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On 11/8/2012 8:20 AM, clipka wrote:
> A side effect of radiosity is that you /can/ also get illumination from
> a sky sphere or background (this is automatic), and even objects (this
> requires a suitable finish setting; previously this could be done by
> using a non-zero "ambient" finish term, but this was originally just a
> hack and has always been troublesome, so as of POV-Ray 3.7 the new
> "emission" term needs to be used instead).
I'm sure that "under-the-hood" emission probably works differently from
ambient, but from where I'm standing it looks like all they did was
trade one word for another.
Todd
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> On 11/8/2012 8:20 AM, clipka wrote:
>> A side effect of radiosity is that you /can/ also get illumination from
>> a sky sphere or background (this is automatic), and even objects (this
>> requires a suitable finish setting; previously this could be done by
>> using a non-zero "ambient" finish term, but this was originally just a
>> hack and has always been troublesome, so as of POV-Ray 3.7 the new
>> "emission" term needs to be used instead).
>
> I'm sure that "under-the-hood" emission probably works differently from
> ambient, but from where I'm standing it looks like all they did was
> trade one word for another.
>
> Todd
>
They look the same, but there is a big difference:
When radiosity is turned on, all ambient to turned off.
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Am 20.06.2013 09:06, schrieb Thomas de Groot:
> Musing...
>
> It is not a /bug/ strictly speaking, as the scene renders correctly in
> its original state, without the radiosity block. The reason is
> adequately described in the wiki Docs (section 1.3.5.3.2).
>
> However, it is something to remain aware of when building scenes from
> pre-defined ones.
Actually, the most important thing to remain aware of when dealing with
older scenes is to make sure that the scene begins with a
#version ...
statement. Normally this should make everything else fall into place
automatically.
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Am 23.06.2013 18:00, schrieb Todd Carnes:
> On 11/8/2012 8:20 AM, clipka wrote:
>> A side effect of radiosity is that you /can/ also get illumination from
>> a sky sphere or background (this is automatic), and even objects (this
>> requires a suitable finish setting; previously this could be done by
>> using a non-zero "ambient" finish term, but this was originally just a
>> hack and has always been troublesome, so as of POV-Ray 3.7 the new
>> "emission" term needs to be used instead).
>
> I'm sure that "under-the-hood" emission probably works differently from
> ambient, but from where I'm standing it looks like all they did was
> trade one word for another.
"They" was me, and there is a fundamental reason why the change was made.
The original intention of the "ambient" term was to provide a means to
(very roughly) approximate what the radiosity feature is doing at much
more computational costs: The light that an object is receiving
indirectly from other objects (IOW ambient illumination, hence the name
of the parameter) and reflecting back diffusely.
(Here's already an initial flaw that has /not/ been tackled yet: How
much ambient light an object reflects in this way obviously does not
only depend on the material properties, but also on how much ambient
light the object is receiving in the first place, which in turn depends
on the geometry of the scene; making the ambient term a part of the
material settings is therefore a questionable decision, and some future
revsion of the SDL - say, some POV-Ray 4.0 SDL - should correct it.)
Users of course found out that this "ambient" term also provided a means
to let objects appear to be glowing - which became even more exciting
when the radiosity feature was added.
Unfortunately, this dual use of the "ambient" term led to a conflicting
situation: While a non-radiosity scene would need all materials to have
a non-zero "ambient" term (and a non-zero ambient_light setting) to look
reasonably ok, a radiosity scene would need all non-glowing materials to
have a zero "ambient" term (or to have ambient_light be zero, but this
would prevent the simulation of glowing objects entirely). So you'd
essentially need two versions of each material: One for non-radiosity
scenes, and one for radiosity scenes.
The introduction of the "emission" setting solves this conflict, by
providing different knobs for each of the purposes "ambient" had come to
be used for: The "ambient" setting can now be used without limitations
to simulate the effect of indirect light in non-radiosity scenes,
without undesired side effects in radiosity scenes (as it is turned off
entirely there, being replaced by the radiosity mechanism); at the same
time, glowing materials can now be modeled without limitations using the
"emission" setting instead, as it is available in both radiosity and
non-radiosity scenes.
(As a side note, another - albeit less important - difference is that
the "emission" setting is insensitive to the ambient_light parameter.)
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On 7/16/2013 4:25 PM, clipka wrote:
> Am 23.06.2013 18:00, schrieb Todd Carnes:
>> On 11/8/2012 8:20 AM, clipka wrote:
>>> A side effect of radiosity is that you /can/ also get illumination from
>>> a sky sphere or background (this is automatic), and even objects (this
>>> requires a suitable finish setting; previously this could be done by
>>> using a non-zero "ambient" finish term, but this was originally just a
>>> hack and has always been troublesome, so as of POV-Ray 3.7 the new
>>> "emission" term needs to be used instead).
>>
>> I'm sure that "under-the-hood" emission probably works differently from
>> ambient, but from where I'm standing it looks like all they did was
>> trade one word for another.
>
> "They" was me, and there is a fundamental reason why the change was made.
>
> The original intention of the "ambient" term was to provide a means to
> (very roughly) approximate what the radiosity feature is doing at much
> more computational costs: The light that an object is receiving
> indirectly from other objects (IOW ambient illumination, hence the name
> of the parameter) and reflecting back diffusely.
>
> (Here's already an initial flaw that has /not/ been tackled yet: How
> much ambient light an object reflects in this way obviously does not
> only depend on the material properties, but also on how much ambient
> light the object is receiving in the first place, which in turn depends
> on the geometry of the scene; making the ambient term a part of the
> material settings is therefore a questionable decision, and some future
> revsion of the SDL - say, some POV-Ray 4.0 SDL - should correct it.)
>
> Users of course found out that this "ambient" term also provided a means
> to let objects appear to be glowing - which became even more exciting
> when the radiosity feature was added.
>
> Unfortunately, this dual use of the "ambient" term led to a conflicting
> situation: While a non-radiosity scene would need all materials to have
> a non-zero "ambient" term (and a non-zero ambient_light setting) to look
> reasonably ok, a radiosity scene would need all non-glowing materials to
> have a zero "ambient" term (or to have ambient_light be zero, but this
> would prevent the simulation of glowing objects entirely). So you'd
> essentially need two versions of each material: One for non-radiosity
> scenes, and one for radiosity scenes.
>
> The introduction of the "emission" setting solves this conflict, by
> providing different knobs for each of the purposes "ambient" had come to
> be used for: The "ambient" setting can now be used without limitations
> to simulate the effect of indirect light in non-radiosity scenes,
> without undesired side effects in radiosity scenes (as it is turned off
> entirely there, being replaced by the radiosity mechanism); at the same
> time, glowing materials can now be modeled without limitations using the
> "emission" setting instead, as it is available in both radiosity and
> non-radiosity scenes.
>
> (As a side note, another - albeit less important - difference is that
> the "emission" setting is insensitive to the ambient_light parameter.)
>
Thank you for the very detailed explanation. I've learned a lot from it. :)
Todd
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