Re: [Maxima-discuss] How do I do just the first half of "plotting contour levels"?

[Robert D. <rob...@gm...>]

On Thu, Nov 17, 2022 at 7:09 PM Eduardo Ochs <edu...@gm...> wrote:

> a few days ago I had to draw some level curves of
>
>   F(x,y) := 2*x^2 - x*y - y^2;
>
> in TikZ.

Gnuplot has an output terminal "tikz". You can tell Gnuplot to use
tikz in Maxima:

set_plot_option ([gnuplot_term, "tikz"]);

and for good measure

set_plot_option ([gnuplot_out_file, "path/to/myplot.tikz"]);

and then plot2d output will go to that file.

About capturing level curves for some analysis or exploration or
something, it's not a bad idea. Maxima doesn't make it easy, but it
looks like it's not too hard to intercept the level curves as
determined in the plotting code by the function DRAW2D-IMPLICIT. Just
to investigate the idea, try this:

:lisp (let ((f (symbol-function 'draw2d-implicit))) (defun
draw2d-implicit (&rest args) (let ((stuff (apply f args))) (displa
stuff) stuff)))

Looks like the stuff generated by DRAW2D-IMPLICIT comprises a list of
curve segments -- not sure at the moment how to interpret it, although
I don't think it's complicated. We can sort it out if you think it's
useful.

Getting a list of curves for a contour plot or implicit plot might be
useful from time to time -- we could consider exposing DRAW2D-IMPLICIT
so that it can be called directly (and returning an intelligible
result).

best,

Robert