In article <421b6daf@news.povray.org>,
 "Zeger Knaepen" <zeg### [at] studentkuleuvenacbe> wrote:

> "Christopher James Huff" <cja### [at] gmailcom> wrote in message
> news:cjameshuff-53CA25.10221222022005@news.povray.org...
> > There are many surfaces that simply can't be seamlessly mapped to a 2D
> > plane. The sphere is the simplest example. How would your hypothetical
> > algorithm UV map a sphere?
> 
> I'm confused, uv-mapping a sphere isn't difficult, is it?

It's very easy, there are several ways to do it, but none are seamless, 
and I see no reasonable way for a general algorithm to automatically UV 
map a sphere. You need to specifically specify that you want to use a 
sphere mapping.

The basic problem is this...given a point where u and v coordinates are 
zero, and basis vectors for the u and v axii at that point, what are the 
UV coordinates at some other point? That's all the information you have, 
unless you somehow collect other information beforehand. You don't have 
the sphere center or radius, because you don't know it's a sphere. You 
could probably do some analysis of the surface to determine if it's 
close enough to spherical and find the approximate center, but that's 
slow, complicated, unreliable, and you'll need to make additional 
special cases for other basic shapes.

The only reasonable solution is to require the user to specify what he 
wants. That's only slightly awkward to do with function patterns...it's 
worse with things like image_maps though. A general (not specific to 
isosurfaces) user-defined UV mapping using functions could be useful, 
but it's hardly a critical feature.

-- 
Christopher James Huff <cja### [at] gmailcom>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: <chr### [at] tagpovrayorg>
http://tag.povray.org/

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