TikZ frameworks in my matrix are skewed

Question

I would like to draw frameworks around certain submatrices of a large matrix, like described in this question: Highlight elements in the matrix.

I tried some of the provided solutions and ended up using this one:

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\usepackage{color}
\definecolor{orange}{RGB}{255,127,0}
\usetikzlibrary{arrows,matrix,positioning}
\usepackage{xfrac}
\begin{document}

\begin{tikzpicture}
\matrix [matrix of math nodes,left delimiter=(,right delimiter=)] (m)
{
1 & 0 & 0 & 0 & 0 & 0 & 0\\ 
0 & 1 & 0 & 0 & 0 & 0 & 0\\ 
0 & \frac{1}{2} & \frac{1}{2} & 0 & 0 & 0 & 0\\ 
0 & \frac{1}{4} & \frac{7}{12} & \frac{1}{6} & 0 & 0 & 0\\ 
0 & 0 & \frac{2}{3} & \frac{1}{3} & 0 & 0 & 0\\ 
0 & 0 & \frac{1}{3} & \frac{2}{3} & 0 & 0 & 0\\ 
0 & 0 & \frac{1}{6} & \frac{2}{3} & \frac{1}{6} & 0 & 0\\ 
0 & 0 & 0 & \frac{2}{3} & \frac{1}{3} & 0 & 0\\ 
0 & 0 & 0 & \frac{1}{3} & \frac{2}{3} & 0 & 0\\ 
0 & 0 & 0 & \frac{1}{6} & \frac{7}{12} & \frac{1}{4} & 0\\ 
0 & 0 & 0 & 0 & \frac{1}{2} & \frac{1}{2} & 0\\ 
0 & 0 & 0 & 0 & 0 & 1 & 0\\ 
0 & 0 & 0 & 0 & 0 & 0 & 1\\
};  
\draw[color=orange,line width=1pt] (m-1-1.north west) -- (m-1-4.north east) -- (m-4-4.south east) -- (m-4-1.south west) -- cycle;
\draw[color=orange,line width=1pt] (m-4-2.north west) -- (m-4-5.north east) -- (m-7-5.south east) -- (m-7-2.south west) -- cycle;
\draw[color=orange,line width=1pt] (m-7-3.north west) -- (m-7-6.north east) -- (m-10-6.south east) -- (m-10-3.south west) -- cycle;
\draw[color=orange,line width=1pt] (m-10-4.north west) -- (m-10-7.north east) -- (m-13-7.south east) -- (m-13-4.south west) -- cycle;
\end{tikzpicture}            

\end{document}

The thing is, when in a certain row there are both plain numbers and fractions, the frameworks are skewed like this:

How can I fix this? I tried to replace each \frac by \sfrac (that's why I added the package xfrac), but this makes the problem even worse.

Answer 1

With fit. (update) I added a new style rec.

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\usepackage{color}
\definecolor{orange}{RGB}{255,127,0}
\usetikzlibrary{arrows,matrix,positioning,fit}
\usepackage{xfrac}
\begin{document}

\begin{tikzpicture}[rec/.style={draw=orange,thick,inner sep=0}]
\matrix [matrix of math nodes,left delimiter=(,right delimiter=)] (m)
{
  1 & 0 & 0 & 0 & 0 & 0 & 0\\ 
  0 & 1 & 0 & 0 & 0 & 0 & 0\\ 
  0 & \sfrac{1}{2} & \sfrac{1}{2} & 0 & 0 & 0 & 0\\ 
  0 & \sfrac{1}{4} & \sfrac{7}{12} & \sfrac{1}{6} & 0 & 0 & 0\\ 
  0 & 0 & \sfrac{2}{3} & \sfrac{1}{3} & 0 & 0 & 0\\ 
  0 & 0 & \sfrac{1}{3} & \sfrac{2}{3} & 0 & 0 & 0\\ 
  0 & 0 & \sfrac{1}{6} & \sfrac{2}{3} & \sfrac{1}{6} & 0 & 0\\ 
  0 & 0 & 0 & \sfrac{2}{3} & \sfrac{1}{3} & 0 & 0\\ 
  0 & 0 & 0 & \sfrac{1}{3} & \sfrac{2}{3} & 0 & 0\\ 
  0 & 0 & 0 & \sfrac{1}{6} & \sfrac{7}{12} & \sfrac{1}{4} & 0\\ 
  0 & 0 & 0 & 0 & \sfrac{1}{2} & \sfrac{1}{2} & 0\\ 
  0 & 0 & 0 & 0 & 0 & 1 & 0\\ 
  0 & 0 & 0 & 0 & 0 & 0 & 1\\
};  
\node[fit=(m-1-1)(m-4-4),  rec] {}; 
\node[fit=(m-4-2)(m-7-5),  rec] {};  
\node[fit=(m-7-3)(m-10-6), rec] {};  
\node[fit=(m-10-4)(m-13-7),rec] {};  
\end{tikzpicture}            

\end{document}

Answer 2

The beta package matrixcells eats this sort of thing for breakfast. Here's your code with a modicum of changes - I'm tempted to set this as a "spot the difference" between the original code and this.

\documentclass{article}
%\url{http://tex.stackexchange.com/q/53938/86}
\usepackage{amsmath}
\usepackage{tikz}
\usepackage{matrixcells}
\usepackage{color}
\definecolor{orange}{RGB}{255,127,0}
\usetikzlibrary{arrows,matrix,positioning}
\usepackage{xfrac}
\begin{document}

\begin{tikzpicture}
\matrix [matrix of math nodes,left delimiter=(,right delimiter=),label cells] (m)
{
1 & 0 & 0 & 0 & 0 & 0 & 0\\ 
0 & 1 & 0 & 0 & 0 & 0 & 0\\ 
0 & \frac{1}{2} & \frac{1}{2} & 0 & 0 & 0 & 0\\ 
0 & \frac{1}{4} & \frac{7}{12} & \frac{1}{6} & 0 & 0 & 0\\ 
0 & 0 & \frac{2}{3} & \frac{1}{3} & 0 & 0 & 0\\ 
0 & 0 & \frac{1}{3} & \frac{2}{3} & 0 & 0 & 0\\ 
0 & 0 & \frac{1}{6} & \frac{2}{3} & \frac{1}{6} & 0 & 0\\ 
0 & 0 & 0 & \frac{2}{3} & \frac{1}{3} & 0 & 0\\ 
0 & 0 & 0 & \frac{1}{3} & \frac{2}{3} & 0 & 0\\ 
0 & 0 & 0 & \frac{1}{6} & \frac{7}{12} & \frac{1}{4} & 0\\ 
0 & 0 & 0 & 0 & \frac{1}{2} & \frac{1}{2} & 0\\ 
0 & 0 & 0 & 0 & 0 & 1 & 0\\ 
0 & 0 & 0 & 0 & 0 & 0 & 1\\
};  
\draw[color=orange,line width=1pt] (m-cell-1-1.north west) -- (m-cell-1-4.north east) -- (m-cell-4-4.south east) -- (m-cell-4-1.south west) -- cycle;
\draw[color=orange,line width=1pt] (m-cell-4-2.north west) -- (m-cell-4-5.north east) -- (m-cell-7-5.south east) -- (m-cell-7-2.south west) -- cycle;
\draw[color=orange,line width=1pt] (m-cell-7-3.north west) -- (m-cell-7-6.north east) -- (m-cell-10-6.south east) -- (m-cell-10-3.south west) -- cycle;
\draw[color=orange,line width=1pt] (m-cell-10-4.north west) -- (m-cell-10-7.north east) -- (m-cell-13-7.south east) -- (m-cell-13-4.south west) -- cycle;
\end{tikzpicture}            

\end{document}

Here's the result:

The matrixcells package puts rectangular nodes over each of the cells of a TikZ matrix in such a way that they tile the matrix. This ensures that their edges line up and so forth. Thus they are better designed for outlining regions of a matrix than the individual cells themselves. For example, if one of the non-corner nodes happens to be extra tall then Altermundus' code won't adapt to this whereas the above will. Here's the comparison with Altermundus' code producing the orange lines and mine producing the green.