http://wiki.povray.org/content/Reference:Union#Split_Union

OK ... narrative says it's used with photons, but made a typo and 
accidentally enabled on a tile floor (many tiles and grout) but I wasn't 
using photons. The object count went down like expected, but render time 
slowed to a crawl ... walked away from computer, and the usual 3 minute 
render had only managed to get almost through the first row. Like I said 
a mistake, and can't off the top of my head think of any use of 
split_union /without/ photons ... could it be that split_union should 
first check for photon usage then ignore the "on" setting if photons 
aren't used?

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> http://wiki.povray.org/content/Reference:Union#Split_Union
>
> OK ... narrative says it's used with photons, but made a typo and
> accidentally enabled on a tile floor (many tiles and grout) but I wasn't
> using photons. The object count went down like expected, but render time
> slowed to a crawl ... walked away from computer, and the usual 3 minute
> render had only managed to get almost through the first row. Like I said
> a mistake, and can't off the top of my head think of any use of
> split_union /without/ photons ... could it be that split_union should
> first check for photon usage then ignore the "on" setting if photons
> aren't used?

Normaly, unions are split into the individual component. It improve 
bounding, making the scene render much faster. Each component have it's 
own bounding box.

With split_union off, you are forced to evaluate the whole union, and 
all of it's elements, for every ray that enter it's bounding box. You no 
loner have the individual bounding boxes, bot only one large one.
You get something similar when you use merge or apply a manual bounding 
on an union.


Alain

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On 12/14/2013 08:55 PM, Alain wrote:

>> http://wiki.povray.org/content/Reference:Union#Split_Union
>>
>> OK ... narrative says it's used with photons, but made a typo and
>> accidentally enabled on a tile floor (many tiles and grout) but I wasn't
>> using photons. The object count went down like expected, but render time
>> slowed to a crawl ... walked away from computer, and the usual 3 minute
>> render had only managed to get almost through the first row. Like I said
>> a mistake, and can't off the top of my head think of any use of
>> split_union /without/ photons ... could it be that split_union should
>> first check for photon usage then ignore the "on" setting if photons
>> aren't used?
>
> Normaly, unions are split into the individual component. It improve
> bounding, making the scene render much faster. Each component have it's
> own bounding box.
>
> With split_union off, you are forced to evaluate the whole union, and
> all of it's elements, for every ray that enter it's bounding box. You no
> loner have the individual bounding boxes, bot only one large one.
> You get something similar when you use merge or apply a manual bounding
> on an union.

Yes I understand the premise, but that really didn't answer the question 
I posed (I don't think) ... is (or should) split_union be exclusive to 
photon usage. That /is/ the only usage the doc passage cites.

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> On 12/14/2013 08:55 PM, Alain wrote:

>>> http://wiki.povray.org/content/Reference:Union#Split_Union
>>>
>>> OK ... narrative says it's used with photons, but made a typo and
>>> accidentally enabled on a tile floor (many tiles and grout) but I wasn't
>>> using photons. The object count went down like expected, but render time
>>> slowed to a crawl ... walked away from computer, and the usual 3 minute
>>> render had only managed to get almost through the first row. Like I said
>>> a mistake, and can't off the top of my head think of any use of
>>> split_union /without/ photons ... could it be that split_union should
>>> first check for photon usage then ignore the "on" setting if photons
>>> aren't used?
>>
>> Normaly, unions are split into the individual component. It improve
>> bounding, making the scene render much faster. Each component have it's
>> own bounding box.
>>
>> With split_union off, you are forced to evaluate the whole union, and
>> all of it's elements, for every ray that enter it's bounding box. You no
>> loner have the individual bounding boxes, bot only one large one.
>> You get something similar when you use merge or apply a manual bounding
>> on an union.
>
> Yes I understand the premise, but that really didn't answer the question
> I posed (I don't think) ... is (or should) split_union be exclusive to
> photon usage. That /is/ the only usage the doc passage cites.
>

Been the only place where it's mentioned don't mean that it can *only* 
apply when photons are used. split_union is an object modifier, just 
like no_image, no_shadow, no_reflection, no_radiosity or hollow that can 
all take an optional on/off parameter.

The only difference been that split_union default to off while the above 
modifiers default to on by their sole presence and off when absent.

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