canonical_ Lommel-Seeliger and Lambert; only /then/ is any correction
factor applied to achieve a specified bihemispherical albedo.)

Post a reply to this message

Am 07.04.2016 um 16:36 schrieb clipka:
> Am 31.03.2016 um 19:27 schrieb clipka:
>> I've just released an experimental version of POV-Ray implementing two
>> alternative models for diffuse reflection, besides the classic so-called
>> Lambertian model:
>>
>> https://github.com/POV-Ray/povray/releases/tag/v3.7.1-alpha.8545805%2Bav122
> 
> A new build is currently in the making:

Actually the build completed faster than I expected:

https://github.com/POV-Ray/povray/releases/tag/v3.7.1-alpha.8555867%2Bav123

Post a reply to this message

On 7-4-2016 16:39, clipka wrote:
> Am 07.04.2016 um 16:36 schrieb clipka:
>> Am 31.03.2016 um 19:27 schrieb clipka:
>>> I've just released an experimental version of POV-Ray implementing two
>>> alternative models for diffuse reflection, besides the classic so-called
>>> Lambertian model:
>>>
>>> https://github.com/POV-Ray/povray/releases/tag/v3.7.1-alpha.8545805%2Bav122
>>
>> A new build is currently in the making:
>
> Actually the build completed faster than I expected:
>
> https://github.com/POV-Ray/povray/releases/tag/v3.7.1-alpha.8555867%2Bav123
>

I have been wondering: How to see the relationship between UberPOV and 
version 3.7.1-alpha, in the context of these diffuse models? The albedo 
and brilliance terms originally in UberPOV are also available in alpha.

Another thing: the alternative diffuse models seem not to be available 
any more in the tonemapping alpha version. I get an error message saying 
undeclared identifier 'oren_nayar'

-- 
Thomas

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On 12-4-2016 10:06, Thomas de Groot wrote:
> I have been wondering: How to see the relationship between UberPOV and
> version 3.7.1-alpha, in the context of these diffuse models? The albedo
> and brilliance terms originally in UberPOV are also available in alpha.
>
> Another thing: the alternative diffuse models seem not to be available
> any more in the tonemapping alpha version. I get an error message saying
> undeclared identifier 'oren_nayar'
>

To be more precise:
-av123-win64 works for diffuse models;
-av124-win64 does not work for diffuse models.

-- 
Thomas

Post a reply to this message

Am 12.04.2016 um 10:06 schrieb Thomas de Groot:

> I have been wondering: How to see the relationship between UberPOV and
> version 3.7.1-alpha, in the context of these diffuse models? The albedo
> and brilliance terms originally in UberPOV are also available in alpha.

UberPOV has been a collection of features that I expect (or expected) to
not be fitting into the philosophy of POV-Ray proper. Some may make it
into official POV-Ray, others may not. (Just the same as with good old
MegaPOV.)

As far as the new features are concerned, I have not much doubt that
they match the spirit of official POV-Ray; the only reason they're not
in the main branch yet is that they come with a high risk of undergoing
some breaking changes before reaching their final shape.

Another difference is that UberPOV is a /collection/ of experimental
features, while the two new features each live in their own happy little
branch, so...

> Another thing: the alternative diffuse models seem not to be available
> any more in the tonemapping alpha version. I get an error message saying
> undeclared identifier 'oren_nayar'

... that's not too much of a surprise to me ;)

Post a reply to this message

On 12-4-2016 20:59, clipka wrote:
> Am 12.04.2016 um 10:06 schrieb Thomas de Groot:
>
>> I have been wondering: How to see the relationship between UberPOV and
>> version 3.7.1-alpha, in the context of these diffuse models? The albedo
>> and brilliance terms originally in UberPOV are also available in alpha.
>
> UberPOV has been a collection of features that I expect (or expected) to
> not be fitting into the philosophy of POV-Ray proper. Some may make it
> into official POV-Ray, others may not. (Just the same as with good old
> MegaPOV.)

I needed the reminder, thanks ;-)

>
> As far as the new features are concerned, I have not much doubt that
> they match the spirit of official POV-Ray; the only reason they're not
> in the main branch yet is that they come with a high risk of undergoing
> some breaking changes before reaching their final shape.
>
> Another difference is that UberPOV is a /collection/ of experimental
> features, while the two new features each live in their own happy little
> branch, so...
>
>> Another thing: the alternative diffuse models seem not to be available
>> any more in the tonemapping alpha version. I get an error message saying
>> undeclared identifier 'oren_nayar'
>
> ... that's not too much of a surprise to me ;)
>

Oh? Two different exe files? My naive understanding was that the 
features were cumulative. If not, I need to take care about which is what.

-- 
Thomas

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