Am 02.09.2018 um 10:07 schrieb Le_Forgeron:

>> There is a page in the wiki about uv-mapping support. (but it is not
>> easy to find in the search engine)
>>
>> http://wiki.povray.org/content/Reference:UV_Mapping#Supported_Objects
>>
>> For the lazy readers, in 3.8:
>> * bicubic_patch
>> * box
>> * cone, cylinder
>> * lathe, sor
>> * lemon
>> * mesh, mesh2
>> * ovus
>> * parametric
>> * sphere
>> * torus
>>
> 
> And in the sleep of the night, come the greatest question ever: why is
> the disc not part of that list ?

Because it doesn't qualify?
Because nobody ever bothered to implement special UV mapping for it?

Because there is a systematic problem with UV mapping of a disc, which -
now that I come to think of it - _should_ also have precluded cylinder,
cone and lemon from being UV-mapped?


The problem is that the parameterization does not mandate a particular
orientation other than the axis of rotational symmetry.

The rotation of the UV pattern around that axis is arbitrary, and purely
emergent from the current implementation of the primitive (most notably
the code computing a transformation to and from a local coordinate
space); if that implementation ever were to change - be it for
performance, numeric stability, code clarity, or sharing of similar code
- it would affect the UV mapping scheme, and it might be a heck of a lot
of work to emulate the current mapping.


The problem does not arise with lathe, sor or torus because although
they are also rotationally symmetric, their orientation is governed by
definition instead of parameterization, so any transformation to and
from local coordinate space is entirely under user control and unambiguous.

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