tikz: opacity of colours

Question

In order to define the opacity of a fill I can use fill opacity = 0.5. But consider this example:

\documentclass[margin=1cm]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \fill[blue](-0.5, -0.5) rectangle(0, 0.5);
  \fill[green](0, -0.5) rectangle(0.5, 0.5);
  \shadedraw[inner color = red, draw=none](0, 0) circle(0.3);
\end{tikzpicture}
\end{document}

The result is:

I would like the outer rim of the inner circle to gradually merge into the background. Hence, I have on purpose evaded the definition of outer color. However, the default is obviously outer color=white. I would like the outer color to be completely translucid. How to make that happen?

Answer 1

Apparently you need to define a fading with the transparency first:

\documentclass[tikz,margin=1pt]{standalone}
\usetikzlibrary{fadings}
\tikzfading
  [name = viesturs fading,
   inner color = transparent!0,
   outer color = transparent!100]
\begin{document}
\begin{tikzpicture}
  \fill[blue](-0.5, -0.5) rectangle(0, 0.5);
  \fill[green](0, -0.5) rectangle(0.5, 0.5);
  \path[path fading = viesturs fading, inner color = red](0, 0) circle(0.3);
\end{tikzpicture}
\end{document}