clipka <nomail@nomail> wrote:
> Warp <war### [at] tagpovrayorg> wrote:
> >   I don't have the motivation to study exactly which mathematical function
> > POV-Ray is using for its splines nor calculating its derivatives.

> You do have the motivation throwing smart statements about, but you don't have
> any motivation to really investigate the issues you are talking about.

> Wise guy.

  Then you, as a wiser person than me, could explain to me why it would be
impossible to calculate the tangent of a spline using its derivative
functions. I'm so stupid that I can't understand why it's impossible.

> You don't have to calculate any particular spline's derivative to find out what
> the problem is. You just have to look at the macros and SDL statements we're
> discussing about, to find out that you're overlooking a much more serious
> obstacle.

  What obstacle?

  The question was: Is it possible to calculate the tangent of a spline with
absolute accuracy? The answer is obviously: Yes, it's possible.

  What does the macro you are talking about have anything to do with this?

> >   I don't see the problem you are having. A derivative function is nothing
> > more than a function. In the exact same way as you can create a macro which
> > returns values of a function, you can create another macro which returns
> > values of its derivative function. Because it's just another function.

> BTW, *I* am not having *any* problem. I'm just describing why a problem the OP
> has is inevitable.

  The original poster asked if it's possible to calculate the direction of
the spline with complete accuracy. You said that it's impossible. That's
clearly not true.

> Anyway, we are talking about macros. SDL statements, you know?

  So what? What is it in SDL that prevents you from evaluating a function?

> So how do you get the function value of an arbitrary spline out of POV-Ray?

  Splines are not "arbitrary". They have a well-defined formula.

> Rrrright - you use an interface built into the SDL, which lets you use the
> spline as a function.

> Fine. Now how do you get the derivative value of an arbitrary spline out of
> POV-Ray?

  Maybe you should go back to school to learn how to calculate derivatives
because you clearly don't seem to grasp the concept.

> Mind you: The spline type and coefficients are buried in the bowels of POV-Ray.

  So what? Does that mean you cannot calculate its derivative? It's not
like POV-Ray would be proprietary closed source.

> If you want to get at them from SDL, you need an interface to do that.

  Why can't you just write the functions in SDL? Is there something in
there that cannot be done in SDL?

> No
> interface - no access to the data. No access to the data - no way to insert the
> data into your derivative formula (let alone decide on which formula to use,
> because we don't even know the type of the spline).

  You are seriously claiming that it's impossible to write a spline function
in SDL? Maybe you should check eg. Colefax's spline includes.

  There's no basic difference between Colefax's spline includes and the
internal splines used in povray, except maybe for some coefficients.

> Now, your turn. Stop blurping and show what you *really* know and understand
> about the OP's question.

> And no excuses: If you're too lazy to really investigate a certain matter,
> better hold your breath about it right from the start.

  You have failed completely to explain the problem. You have just rambled
about how something is impossible in SDL without giving any actual example.

-- 
                                                          - Warp

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