# How can I highlight elements in a matrix?

I'm about to use this diagram to make computing 3x3 determinants clear and easy for my students. I tried to add this picture to my lecture but it's blurred due to the bad quality of the picture. So i want to create exactly the same figure using latex. I've searched and read a few topics about highlighting elements and diagonals in matrix but it does not work in this case. I want to add plus signs and minus signs as in the picture, and highlight elements in the matrice with elements outside the matrice. I appreciate all your help and your advice. Thank you in advance ! An alternative "pure" TikZ solution:

\documentclass[border=1mm,tikz]{standalone}
\usetikzlibrary{calc,matrix}

\begin{document}
\begin{tikzpicture}[
strip/.style args = {#1/#2}{
draw=#1, very thin, fill=#1, opacity=0.2,
text width=#2,% will be calculated in path
rounded corners,% only if you like them ...
minimum height=1em,% adjust to desired height,
inner xsep=-1mm,% adjust to desired length of strip
inner ysep=2mm,
yshift=-0.6ex,
sloped},
]
\matrix (mtrx)  [matrix of math nodes,
column sep=1em,
nodes={text height=2ex,text width=2ex}]
{
|[red]|+
& |[red]|+
& \color{red}+\color{blue}-
& |[blue]|-
& |[blue]|-   \\
a_1 & b_1 & c_1 & a_1 & a_2   \\
a_2 & b_2 & c_2 & a_2 & b_2   \\
a_3 & b_3 & c_3 & a_3 & b_3   \\
};
\draw[thick] (mtrx-2-1.north) -| (mtrx-4-1.south -- (mtrx-4-1.south);
\draw[thick] (mtrx-2-3.north) -| (mtrx-4-3.south -- (mtrx-4-3.south);
\path let   \p1 = (mtrx-4-1.center),
\p2 = (mtrx-2-4.center),
\n1 = {veclen(\y2-\y1,\x2-\x1)} in
(mtrx-4-1.center) -- node[strip=blue/\n1] {} (mtrx-2-3.center)
(mtrx-4-2.center) -- node[strip=blue/\n1] {} (mtrx-2-4.center)
(mtrx-4-3.center) -- node[strip=blue/\n1] {} (mtrx-2-5.center)
%
(mtrx-2-1.center) -- node[strip=red/\n1] {} (mtrx-4-3.center)
(mtrx-2-2.center) -- node[strip=red/\n1] {} (mtrx-4-4.center)
(mtrx-2-3.center) -- node[strip=red/\n1] {} (mtrx-4-5.center);
\end{tikzpicture}
\end{document} If you like to have different colors, you can defined similarly as this do @MyUserlsThis in his/her answer.

Edit: I wasn't aware for this answer, @egreg thank you for point it. Considered it, the solutions is more simple (and beautiful):

\documentclass[border=1mm,tikz]{standalone}
\usetikzlibrary{calc,matrix}

\begin{document}
\begin{tikzpicture}[
strip/.style = {
draw=#1,%color
line width=1em, opacity=0.2,
line cap=round ,% only if you like them ...
},
]
\matrix (mtrx)  [matrix of math nodes,
column sep=1em,
nodes={text height=2ex,text width=2ex}]
{
|[red]|+
& |[red]|+
& \color{red}+\color{blue}-
& |[blue]|-
& |[blue]|-   \\
a_1 & b_1 & c_1 & a_1 & a_2   \\
a_2 & b_2 & c_2 & a_2 & b_2   \\
a_3 & b_3 & c_3 & a_3 & b_3   \\
};
\draw[thick] (mtrx-2-1.north) -| (mtrx-4-1.south west)
-- (mtrx-4-1.south);
\draw[thick] (mtrx-2-3.north) -| (mtrx-4-3.south east)
-- (mtrx-4-3.south);
\path[draw,strip=red,shorten <=-1mm]
(mtrx-4-1.center) edge (mtrx-2-3.center)
(mtrx-4-2.center) edge (mtrx-2-4.center)
(mtrx-4-3.center)  --  (mtrx-2-5.center);
\path[draw,strip=blue,shorten >=-1mm]
(mtrx-2-1.center) edge (mtrx-4-3.center)
(mtrx-2-2.center) edge (mtrx-4-4.center)
(mtrx-2-3.center)  --  (mtrx-4-5.center);

\end{tikzpicture}
\end{document}


which gives: Edit (2): Regarding to comment of @BarbaraBenton: signs + and - can be moved nearly line width reducing distance between the first and second row. This and also lookout, when strips is rectangular one can obtain with next MWE (where also colors are interchanged):

\documentclass[border=1mm,tikz]{standalone}
\usetikzlibrary{calc,matrix}

\begin{document}
\begin{tikzpicture}[
strip/.style = {
draw=#1,%color
line width=1em, opacity=0.2,
shorten <=-2mm,shorten >=-2mm,
},
]
\matrix (mtrx)  [matrix of math nodes,
column sep=1em,
nodes={text height=1ex,text width=2ex}]
{
|[red]|+
& |[red]|+
& \color{red}+\color{blue}-
& |[blue]|-
& |[blue]|-   \$3.3mm,between origins] a_1 & b_1 & c_1 & a_1 & a_2 \\ a_2 & b_2 & c_2 & a_2 & b_2 \\ a_3 & b_3 & c_3 & a_3 & b_3 \\ }; \draw[thick] (mtrx-2-1.north) -| (mtrx-4-1.south west) -- (mtrx-4-1.south); \draw[thick] (mtrx-2-3.north) -| (mtrx-4-3.south east) -- (mtrx-4-3.south); \path[draw,strip=blue] (mtrx-4-1.center) edge (mtrx-2-3.center) (mtrx-4-2.center) edge (mtrx-2-4.center) (mtrx-4-3.center) -- (mtrx-2-5.center); \path[draw,strip=red] (mtrx-2-1.center) edge (mtrx-4-3.center) (mtrx-2-2.center) edge (mtrx-4-4.center) (mtrx-2-3.center) -- (mtrx-4-5.center); \end{tikzpicture} \end{document} Of course, there is still place to make fine adjustment. For example smaller signs + and -, or different positioning, or etc, but something should be left to OP (if he will accept the answer). Edit (3): For use with document class beamer, here is complete and many time tested MWE (with recent Beamer: version 3.36):  \documentclass{beamer} \documentclass{beamer} \usepackage{tikz} \usetikzlibrary{calc,matrix} \begin{document} \begin{frame}[fragile] \begin{tikzpicture}[ strip/.style = { draw=#1,%color line width=1em, opacity=0.2, shorten <=-2mm,shorten >=-2mm, }, ] \matrix (mtrx) [matrix of math nodes, column sep=1em, nodes={text height=1ex,text width=2ex}] { |[red]|+ & |[red]|+ & \color{red}+\color{blue}- & |[blue]|- & |[blue]|- \\[3.3mm,between origins] a_1 & b_1 & c_1 & a_1 & a_2 \\ a_2 & b_2 & c_2 & a_2 & b_2 \\ a_3 & b_3 & c_3 & a_3 & b_3 \\ }; \draw[thick] (mtrx-2-1.north) -| (mtrx-4-1.south west) -- (mtrx-4-1.south); \draw[thick] (mtrx-2-3.north) -| (mtrx-4-3.south east) -- (mtrx-4-3.south); \path[draw,strip=blue] (mtrx-4-1.center) edge (mtrx-2-3.center) (mtrx-4-2.center) edge (mtrx-2-4.center) (mtrx-4-3.center) -- (mtrx-2-5.center); \path[draw,strip=red] (mtrx-2-1.center) edge (mtrx-4-3.center) (mtrx-2-2.center) edge (mtrx-4-4.center) (mtrx-2-3.center) -- (mtrx-4-5.center); \end{tikzpicture} \end{frame} \end{document}  • Thanks for the answer, I didn't know the \matrix tikz command. – MyUserIsThis Jul 26 '15 at 10:08 • the pluses and minuses are not placed in proper relation to the bars they should be associated with, and in the second example, the colors are reversed. – barbara beeton Jul 26 '15 at 12:06 • @barbarabeeton, regarding colors: rush is newer good ... fortunate<y, this is easy to correct (and use colors on own taste), But the signs + and -: to put them closer to lines is require some effort. I will try do this. – Zarko Jul 26 '15 at 12:32 • I use beamer class and i put the code in a frame but i did not work. It seems to me that Tikz does not work in beamer. Any suggestion ? – Bungbu Jul 27 '15 at 15:58 • @Bungbu, it works. Tested. Instead of first two lines in MWE use: \documentclass{beamer} \usepackage{tikz} .... – Zarko Jul 27 '15 at 18:17 After playing around with tikz, I could get it, I believe, decently. I'm just a tikz beginner, so I am 100% sure this can be done better, without needing so much fine-tuning of positions. You can adjust all positions and colors though: \documentclass{article} \usepackage{tikz} \usepackage{amsmath} \usepackage{xcolor} \definecolor{myblue}{rgb}{0,0,0.8} \definecolor{myred}{rgb}{0.8,0,0} \begin{document} \begin{tikzpicture} \node[fill=none,draw=none] (matrix) { \begin{minipage}{\textwidth} \[ \begin{bmatrix} a_1 & b_1 & c_1 \\ a_2 & b_2 & c_2 \\ a_3 & b_3 & c_3 \end{bmatrix} \begin{matrix} a_1 & b_1 \\ a_2 & b_2 \\ a_3 & b_3 \end{matrix}$
\end{minipage}
};
\draw[myblue,fill=myblue,opacity=0.2] (-1.4,-0.4)--(0.25,0.6)--(0.4,0.4)--(-1.25,-0.6);
\draw[myblue,fill=myblue,opacity=0.2] (-0.7,-0.4)--(0.9,0.6)--(1.05,0.4)--(-0.55,-0.6);
\draw[myblue,fill=myblue,opacity=0.2] (0,-0.4)--(1.55,0.6)--(1.7,0.4)--(0.15,-0.6);
\draw[myred,fill=myred,opacity=0.2] (-1.4,0.4)--(0.25,-0.6)--(0.4,-0.4)--(-1.25,0.6);
\draw[myred,fill=myred,opacity=0.2] (-0.7,0.4)--(0.85,-0.6)--(1.0,-0.4)--(-0.55,0.6);
\draw[myred,fill=myred,opacity=0.2] (0,0.4)--(1.55,-0.6)--(1.7,-0.4)--(0.15,0.6);
\node[above of=matrix,xshift=-1.3cm,yshift=-0.2cm] {\textcolor{myred}{$+$}};
\node[above of=matrix,xshift=-0.6cm,yshift=-0.2cm] {\textcolor{myred}{$+$}};
\node[above of=matrix,xshift=-0cm,yshift=-0.2cm] {\textcolor{myred}{$+$}};
\node[above of=matrix,xshift=0.4cm,yshift=-0.2cm] {\textcolor{myblue}{$-$}};
\node[above of=matrix,xshift=1cm,yshift=-0.2cm] {\textcolor{myblue}{$-$}};
\node[above of=matrix,xshift=1.6cm,yshift=-0.2cm] {\textcolor{myblue}{$-$}};
\end{tikzpicture}
\end{document} EDIT: In view of the comment by barbara, I've tried to enlarge the space between rows, to make it a little easier to see. I've done it following exactly this: incrementing matrix space. The only problem is that the matrix's bracket is too tall (it ends up being above the plus and - signs), and I don't know how to fix it. I leave the code and result, and if anyone knows how to do so, I'll happily edit this. Or you can just increase the yshift value of the plus and minus signs to put them above the matrix.

\documentclass{article}

\usepackage{tikz}
\usepackage{amsmath}
\usepackage{xcolor}
\definecolor{myblue}{rgb}{0,0,0.8}
\definecolor{myred}{rgb}{0.8,0,0}

\begingroup
\renewcommand*{\arraystretch}{1.5}
% your pmatrix expression
\endgroup

\makeatletter
\renewcommand*\env@matrix[\arraystretch]{%
\edef\arraystretch{#1}%
\hskip -\arraycolsep
\let\@ifnextchar\new@ifnextchar
\array{*\c@MaxMatrixCols c}}
\makeatother

\begin{document}
\begin{tikzpicture}
\node[fill=none,draw=none] (matrix)
{
\begin{minipage}{\textwidth}
$\begin{bmatrix}[1.5] a_1 & b_1 & c_1 \\ a_2 & b_2 & c_2 \\ a_3 & b_3 & c_3 \end{bmatrix} \begin{matrix}[1.5] a_1 & b_1 \\ a_2 & b_2 \\ a_3 & b_3 \end{matrix}$
\end{minipage}
};
\draw[myblue,fill=myblue,opacity=0.2] (-1.4,-0.7)--(0.2,0.75)--(0.35,0.55)--(-1.25,-0.9);
\draw[myblue,fill=myblue,opacity=0.2] (-0.7,-0.7)--(0.8,0.75)--(0.95,0.55)--(-0.55,-0.9);
\draw[myblue,fill=myblue,opacity=0.2] (0,-0.7)--(1.45,0.75)--(1.6,0.55)--(0.15,-0.9);
\draw[myred,fill=myred,opacity=0.2] (-1.4,0.55)--(0.25,-0.9)--(0.4,-0.7)--(-1.25,0.75);
\draw[myred,fill=myred,opacity=0.2] (-0.7,0.55)--(0.85,-0.9)--(1.0,-0.7)--(-0.55,0.75);
\draw[myred,fill=myred,opacity=0.2] (0,0.55)--(1.55,-0.9)--(1.7,-0.7)--(0.15,0.75);
\node[above of=matrix,xshift=-1.3cm,yshift=-0.1cm] {\textcolor{myred}{$+$}};
\node[above of=matrix,xshift=-0.6cm,yshift=-0.1cm] {\textcolor{myred}{$+$}};
\node[above of=matrix,xshift=-0cm,yshift=-0.1cm] {\textcolor{myred}{$+$}};
\node[above of=matrix,xshift=0.4cm,yshift=-0.1cm] {\textcolor{myblue}{$-$}};
\node[above of=matrix,xshift=1cm,yshift=-0.1cm] {\textcolor{myblue}{$-$}};
\node[above of=matrix,xshift=1.6cm,yshift=-0.1cm] {\textcolor{myblue}{$-$}};
\end{tikzpicture}
\end{document} • Thank you. How can i lengthen or shorten the diagonal highlights ? – Bungbu Jul 26 '15 at 8:16
• @Bungbu The \draw lines are the ones drawing the rectangles, and the four pairs of numbers are the coordinates of the corners of each rectangle. If you want to change the sizes, you will have to change those coordinates and do some fine-tuning until you get the desired results. – MyUserIsThis Jul 26 '15 at 10:05
• very nice. i think a little more vertical space between the rows would improve the appearance, also, i interpret the plus signs as being applicable to the pink bars, so they might be placed a bit more to the left, along the axes of those bars; similarly for the minus signs and the blue bars, shifted a bit to the right. (shifting pluses and minuses may not be necessary if the bars are more upright as a result of additional vertical space.) – barbara beeton Jul 26 '15 at 12:04