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This is my minimal working example.

\documentclass[12pt,a4paper]{mwrep}
%\usepackage{amsfonts}
\usepackage{amsmath}
%\renewcommand\baselinestretch{1.5}

\begin{document}
\chapter{Example}
\begin{equation}
P=
\begin{bmatrix} p_{11} & p_{12} & \cdots & p_{1r} \\
                p_{21} & p_{22} & \cdots & p_{2r} \\
                \vdots & \vdots & \ddots & \vdots \\
                p_{r1} & p_{r2} & \cdots & p_{rr}
\end{bmatrix}
\end{equation}
\end{document}

This is view of the matrix when lines 2 and 4 are left commended (200%)
This is what you get when you uncomment them (200%)

As you can see the brackets are thinner and size is different. I don't mind it. But I don't like those gaps in brackets. How can I get rid of them?
How can I make bmatrix behave like lines 2 and 4 were left commented and the rest of document like they were uncommented?

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2  
By 'gaps', are you referring to the noticeable 'white separations' in the [] surrounding the matrix? For one, this may just be an artifact of the PDF viewer you're using. For example, at 200% zoom in Adobe Acrobat 9, there is no difference in the line thickness and 'gaps'. In fact, when I look closely at your figures, I see similar (albeit less noticeable, but still there) 'white separations' in []. –  Werner Jul 18 '11 at 20:56
1  
This is possibly distribution/system/viewer dependent. I see no such gaps with your example. (TeXLive 2010 on a Mac), viewed with Preview or Adobe Reader. For changing the line spacing it's better to use the setspace package, and then use \onehalfspacing. –  Alan Munn Jul 18 '11 at 20:57
    
@Ichibann: Please tell us if the comments of Werner and/or Alan were helpful. If yes, also consider to write a short (self-)answer. –  lockstep Sep 4 '11 at 11:02
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2 Answers

up vote 9 down vote accepted

Whilst I agree with the comments that this is most likely due to the viewer, it is nonetheless irritating. I believe that when printed this effect is not seen, but since TeX is used quite a lot nowadays for presentations then how it looks in a particular viewer is becoming more important than it was. So despite it likely to be an issue with the viewer, it is still an issue worth solving.

The problem is that TeX is building the delimiters out of pieces and the viewer is rendering these pieces in a non-optimal manner and showing the spaces between them. (Unfortunately, I also dislike it when it does the opposite and overlaps the pieces as I can see the overlaps as extra thick pieces!).

A simple way to solve it is to use a font that has proper scalable delimiters, as mentioned in the accepted answer to Vertical vectors in angle brackets.

Another way of solving it would be to draw the delimiters as a single entity, rather than build them piecemeal. We had a question about replacing the AMS matrices with TikZ matrices: Obstacles to simulating an amsmath matrix by a TiKZ matrix of math nodes. In answering that question, I figured out a drop-in replacement for the standard AMS matrix environments using TikZ matrices instead. However, for the delimiters then it uses the standard delimiters so doesn't solve this problem.

But it wasn't too difficult to modify the code there to make it so that it draws the delimiters using TikZ code. By using my calligraphy package, I could even get nice tapering on the brace and parenthesis versions. At time of writing, the calligraphy package is not on CTAN, but you can get it from http://bazaar.launchpad.net/~tex-sx/tex-sx/development/files (you also need the spath.dtx file).

I'll put the code in a moment, first I want to show the result. The following is a matrix of matrices. Each row is a particular type (bmatrix, pmatrix, etc). The first in each row has TikZ draw the delimiters. The second has TikZ use "standard" delimiters. The third is the corresponding matrix command from the AMS packages.

TikZ replacements for matrices

The code follows.

\documentclass{article}
%\url{http://tex.stackexchange.com/q/26866/86}
\usepackage{amsmath,tikz}
\usetikzlibrary{matrix,decorations}
\usepackage{calligraphy}

\newlength\mtxrowsep
\setlength\mtxrowsep{1.5ex}
\newlength\mtxcolsep
\setlength\mtxcolsep{2\arraycolsep}

\makeatletter
\pgfdeclaredecoration{doubled lineto}{brace}
{
  \state{brace}[width=+\pgfdecoratedremainingdistance,next state=final]
  {
    \pgfsyssoftpath@setcurrentpath{\pgfutil@empty}
    \pgfpathmoveto{\pgfpointorigin}
    \pgfpathlineto{\pgfqpoint{\pgfdecoratedremainingdistance}{0pt}}
    \pgfpathmoveto{\pgfqpoint{0pt}{\pgfdecorationsegmentamplitude}}
    \pgfpathlineto{\pgfqpoint{\pgfdecoratedremainingdistance}{\pgfdecorationsegmentamplitude}}
  }
  \state{final}{}%
}
\pgfdeclaredecoration{bracket}{brace}
{
  \state{brace}[width=+\pgfdecoratedremainingdistance,next state=final]
  {
    \pgfsyssoftpath@setcurrentpath{\pgfutil@empty}
    \pgfpathmoveto{\pgfpointorigin}
    \pgfpathlineto{\pgfqpoint{0pt}{\pgfdecorationsegmentamplitude}}
    \pgfpathlineto{\pgfqpoint{\pgfdecoratedremainingdistance}{\pgfdecorationsegmentamplitude}}
    \pgfpathlineto{\pgfqpoint{\pgfdecoratedremainingdistance}{0pt}}
  }
  \state{final}{}%
}
\makeatother

\expandafter\def\csname delimiter \string\lbrace\endcsname{calligraphic brace}
\expandafter\def\csname delimiter \string(\endcsname{calligraphic curved parenthesis}
\expandafter\def\csname delimiter \string|\endcsname{lineto}
\expandafter\def\csname delimiter \string\|\endcsname{doubled lineto}
\expandafter\def\csname delimiter \string[\endcsname{bracket}
\expandafter\def\csname delimiter \string\rbrace\endcsname{calligraphic brace}
\expandafter\def\csname delimiter \string)\endcsname{calligraphic curved parenthesis}
\expandafter\def\csname delimiter \string]\endcsname{bracket}

\tikzset{
  ams/.style={
    baseline=-.7ex,
    every delimiter/.style={yshift=-1pt},
    every left delimiter/.style={xshift=2pt},
    every right delimiter/.style={xshift=-2pt},
    every node/.style={inner sep=0pt},
    execute at end picture={
      \path (current bounding box.east) ++(\pgfkeysvalueof{/tikz/ams matrix xsep},0) (current bounding box.west) ++(-\pgfkeysvalueof{/tikz/ams matrix xsep},0);
    },
  },
  ams matrix xsep/.initial={.5ex},
  ams matrix/.style={
    inner sep=1pt,
    column sep=\mtxcolsep,
    row sep=\mtxrowsep,
%    ampersand replacement=\&,
    matrix of math nodes,
  },
  ams delimiters/.style args={#1,#2}{
    left delimiter={#1},
    right delimiter={#2},
  },
  delimiters/.style args={#1,#2}{
    \pgfkeysvalueof{/tikz/matrix delimiter type} delimiters={{#1},{#2}},
  },
  matrix delimiter type/.initial={tikz},
  tikz delimiter style/.style={
    thick,
  },
  tikz delimiters/.style args={#1,#2}{
    left tikz delimiter={#1},
    right tikz delimiter={#2},
  },
  left tikz delimiter/.style={
    render left tikz delimiter/.expand once={\csname delimiter \string#1\endcsname}
  },
  render left tikz delimiter/.style={
    append after command={(\tikzlastnode.north west) edge[tikz delimiter style,decorate,decoration={mirror,#1}] (\tikzlastnode.south west)}
  },
  right tikz delimiter/.style={
    render right tikz delimiter/.expand once={\csname delimiter \string#1\endcsname}
  },
  render right tikz delimiter/.style={
    append after command={(\tikzlastnode.north east) edge[tikz delimiter style,decorate,decoration={#1}] (\tikzlastnode.south east)}
  },
  bmatrix/.style={
    ams,
    every matrix/.style={
      ams matrix,
      delimiters={[,]},
    }
  },
  Bmatrix/.style={
    ams,
    every matrix/.style={
      ams matrix,
      delimiters={\lbrace,\rbrace},
    }
  },
  pmatrix/.style={
    ams,
    every matrix/.style={
      ams matrix,
      delimiters={(,)},
    }
  },
  vmatrix/.style={
    ams,
    every matrix/.style={
      ams matrix,
      delimiters={|,|},
    }
  },
  Vmatrix/.style={
    ams,
    every matrix/.style={
      ams matrix,
      delimiters={\|,\|},
    }
  },
}

\let\matamp=&

\catcode`\&=13
\makeatletter
\def&{\iftikz@is@matrix
  \pgfmatrixnextcell
  \else
  \matamp
  \fi}
\makeatother

%\usepackage{environ}
\def\endtikzmatrix{\\\egroup;\end{tikzpicture}}
\foreach \mtype in {b,B,p,v,V} {

\expandafter\xdef\csname tikz\mtype matrix\endcsname{%
    \noexpand\begin{tikzpicture}[\mtype matrix,matrix delimiter type={tikz}]
    \noexpand\matrix \noexpand\bgroup}
\expandafter\xdef\csname ams\mtype matrix\endcsname{%
    \noexpand\begin{tikzpicture}[\mtype matrix,matrix delimiter type={ams}]
    \noexpand\matrix \noexpand\bgroup}
\expandafter\global\expandafter\let\csname endtikz\mtype matrix\endcsname=\endtikzmatrix
\expandafter\global\expandafter\let\csname endams\mtype matrix\endcsname=\endtikzmatrix
}

\begin{document}
\foreach \mtype in {b,B,p,v,V} {
  \edef\metype{\mtype matrix}
  \edef\tmetype{tikz\mtype matrix}
  \edef\ametype{ams\mtype matrix}
\begin{gather*}
\begin{\tmetype} a & b \\ c & d \end{\tmetype}
\begin{\ametype} a & b \\ c & d \end{\ametype}
\begin{\metype} a & b \\ c & d \end{\metype}
\end{gather*}
}

\end{document}

The change from the code as in Obstacles to simulating an amsmath matrix by a TiKZ matrix of math nodes is that after rendering the matrix, we draw a line down each side. But we decorate the path according to the type of delimiter chosen. If we didn't use the calligraphy package, we'd have to change some of the choices of decorations in the list near the beginning:

\expandafter\def\csname delimiter \string\lbrace\endcsname{calligraphic brace}
\expandafter\def\csname delimiter \string(\endcsname{calligraphic curved parenthesis}
\expandafter\def\csname delimiter \string\rbrace\endcsname{calligraphic brace}
\expandafter\def\csname delimiter \string)\endcsname{calligraphic curved parenthesis}

calligraphic brace could be replaced by brace and calligraphic curved parenthesis by bent.

One advantage of this approach is that it allows for more variation in the delimiters. We can define new decorations, and can style the paths in a variety of different ways.

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that is only a problem with the viewer not with the font! However, I cannot see it with the adobe reader for 200% and the \left[...\right] sequence for the matrix:

\documentclass{article}
\usepackage{amsmath,amsfonts}

\begin{document}

\[\def\arraystretch{2} \left[\begin{matrix} a & b \\ c & d \end{matrix}\right] 
\]

\end{document}

enter image description here

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If you zoom into your image, half-way along the brackets you will see one pixel of darker antialiasing, indicating that the pieces overlap slightly. The behaviour in Adobe Reader is zoom-dependent. For me, I can see gaps at 100% zoom, but not at 110%, but then I can see them again at 112% zoom. At some zoom levels, overlap appears. So while TeX must be very precise in drawing these piece shapes to have zero gap between them, the renderer might be using some approximations (rounding? alignment to the pixel grid?) to speed up rendering. –  Evgeni Sergeev Aug 6 '13 at 6:29
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