[Tex/LaTex] Problem in adjusting contour label color and numbr of samples in pgfplot

Question

I am trying to create a contour plot using TikZ. I have two questions:

1. How can I change the contour labels color to all black?
2. Is it normal that gnuplot responds slow for creating smoother contour lines? what is the best way of creating a smooth contour line?

The code that I used is as follows:

\begin{tikzpicture}
    \begin{axis}[
        title = {$\omega_\phi\left(x, \sigma\right) = \frac{\Gamma}{\pi{R_c}^2}\exp\left[-\frac{\left(x-x_0\right)^2+\left(\sigma-R_0\right)^2}{{R_c}^2}\right]$},
        xmin=1.5, xmax=3.5,
        ymin=1.5, ymax=3.5,
        view={0}{90},
    ]
    \addplot3[
        contour gnuplot = {contour label style={draw=black},},
        samples=10,
        contour/draw color={black},
        ]
    {1/3.1415/0.25^2*exp(-((x-2.5)^2+(y-2.5)^2)/0.25^2)};
    \end{axis}
\end{tikzpicture}

When I want to create a smoother plot by using for example 200 samples, It takes a long time to respond. Is this normal? Because I have a super powerful computer. Even with 200 sample points, it does not produce the smooth contour line that I want. I am thinking of using 1000 sample points which takes forever. Everytime I want to compile my paper, I have to wait for a long time for a figure to see the whole PDF of my paper. Is there a better and faster way of creating a smooth contour lines?

Also, the label colors are mapped. I tried to force it to black but it used mapped color. The following is result of the above code using 10 samples.

Could someone help me?

Answer 1

The initial value for the /pgfplots/contour/every contour label key—to which you append with the contour label style key—is ^{_{(pgfplots manual, p. 117,and pgfplotsplothandler.code.tex, ll. 1877ff.)}}:

\pgfplotsset{
  contour/every contour label/.style={
    sloped,
    transform shape,
    inner sep=2pt,
    every node/.style={mapped color!50!black,fill=white},
    /pgf/number format/relative={\pgfplotspointmetarangeexponent},
  }
}

Interesting enough, the initial color (mapped color!50!black) as well as the fill color (to overdraw the plot itself) is set inside the every node style. Why is that so?

Taking a look into pgfplotsplothandlers.code.tex, lines 2071ff., we can see that the every contour label is in fact applied to a scope and not a node. Sneaky!

We can also add stuff to the every node style with either every node/.append style or its short-cut nodes. But use text=black because solely black will also activate a black fill color, and draw=black will only draw the shape’s border.

Now the samples option does actually take effect.

Code

\documentclass[tikz]{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
    \begin{axis}[
        title = {$\omega_\phi\left(x, \sigma\right) = \frac{\Gamma}{\pi{R_c}^2}\exp\left[-\frac{\left(x-x_0\right)^2+\left(\sigma-R_0\right)^2}{{R_c}^2}\right]$},
        xmin=1.5, xmax=3.5,
        ymin=1.5, ymax=3.5,
        view={0}{90},
    ]
    \addplot3[
        contour gnuplot={contour label style={every node/.append style={text=black}}},
        samples=100,
        contour/draw color={black},
        ]
    {1/3.1415/0.25^2*exp(-((x-2.5)^2+(y-2.5)^2)/0.25^2)};
    \end{axis}
\end{tikzpicture}
\end{document}

Output