Glowing path in tikz

Question

I want to draw in tikz a line that diffuses and a dashed path that is basically the contour of this diffused area. I think that in photoshop the diffusing effect can be done by a blurred stroke.

Is this possible?

Here is a very badly hand-drawn example:

EDIT

Thanks to the help of @percusse and @Alenanno I was able to produce almost exactly what I wanted. The only thing missing is a dashed line around the shaded area instead of the full gray line that I hacked in.

Code here:

\documentclass[margin=10pt]{standalone}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}[>=latex]

    \def\lc{0.6} %Transition: 5-linear %0-infinitely sharp
    \def\psz{1.0} %Potato size
    \def\smss{0.92} %Smoothness: number of colors=1/(1-\smss)

    %Potato
    \newcommand{\potato}[1]{(0*#1,0*#1)(10*#1,-1*#1)(12*#1,10*#1)(4*#1,11*#1)(-1*#1,7*#1)}

    %Crack path
    \def\Ax{0} \def\Ay{0}
    \def\Bx{1.5}    \def\By{2}
    \def\Cx{4.5}    \def\Cy{4}
    \def\Dx{6}  \def\Dy{6}  
    \def\Ex{6}  \def\Ey{4}  
    \def\Fx{8}  \def\Fy{3}  
    \def\linepath{(\Ax,\Ay)(\Bx,\By)(\Cx,\Cy)(\Dx,\Dy)}
    \def\linepathnb{(\Cx,\Cy)(\Ex,\Ey)(\Fx,\Fy)}

    \begin{scope}
        \clip plot [smooth cycle, tension =0.8] coordinates{ \potato{\psz}};
        \draw[line cap=round,line width=105pt, black!20] plot [smooth,tension=1] coordinates{ \linepath};
        \draw[line cap=round,line width=105pt, black!20] plot [smooth,tension=1] coordinates{ \linepathnb};
        \foreach \x[evaluate={\xc=90*(exp(-\x/\lc)-0.9*exp(-1/\lc)*\x;}] in {1,\smss,...,0}{
            \draw[line cap=round,line width=\x*100pt,black,draw=black!\xc] plot [smooth,tension=1] coordinates{ \linepath};
            \draw[line cap=round,line width=\x*100pt,black,draw=black!\xc] plot [smooth,tension=1] coordinates{ \linepathnb};
        }
        \draw[line cap=round,line width=2pt, black] plot [smooth,tension=1] coordinates{ \linepath};
        \draw[line cap=round,line width=2pt, black] plot [smooth,tension=1] coordinates{ \linepathnb};
    \end{scope}
    \draw [line width=5pt] plot [smooth cycle, tension =0.8] coordinates{ \potato{\psz}};

\end{tikzpicture}

\end{document}

Answer 1

You can use the idea behind the pgf-blur package (originated from the question Reuse of soft path in fading declaration? Transformation of fadings? ) that utilizes for smooth shadows : draw over and over with changing the color (or opacity as you wish) and shape size (or line width for paths). The step size can be changed to make it smoother.

I cooked up a formula that looked OK to my eyes but you can go more rigorous for the decay rate and so on.

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
\foreach \x[evaluate={\xc=0.5*100*ln(10/\x);}] in {10,9.9,...,1}{
\draw[line cap=round,line width=\x*1pt,draw=black!\xc]
(0,0) arc (0:30:1 and 2) to[bend left] (3,2) arc (0:250:1 and 1) -- cycle;
}
\end{tikzpicture}
\end{document}

Answer 2

As far as I know, you cannot shade a path the way you have it in your question. You could use a path to clip a shaded shape beneath it, but it wouldn't achieve your result because it's curved and the shading follows the curve.

However, you can blur the line elsewhere and include it as an external image. If it's your only instance of such a graphic, then it's not that time-consumming, but if you don't have multiple types throughout a document, then you might do the dashed line in an external program (it's easy to select a -blurred- shape and apply a contour).

If you did the dashed curve using a graphics editing program along the contour of a blurred stroke, then it would be much more precise than the example below. After that, you would only need to include it in a simple shape in Tikz.

This solution uses a tweaked version of JLDiaz's solution.

Output

Code

\documentclass[margin=10pt]{standalone}
\usepackage{tikz}
\usepackage{graphicx}
\usetikzlibrary{shapes,calc}   

\newcommand\fillshape[3]{ % #1 = shape, #2 = filename of texture, #3 = includegraphics options
    \begin{scope}
        \clip #1;
        \node[yshift=-2.6em] {\includegraphics[#3]{#2}};
        \draw[densely dashed,line width=.2pt] (.43,-1.5) 
            to[out=77,in=255,looseness=1.6,yshift=1.3] (.1,-.2) 
            to[out=65,in=69,looseness=1.7] (-.45,.05)
            to[out=249,in=95,looseness=.7] (-.55,-.6)
            to[out=275,in=70,looseness=1.1,yshift=1.3] (-.15,-1.6);
    \end{scope}
    \draw[line width=.5pt] #1;

}

\begin{document}
\begin{tikzpicture}
\fillshape{(0,0) circle (1.5cm)}{line}{width=2cm};
\end{tikzpicture}
\end{document}