[Tex/LaTex] Inconsistent size of parentheses in Latin Modern and Computer Modern

computer-moderndelimiterslatin-modern

I have noticed that the sizes of delimiters change when I switch from Computer Modern to Latin Modern. In 12pt font size documents, this causes inline math with \bigl( to be too large to fit, and expands the vertical space taken by that line. This was not a problem with Computer Modern. Is this a bug?

\documentclass[12pt]{article}

\usepackage{lmodern}
\usepackage{amsmath}

\begin{document}
An equivalence class~$[a]_{\sim} \in A / {\sim}$ consists of all the elements
in $A$ that are mapped to $b = f(a)$. By the axiom of choice, there exists a
choice function~$c \colon A / {\sim} \to A$ which selects a representative
element of each equivalence class. There exists a
function~$h \colon B \to A / {\sim}$, so that
$h(b) = h\bigl(f(a)\bigr) = [a]_{\sim}$. This allows us to construct the
function~$g = c \circ h$, which is in fact a right-inverse of $f$.
\[
    \Biggl(\biggl(\Bigl(\bigl((X)\bigr)\Bigr)\biggr)\Biggr)
\]
\end{document}

Computer Modern:
Computer Modern

Latin Modern:
Latin Modern

There is a gap above the fourth line in the Latin Modern case, which is not there in the Computer Modern case.

Edit:

This post by egreg tackles the same problem.

Best Answer

For historical reasons a fixed size is used for the scalable delimiters, even for cm that's not clearly a good idea (see exscale package), but here it's not good at all, if you use the font at its natural scaled size then....

enter image description here

\documentclass[12pt]{article}

\DeclareFontFamily{OMX}{lmex}{}
\DeclareFontShape{OMX}{lmex}{m}{n}{%
   <->lmex10%
   }{}
\usepackage{lmodern}
\usepackage{amsmath}

\begin{document}



\show\big
\showthe\baselineskip
\setbox0\hbox{$\big($}\showthe\dimexpr\ht0+\dp0
\showoutput
\showbox0

An equivalence class~$[a]_{\sim} \in A / {\sim}$ consists of all the elements
in $A$ that are mapped to $b = f(a)$. By the axiom of choice, there exists a
choice function~$c \colon A / {\sim} \to A$ which selects a representative
element of each equivalence class. There exists a
function~$h \colon B \to A / {\sim}$, so that
$h(b) = h\bigl(f(a)\bigr) = [a]_{\sim}$. This allows us to construct the
function~$g = c \circ h$, which is in fact a right-inverse of $f$.
\[
    \Biggl(\biggl(\Bigl(\bigl((X)\bigr)\Bigr)\biggr)\Biggr)
\]
\end{document}

Now the size is shown as

> 14.5pt.
l.17 \showthe\baselineskip

? 

> 14.40013pt.

with \big just less than \baselineskip

You can redefine the \big size keeping it within \baselineskip and so avoid \lineskip glue being used within the paragraph.

\documentclass[12pt]{article}

\usepackage{lmodern}
\usepackage{amsmath}

\begin{document}


\makeatletter
\renewcommand{\big}{\bBigg@{0.92}}
\makeatother
\show\big
\showthe\baselineskip
\setbox0\hbox{$\big($}\showthe\dimexpr\ht0+\dp0
\showoutput

An equivalence class~$[a]_{\sim} \in A / {\sim}$ consists of all the elements
in $A$ that are mapped to $b = f(a)$. By the axiom of choice, there exists a
choice function~$c \colon A / {\sim} \to A$ which selects a representative
element of each equivalence class. There exists a
function~$h \colon B \to A / {\sim}$, so that
$h(b) = h\bigl(f(a)\bigr) = [a]_{\sim}$. This allows us to construct the
function~$g = c \circ h$, which is in fact a right-inverse of $f$.
\[
    \Biggl(\biggl(\Bigl(\bigl((X)\bigr)\Bigr)\biggr)\Biggr)
\]
\end{document}

The \show at the top of the document show

> 14.5pt.
l.13 \showthe\baselineskip

? 

> 13.24792pt.
<to be read again>

showing that \big( is less than the 14.5pt baselinekip.

The .92 chosen empirically: .93 makes the font jump to the next available size, which is too big here.

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