TonyB wrote:
> 
> I was wondering why splines, within POV, are not all interchangeable and
> applicable everywhere.  Are not all splines the same basic thing?

  They certainly are not all the same thing. Each spline type performs
a different type of function and each will influence the curvature
of a spline constructed surface in it's own unique way. Some more so
than others and some with little of no merit for 3D surface modelling.

> Could all these different types be added to POV in such a way that they
> could be used everywhere? For example, that I could use a bezier spline
> in a sphere_sweep, or a catmull-rom spline in a prism?

  I see no reason that with your basic everyday knowledge of writing
math intensive algorithms, for a complex raytracing system like Pov,
that it would an overly difficult task to add in sometime before lunch
tommorrow (humour intended).

  Seriously though as you pointed out thre are a wide variety of spline
types that have been been developed into a standard recognized format.
Each offers it's own unique function and there are applied with varying
degrees of difficulty in their implementation and the complexity of their
use by the end user. The reason I see your suggestion as impractical is
because the spline types currently available offer a good average of the
many types of splines commonly used used in these types of applications.

  There are other considerations besides just the existence of the
splines too. When you are using splines to create a surface for a 3D
object it must have mathematical compatibility with the software that
will be appying it. Some of these spline types are extremely math
intensive in the way their functions are evaluated. This makes them
difficult to use use and/or they are so time intensive from a raytracing
point of view it would not be worth the sacrifices needed to implement
them.

  The simple linear spline type in Pov for example requires a quadratic
polynomial equation to be solved in order to perform correctly. The cubic
and bezier splines each require a 6th order polynomial to be solved.
These types of high precision splines have a lot of processing overhead
and not all of the splines you mentioned are necessarily optimized or
can be optimized for use in this environment and for these type of
surface modelling applications.

  There is also the issue of whether or not you would gain very much by
providing support for every type in existence just because it exists.
There are places that you would end up with too much redundancy from
offering so many that I think you would find that in most case the
splines already provided can and will accomplish the same task for the
operation they are associated with.

  Please remember that I am speaking from an experienced Pov users point
of view and I have absolutely no clue as to what actually goes on inside
the program nor do I really have any programming or math skills to speak
of. I speak with the advantage of common sense and a few years of practical
experience using splines in Cad and Drafting software plus the experience
gained from using them in this program. There may be some glaring errors
in my reply but I tried not to exaggerate my opinions in this regard.
Take them for what they are worth and do not consider me the harbinger of
doom for your suggestions.



  Please also remember that when you post a question on a news group
no one is obligated to reply to you. A simple answer to your questions
this time was not possible and my reply has taken more time than I
really cared to devote to it.  I hate to answer with short unhelpful
replies and answering questions like these, make it a choice of
weather or not I want to spend the amount of time it takes to respond
to them in the thorough manner they deserve of just ignore it and move
on. Sometimes I just don't feel like it and there are other times I
simply do not have the time to do so.

  There is also the issue that most of the people around here who
answer to replies are not associated with the Pov team in any way
and are not able to dictate to them what can and or cannot be added
to their software. It gets a little tiring to wish too much about
things you are not even sure are reaching the right ears. If you
think your idea has enough merit send Chris Young a proposal or
feature request and let him decide if it's worth the effort.

  Others who frequently respond to peoples questions here have similar
restrictions and you just are not going to get a large response to
every question you can think of to post. Try to keep this in mind when
your questions seem to go unnoticed or only receive a single reply.
It is not that they are poor questions but more likely are poorly
timed as to when they are posted. Catch me on a good day and I will
write the whole pov manual into a reply for you. Catch me on a bad
day and you will hear not a word from me.

Regards,

-- 
Ken Tyler

mailto://tylereng@pacbell.net

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