Am 06.04.2018 um 08:39 schrieb Kenneth:
> clipka <ano### [at] anonymousorg> wrote:
>> Am 05.04.2018 um 19:09 schrieb kurtz le pirate:
>>
>>>  - the floor currently uses
>>>     finish {
>>>         ambient 0.00
>>>         emission 0.10
>>
>> No. No. Absolutely no. Unless you /specifically/ want to model something
>> that glows in the dark, do NOT EVER use "emission".
>>
>> If you want to brighten up shadows, use radiosity or "ambient".
>>
> 
> That's interesting; I thought ambient and emission were identical in their
> effect (in a non-radiosity scene, that is.) Basically that they are
> interchangable.

And that's /exactly/ the reason: From their effect in /non-radiosity/
scenes they are interchangable, but from both their /intention/ and
their effect in /radiosity/ scenes the two are worlds apart.

Using "emission" to model indirect illumination of shadowy areas so
/utterly/ defies the purpose of the feature that I /must/ rant about it,
as a matter of principle. Kind of like if you'd use "assumed_gamma 500"
or something like that.

Here's some historic background:

Brightening up shadows, to model the indirect illumination they receive
from other objects, is the very thing the "ambient" mechanism was
originally designed to do. The mechanism was literally named for a
technical term denoting this type of illumination: "ambient light". It
was the very first mechanism in a triad of mechanisms.

Next in the triad came radiosity, which serves /exactly/ the same
purpose, albeit with much higher quality. So theoretically, the
"ambient" mechanism and radiosity shouldn't be used together.

However, people figured out that radiosity still picked up the light
from the "ambient" mechanism, which had two consequences:

(1) "Classic" materials (i.e. materials originally designed for
non-radiosity scenes, and therefore using a non-zero "ambient" value)
could not be used in a radiosity scene "as is" -- unless you disabled
the "ambient" mechanism altogether by using "ambient_light 0" in the
global settings.

(2) Light-emitting materials could easily be modeled in radiosity scenes
by co-opting the "ambient" mechanism, setting it to non-zero for that
particular material -- but only if you kept the "ambient" mechanism
enabled by using a non-zero "ambient_light" in the global settings.

So in radiosity scenes, there was frequently a conflict of interest
between disabling the "ambient" mechanism (to re-use existing
materials), and keeping it enabled (to model glowing materials).

To solve this conflicts of interest, the third mechanism in the triad
was introduced in v3.7: The "emission" mechanism was /specifically/
designed to model glowing materials. It does the same in both radiosity
and non-radiosity scenes. "ambient", on the other hand, is now
explicitly intended solely to model ambient light, and is disabled
automatically in radiosity scenes, so that materials can be designed to
work equally well in both radiosity scenes (where ambient light is taken
care of by radiosity) and non-radiosity scenes (where ambient light can
be approximated using the simpler "ambient" mechanism), without changing
a jot in the material definition.


> I haven't yet seen an indication in the docs that suggests
> otherwise(?)

The tutorial talks a bit about the differences ("Using Ambient" and
"Using Emission").

Also, the reference section on ambient talks a lot about "Ambient
Lighting in shadowed areas", whereas the reference section on emission
talks about "glowing materials". In the case of your floor it is pretty
clear which one is more applicable.


> Is there a visual difference? Or does it have to do with making sure that
> POV-Ray's internal computations are performed correctly?

It has to do with not co-opting a mechanism that was designed to avoid
having to co-opting the very same mechanism you should actually be using.

It has to do with making sure your materials will work equally well in
both radiosity and non-radiosity scenes. If you use the "ambient"
mechanism to model the brightening of shadows from ambient light, your
material has a fair chance of continuing to work properly if you ever
turn on radiosity. If you use the "emission" mechanism instead, your
material is /guaranteed/ to screw it all up.

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