On 11/15/2012 8:43 AM, Bruno Cabasson wrote:
> "Cousin Ricky" <rickysttATyahooDOTcom> wrote:
>> "Bruno Cabasson" <bru### [at] cabassoncom> wrote:
>>> Hi all!
>>>
>>> Considering that many 3D applications (eg games) use single precision for their
>>> renders, I am wondering where we really need double precision for 'normally'
>>> sized scenes. I mean scenes that do not mix microscopic and gigantic distances.
>>>
>>> For sure, if the dynamic is huge, we absolutely need double precision, at least
>>> for object coordinates. But, if the dynamic in distances is reasonable, is it
>>> mandatory to use double precision for the following features :
>>>
>>>      ray - bounding box intersection tests ?
>>>      ray - object intersection computation ?
>>>      others (TBD) ?
>>>
>> [snip]
>>>
>>> What do you think ? Has the subject already been debated here ?
>>
>> The dynamic doesn't even have to be huge.  See:
>>
>>
http://news.povray.org/povray.binaries.images/thread/%3Cweb.47e40ce72d3a644685de7b680@news.povray.org%3E/?ttop=304745
> &t
>> off=500
>>
>> Since the error was not lighting-related, I don't think the 37,417 POV-unit
>> distance to the light source should be considered in the dynamic.  What sort of
>> dynamic do you have in mind?
>>
>> POV-Ray does not seem suited to single precision.  Perhaps it is the nature of
>> mathematically defined shapes that they require higher precision than mesh-based
>> systems.
>
> Meshes also involve maths ...
>
> The kind of dynamic I have in mind could be a 1 millimeter detail close to the
> camera, and a million miles away huge object. Take the recent post with the
> toroidal planet in pbi. Imagin the camera sees a flower on its ground with small
> petals.
>
> My question was about the fundamental need of double precision in Ray-Tracing
> for 'normal' scenes. What features need DP, and what need not.
>
Uh.. All of them? lol

Seriously though, the problem is close distances. Things like 
"coincident surfaces" can appear much easier in cases where the 
precision is lower. The higher the precision, the less likely you are to 
get miscalculations. I presume there are other cases as well. But, in 
this case, in principle, you can have objects layered "much closer" to 
each other, and not have accidental overlaps, due to attempts to find 
which one got hit first in the math.

Still think there has got to be some better solution to those things 
though, than just throwing bigger and bigger numbers at them... lol

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