"Bald Eagle" <cre### [at] netscapenet> wrote:
> So, I was going to spend some time converting some cool Shadertoy code into SDL,
> and I was annoyed (as usual) that I couldn't do certain things.
> I specifically wanted to create a function that performed a rotational matrix
> transform on a vector given a variable angle.
>
> Well, the function VM doesn't allow transform functions to take user-defined
> arguments ("parameter names").
> And we all "know" that you can't use vectors in functions.
> Hell you can't even use vector dot-notation in functions.
>
> 2.2.1.6.4 Declaring User-Defined Vector Functions
> Right now you may only declare vector functions using one of the special
> function types. Supported types are transform and spline functions. For example:
>
>  #declare foo = function {
>    transform {
>      rotate <90, 0, 0>
>      scale 4
>    }
>  }
>
>  #declare myvector = foo(4, 3, 7);
>
>  #declare foo2 = function {
>    spline {
>      linear_spline
>      0.0, <0,0,0>
>      0.5, <1,0,0>
>      1.0, <0,0,0>
>    }
>  }
>
>  #declare myvector2 = foo2(0.7);
>
> Function splines take the vector size into account. That is, a function
> containing a spline with five components will also return a five component
> vector (aka a color), a function containing a spline with two components will
> only return a two component vector and so on.
>
> Note: the number of parameters of special function types is determined
> automatically, so you do not need to specify parameter names.
>
>
> But, me being ME, that got a few things wiggling up in the ole' brain, and I
> started to follow a few tenuous ides.
> Round and round I went in circles, trying to make vector functions work, and
> trying to bridge the gap between the SDL parser and the function VM.
>
> And I have mostly done that as you'll see in the attached file.
>
> The only thing that would make this FULLY functional would be to find a way to
> shoehorn the actual vector into an argument allowable by some sort of special
> function syntax.
>
> But I hope that this inspires people to start playing around with the functions,
> macros, reading the docs, exploring the various include files in the
> distribution, and doing some experiments that reveal things that we can do (but
> were told we couldn't).

Hi Bill

I once did some work on something similar. See this thread:

From: Tor Olav Kristensen
Subject: Visualizing vector fields with a single isosurface
Date: 2010-01-30 08:35:01
http://news.povray.org/povray.binaries.images/thread/%3Cweb.4b6434d658c2698ed6291f840%40news.povray.org%3E/

Here's my (not so good) try at an explanation of what is going on:

In this isosurface, a grid of rounded boxes are created with a function.
And each of the x, y and z arguments to this function are processed by
other functions that does inverse transformations to them. I.e.: For each
points (x, y, z) in space these "preprosessing functions" calculates a
transform to apply to the x, y, z values that are fed to the function
that creates the the grid of rounded boxes.

Also have a look at how the function created by the RotateXYZ_Function()
macro is used in these SDL files:

https://dataduppedings.no/subcube/POV-Ray/Isosurface-Rotated_Noise.txt
https://dataduppedings.no/subcube/POV-Ray/Isosurface-Cubic_Space_Division.txt

--
Tor Olav
http://subcube.com
https://github.com/t-o-k

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