Highlighting rows/columns/diagonals in matrix

Question

I'm trying to highlight rows/columns/diagonals in a matrix. I came across this example, which looks very clean. Unfortunately, it seems to cause some issue with spacing in the matrix, and I can't figure out why. Below is a MWE and the output:

documentclass{article}
usepackage{amsmath}
usepackage{tikz}

newcommand{tikzmark}[2]{
    tikz[overlay,remember picture,baseline] 
    node[anchor=base] (#1) {$#2$};
}

begin{document}

No highlighting:
begin{equation}
  A =
  begin{bmatrix}
    a_{11} & 0 & 0 
    0 & a_{22} & 0 
    0 & 0 & a_{33}
  end{bmatrix}
end{equation}

Highlighting:
begin{equation}
  A =
  begin{bmatrix}
    tikzmark{top}{a_{11}} & 0 & 0 
    0 & a_{22} & 0 
    0 & 0 & tikzmark{bottom}{a_{33}}
  end{bmatrix}
end{equation}
begin{tikzpicture}[overlay,remember picture]
  draw[opacity=.4,line width=3mm,line cap=round] (top.center) -- (bottom.center);
end{tikzpicture}

end{document}

What is the tikzmark command doing, and is there anything that can be done to preserve the standard matrix spacing?

user194703 · Accepted Answer

Here is a version with the tikzmark library (and the eso-pic package to get the highlighting on the background).

documentclass{article}
usepackage{amsmath}
usepackage{tikz}
usetikzlibrary{tikzmark,calc}
usepackage{eso-pic}
begin{document}

No highlighting:
begin{equation}
  A =
  begin{bmatrix}
    a_{11} & 0 & 0 
    0 & a_{22} & 0 
    0 & 0 & a_{33}
  end{bmatrix}
end{equation}

Highlighting:
begin{equation}
  A =
  begin{bmatrix}
    tikzmarknode[circle]{top}{a_{11}} & 0 & 0 
    0 & a_{22} & 0 
    0 & 0 & tikzmarknode[circle]{bottom}{a_{33}}
  end{bmatrix}
end{equation}
AddToShipoutPictureBG*{%
begin{tikzpicture}[overlay,remember picture]
  draw let  p1=($(top.north)-(top.center)$),
  p2=($(bottom.north)-(bottom.center)$), n1={2*max(y1,y2)-2pt} in
  [opacity=.4,line width=n1,line cap=round,
  shorten >=-y2/3,shorten <=-y1/3] (top.center) -- (bottom.center);
end{tikzpicture}}
end{document}

Sebastiano · Answer

An adapt with variants of the nice answer of @Zarko Rectangle box around elements of matrix using nicematrix package as also suggested by the very good user @Schrödinger's cat in your comment. If you want to have more rounded corners increase the parameter rounded corners=2pt. I add a MWE and a screenshot:

documentclass[12pt]{article}
usepackage{nicematrix,tikz}
usetikzlibrary{fit}
begin{document}
[
begin{bNiceArray}{>{strut}ccc}[margin]    
a_{11} & 0 & 0 
0 & a_{22} & 0 
0 & 0 & a_{33} 
CodeAfter 
tikz
node [fill=gray, rounded corners=2pt, opacity=0.2, 
       rotate fit=-31, inner xsep=1.2pt, inner ysep = -0.8pt,
       fit = (1-1) (3-3)] {} ;
end{bNiceArray}
]
end{document}

Highlighting rows/columns/diagonals in matrix

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