Re: vrml/povray output ("terminal")

[Ethan A M. <sf...@us...>]

On Friday, 22 January, 2016 12:41:39 folkert wrote:
> > >At http://t-kita.net/gnuplot_povrml/ I found a patch for gnuplot to add
> > >povray and/or vrml output. Especially the last one I think is useful
> > >because it allows one to easily view for example an splot from all
> > >kinds of directions.
> > >Now I'm wondering: what does it take to get it to be included in the
> > >main gnuplot distribution?
> > 
> > That looks like code outside the project, no license infringement
> > technically or otherwise.  Patch and compile.
> 
> Yes but what does it take to get it integrated in the main gnuplot
> distribution?

I do not think that this patch is suitable for inclusion in gnuplot.

The web page you link to gives a misleading impression of what the patch
actually does. Let me explain.

Gnuplot can be viewed as having 3 layers.

(1) An input layer in which data enters the program.  Currently the
program can read ascii files (include csv format files), binary files,
and some specific image formats. Once the data enters the program it is
manipulated by ...

(2) A data-processing layer. This is primarily the "plot" and "splot"
commands, but also "fit" and "stats".  These latter two options output
analysis results and stop. The two plotting commands feed their output
though ...

(3) A third layer consisting of various "terminals" that format the
composed plot for a specific output device.  It is important to note
that the interface from layer 2 to layer 3 passes only 2D coordinates.
This is a historical artifact.  Gnuplot was originally designed in the
days of of pen plotters and character cell terminals.  Today we just
assume that graphics is done in 3D, but that was not true 40 years ago. 

The important thing is that all of the various plot styles
(lines, dots, boxes, surfaces, histograms, etc) are implemented in 
layer 2.  You can mix-and-match any choice of plot style (layer 2)
with any output device (layer 3).  Of course a hidden-surface plot
displayed to "set term dumb" won't look very good, but you can do it
if you want.

OK, now to the vrml/povray patch.
Although the patch claims to provide new terminals (vrml and povray),
this is not true and would not be possible because a gnuplot
terminal only knows about 2D coordinates.  Instead it inserts a 
new chunk of code into layer 2 that dumps a bunch of vrml or povray
header text, a formatted version of selected coordinates that were
previously read in by layer 1, a trailing bunch of vrml text,
and that's it.  It mostly ignores all the gnuplot plot styles.

I won't say this is useless.  As the web page shows, you can create
certain 3D plots this way.  But I will say that if this is what you
want to do, there are better ways to do it. Most of those better
ways don't involve gnuplot at all, but if for whatever reason you
want the solution to be based on gnuplot then a better approach would
be to write a separate script (your choice of scripting languages)
that dumps
- the vrml/povray header lines
- tabular output from gnuplot run as a separate process
- the vrml/povray trailing lines

This would have many advantages, not least that you could tweak
the script without having to patch and recompile gnuplot.

It would still be tricky to dump 3D coordinates, but I think it might
be possible using the plot style "with table" in version 5.  If not,
perhaps a patch to extend that capability would be appropriate.
If this works at all it would be useful for more things than just
the hypothetical vrml script.

		Ethan


> 
> 
> Folkert van Heusden
> 
>