tikz: placing nodes relative to the bounding box coordinates

Question

I have a tikzpicture,in this case a blue rectangle, that will produce a bounding box, here visualized by the green background. I would now like to place a node in the top left corner of the bounding box. In this case, since I know the absolute coordinates it is easy to do, but that is not always the case, especially when using external graphics or drawing three dimensional objects in rotated coordinate systems. It would therefore be handy, to be able to place the nodes relative to the bounding box. In the axis environment, nodes can be placed in relative coordinates using rel axis cs. In this case the code would be \node at (rel axis cs:0,1) [anchor=north west] {node content}. Since in my example, there is no pre-defined axis,this is not possible here. Is there something similar for a plain tikzpicture?

\documentclass[10pt,crop]{standalone}

\usepackage{tikz}

\usetikzlibrary{backgrounds}

\begin{document}

\begin{tikzpicture}[tight background,background rectangle/.style=
 {fill=green},show background rectangle] \path[draw=black,fill=blue]
 (0,-1) -- (-1,0) -- (1,2) -- (2,1) -- cycle; 
\end{tikzpicture}

\end{document}

Answer 1

You can use current bounding box.north west with proper anchor. Since the bounding box will vary when we add something, it will be better to fix it by adding use as bounding box (thanks to zeroth). Then we are let loose to use all options like positioning library etc. A little example:

\documentclass[10pt,crop]{standalone}

\usepackage{tikz}

\usetikzlibrary{backgrounds,calc,positioning}

\begin{document}

\begin{tikzpicture}[tight background,background rectangle/.style= {fill=green},show background rectangle, ] 
\path[draw=black,fill=blue,use as bounding box] (0,-1) -- (-1,0) -- (1,2) -- (2,1) -- cycle;    %%% note the use as bounding box here
\node [draw,rectangle,inner sep=0pt,anchor=north west] at (current bounding box.north west)(me) {me};
\node [draw,rectangle,inner sep=0pt,anchor=north west,below=.7cm of me]  {me};
\node [draw,circle,inner sep=0pt,anchor=south] at ($(current bounding box.south east)+(-.5,.5)$) {you};
\end{tikzpicture}
\end{document}

Relative positioning can be done by making use of the available anchors of the bounding box. A simple demonstration will be:

\documentclass[10pt,crop]{standalone}

\usepackage{tikz}

\usetikzlibrary{backgrounds,calc,positioning}

\begin{document}

\begin{tikzpicture}[tight background,background rectangle/.style= {fill=green},show background rectangle, ] 
\path[draw=black,fill=blue,use as bounding box] (0,-1) -- (-1,0) -- (1,2) -- (2,1) -- cycle;    %%% note the use as bounding box here
\node [draw,rectangle,inner sep=0pt,anchor=north west] at (current bounding box.north west)(me) {me};
\node [draw,rectangle,inner sep=0pt,anchor=west] at ($(current bounding box.north west)!.37!(current bounding box.south west)$) {me};  %% 37% from north west
\node [draw,circle,inner sep=0pt,anchor=center] at ($(current bounding box.south)!.5!(current bounding box.east)$) {you};
\end{tikzpicture}
\end{document}

Also, please note the comment by zeroth below to disable the updating of bounding box.