clipka <ano### [at] anonymousorg> wrote:
> As good as MCPov renders tend look - they're actually physically wrong.
> Have a look at this:
>
> The following images show a 100% white diffusely reflecting plane
> (actually a large slab; left side),...
> Reflection is perfect, i.e. 100%.

I don't think perfect reflection is physically possible.  100% white is tough
too.

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nemesis schrieb:
> clipka <ano### [at] anonymousorg> wrote:
>> As good as MCPov renders tend look - they're actually physically wrong.
>> Have a look at this:
>>
>> The following images show a 100% white diffusely reflecting plane
>> (actually a large slab; left side),...
>> Reflection is perfect, i.e. 100%.
> 
> I don't think perfect reflection is physically possible.  100% white is tough
> too.

Nonetheless computations with such theoretical maximum values give clear 
indications about the general quality of an algorithm. As far as that 
goes, MCPov apparently sucks: It can do no more than 50% white without 
adversely affecting some feature or the other, and that's /definitely/ 
below even the /practical/ maximum.

Whole milk, for instance, ranges at around 85%. And special materials do 
indeed achieve a diffuse reflectance of 99% and higher in the visble 
spectrum (e.g. Labsphere Spectralon).

As for perfect specular reflectance, I only used it to create a 
reference surface in the image anyway, but for instance water-to-air 
interfaces come extraordinary close to perfect reflection.

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nemesis <nam### [at] gmailcom> wrote:

> clipka <ano### [at] anonymousorg> wrote:
> > As good as MCPov renders tend look - they're actually physically wrong.
> > Have a look at this:
> >
> > The following images show a 100% white diffusely reflecting plane
> > (actually a large slab; left side),...
> > Reflection is perfect, i.e. 100%.

> I don't think perfect reflection is physically possible.  100% white is tough
> too.

  Talk about missing the point... ;)

  The point is, if mcpov is rendering a 100% white surface wrongly, it's
probably going to render a 50% white surface (perfectly possible in the
real world) wrongly too.

-- 
                                                          - Warp

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Warp <war### [at] tagpovrayorg> wrote:
> nemesis <nam### [at] gmailcom> wrote:
>
> > clipka <ano### [at] anonymousorg> wrote:
> > > As good as MCPov renders tend look - they're actually physically wrong.
> > > Have a look at this:
> > >
> > > The following images show a 100% white diffusely reflecting plane
> > > (actually a large slab; left side),...
> > > Reflection is perfect, i.e. 100%.
>
> > I don't think perfect reflection is physically possible.  100% white is tough
> > too.
>
>   Talk about missing the point... ;)

I think it's perfectly on topic:  how is a "physically-based" renderer supposed
to behave in the face of a situation beyond the realm of physics?  Perhaps they
should do some phong shading?

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nemesis <nam### [at] gmailcom> wrote:
> >   Talk about missing the point... ;)

> I think it's perfectly on topic:  how is a "physically-based" renderer supposed
> to behave in the face of a situation beyond the realm of physics?  Perhaps they
> should do some phong shading?

  So he should have used eg. 50% white instead of 100% to keep you happy?

-- 
                                                          - Warp

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Warp <war### [at] tagpovrayorg> wrote:
> nemesis <nam### [at] gmailcom> wrote:
> > >   Talk about missing the point... ;)
>
> > I think it's perfectly on topic:  how is a "physically-based" renderer supposed
> > to behave in the face of a situation beyond the realm of physics?  Perhaps they
> > should do some phong shading?
>
>   So he should have used eg. 50% white instead of 100% to keep you happy?

95% white is ok, I guess, but I'm more concerned about the 100% reflexive
surface.

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nemesis schrieb:

> I think it's perfectly on topic:  how is a "physically-based" renderer supposed
> to behave in the face of a situation beyond the realm of physics?  Perhaps they
> should do some phong shading?

If physics has a /theoretical/ answer to a certain limit-case situation, 
then a "physically-based" renderer should end up producing exactly that 
answer when confronted with that case - or at least approach it.

What, btw, has phong shading to do with the test setup??

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clipka <ano### [at] anonymousorg> wrote:
> If physics has a /theoretical/ answer to a certain limit-case situation,
> then a "physically-based" renderer should end up producing exactly that
> answer when confronted with that case - or at least approach it.

I see.  What I see from physically-based renderer forums though is that in the
face of extreme conditions like that (way too high, unrealistic reflectance for
instance) renders tend to break and be very poor and noisy.

> What, btw, has phong shading to do with the test setup??

It's called irony. :)

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nemesis schrieb:
> clipka <ano### [at] anonymousorg> wrote:
>> If physics has a /theoretical/ answer to a certain limit-case situation,
>> then a "physically-based" renderer should end up producing exactly that
>> answer when confronted with that case - or at least approach it.
> 
> I see.  What I see from physically-based renderer forums though is that in the
> face of extreme conditions like that (way too high, unrealistic reflectance for
> instance) renders tend to break and be very poor and noisy.

This is not a case of an algorithm becoming too noisy: This is a case of 
an algorithm returning too dark colors. The phenomenon of MCPov getting 
the brightness wrong by a factor of 2 is not just limited to 100% white 
surfaces - I just chose that particular level of reflectivity because it 
is easy to model a reference surface for it.

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clipka <ano### [at] anonymousorg> wrote:
> nemesis schrieb:
> > clipka <ano### [at] anonymousorg> wrote:
> >> If physics has a /theoretical/ answer to a certain limit-case situation,
> >> then a "physically-based" renderer should end up producing exactly that
> >> answer when confronted with that case - or at least approach it.
> >
> > I see.  What I see from physically-based renderer forums though is that in the
> > face of extreme conditions like that (way too high, unrealistic reflectance for
> > instance) renders tend to break and be very poor and noisy.
>
> This is not a case of an algorithm becoming too noisy: This is a case of
> an algorithm returning too dark colors. The phenomenon of MCPov getting
> the brightness wrong by a factor of 2 is not just limited to 100% white
> surfaces - I just chose that particular level of reflectivity because it
> is easy to model a reference surface for it.

ah, well.  Then it's a bug alright.

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