Tikz: use opacity for fill, yet leave draw (lines) untouched

Question

When I use the tikz key opacity for a polygon in a tikzpicture, it applies to the drawn fill as well as to the draw lines.

I would like to have it this way that the lines remain untouched by the opacity key, so that they behave as if they had opacity=1.

(What I do now is redraw the polygon with fill=none, but that is a crippled solution to the problem).

Answer 1

You can specify the fill opacity separately from the draw opacity. In the first example below I used opacity and in the second I used fill opacity which will only affect the fill and not the draw.

Note:

• As pointed out by Martin Scharrer: Specifying \opacity=<x> affects the fill, draw and text so is equivalent to saying fill opacity=<X>, draw opacity=<X>, text opacity=<X>.
• fill opacity applies not only to the fill, but also applies to text. To override this behavior, you can separately set the text opacity which applies only to the text labels. So, apply text opacity=1 to ensure that opacity is not applied to the text.

Code:

\documentclass[border=3pt]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
    \draw [ultra thick, draw=black, fill=yellow, opacity=0.2]
       (0,0) -- (0,1) -- (1,1) -- cycle;
\end{tikzpicture}
\begin{tikzpicture}
    \draw [ultra thick, draw=black, fill=yellow, fill opacity=0.2]
       (0,0) -- (0,1) -- (1,1) -- cycle;
\end{tikzpicture}
\end{document}