Changing the style of the first *typeset* line of a paragraph

Question

I've been playing around with dropcaps in LaTeX (with the yfonts and lettrine packages), and one typographical effect you often see after dropcaps is having the entire first line of the paragraph typeset in small caps. It's clear how to get this effect by trial and error—run LaTeX, see what's on the first line, \textsc that, rerun LaTeX, see what's moved, and re-\textsc the appropriate text—but that's a royal pain, and it's too easy to forget to do. Is there a good way to get (La)TeX to detect the location of linebreaks in its typeset output? I presume I'd have to run LaTeX at least twice, since things could change mid-processing, but such is life with LaTeX.

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Edit 1: I need this to work for math-mode ($...$) code as well—from the solutions so far suggested, this is not necessarily non-trivial.

Answer 1

Donald Arseneau's magaz package will do this for you. From memory, it uses roughly the same idea shown by Yiannis — that is, only a single pass is necessary — but I think it may be slightly more robust.

This solution will work with inline maths provided the maths does not break over the end of the line, as here:

\documentclass{article}
\usepackage{magaz}
\begin{document}
\FirstLine{There is no just \emph{ground}, therefore, $a+b=c$ for the charge $x\in\{0,1,2\}$ brought against me by certain ignoramuses.}
\end{document}

But putting the maths inside an \mbox seems to fix this problem.

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Readers of this answer may wish to select a color for the first line. This requires a slightly different approach which is discussed in David Carlisle's answer in the thread Why does color meant for first typeset line ONLY (with package magaz) affect ENTIRE document?

Answer 2

Here is a TeXie solution!

    \documentclass[11pt]{article} 
    \begin{document}

    \def\firstline{\par\begingroup\Firstlinefont
      \def\zzz{}\dimen0\parindent
      \setbox0=\hbox{ }\advance\dimen0by-\wd0
      \x}
    \def\x{\afterassignment\y\z}
    \def\z#1 {\xdef\zz{#1\ }}
    \def\y{\setbox0=\hbox{\zz}
      \advance\dimen0 by\wd0
      \ifdim\dimen0<\hsize\let\next=\x
        \edef\zzz{\zzz\zz}
      \else\let\next=\endgroup
        \leavevmode\zzz\unskip\break\aftergroup\zz
      \fi
      \next}


    \let\Firstlinefont=\scshape  

    \firstline The first line of this paragraph is in small caps font series. You
    will need to hack the rest of the code to get it working with the lettrine package!

\end{document}

Answer 3

(I came to this from the duplicate question Emphasize first line of paragraph in LaTeX)

Yiannis solution is very nice. Here is another method, which leads to less visual differences between the first two lines in the way the inter-word spaces are treated. My code presumably leads to inferior results in general compared to Yiannis's (the emphasizing can extend to the beginning of the second line or end a bit too short in the first line), but I do propose it as it is simpler minded than Yiannis's sophisticated macros! Also it seems to work with the example including inline mathematics, and allows hyphenation at the end of the first line (but the emphasizing applies to entire words).

Inline math should be written (whenever there is a chance it will belong to the emphasized text) either without spaces (e.g. $a+b=c$) or be enclosed in doubled braces (e.g. {{$ a + b = c $}}). Then, breaking at the end of lines will work correctly.

The code defines two user commands \decorate and \Decorate (as well as other commands to decide the style of the emphasized text). \decorate is used as \decorate{TEXT} (where TEXT cannot contain a \par token), and \Decorate is to be put at the beginning of a paragraph (so can't be used in a \parbox for example, contrarily to the lowercase form).

Both admit an optional parameter (default value 0.95) which gives a rough idea of the number of lines to which the emphasizing will be applied.

And \Decorate has a starred version \Decorate* which tries to provoke expansion of the paragraph to which it is applied (see the end of the code for a comparative illustration). It is not possible though to use \lipsum[1] as an argument (even with option nopar to package lipsum), but lipsum@i is ok.

\documentclass[a4paper]{article}
\makeatletter

\def\addtobufferwithaspace#1#2{\expandafter\def\expandafter#1%
               \expandafter{#1 #2}}

\def\deco@step#1{%
   \setbox1=\hbox{\deco@buffer{} #1}%
   \dimen0\hsize\advance\dimen0-\parindent\relax
   #1
   \ifdim\wd1>\deco@scale\dimen0
     \expandafter\deco@trail
   \else
     \addtobufferwithaspace\deco@buffer{#1}%
     \expandafter\deco@parsei
   \fi
}

\def\deco@trail#1\@nil{\deemphasizingcommand #1}

\def\deco@parsei#1{\ifnot@nil{#1}{\deco@parse #1}}

\def\deco@parse #1 {\deco@step{#1}}

\newcommand*{\decorate}[2][.95]{%
   \def\deco@scale{#1}%
   \def\deco@buffer{\emphasizingcommand}%
   \emphasizingcommand
   \deco@parse #2 \@nil
   \deemphasizingcommand
}

\def\@@Decorate[#1] #2\par{\decorate[#1]{#2}\par}

\def\@@eDecorate[#1] #2\par{%
     \def\@decorate{\decorate[#1]}%
     \edef\@Decorated{\noexpand\@decorate{#2}}%
     \@Decorated\par}

\newcommand*\@Decorate[1][.95]{\@@Decorate[#1] }
\newcommand*\@eDecorate[1][.95]{\@@eDecorate[#1] }

\newcommand*\Decorate{\@ifstar{\@eDecorate}{\@Decorate}}

\makeatother

\newcommand{\emphasizingcommand}{\bfseries} 
\newcommand{\deemphasizingcommand}{\normalfont\normalsize} 
\usepackage[nopar]{lipsum}

\begin{document}

\Decorate
  I am (of course) shocked! and appalled! I have a particular typesetting
  task, described below. Trying to find out how to emphasize the line of text
  automatically. The issue here is that I have no idea where the line would
  break.  Ideally it would be start paragraph with slanted shape and then
  revert to upright shape upon first line break.  

\Decorate[.5] I am (of course) shocked! and appalled! I have a particular
  typesetting task, described below. Trying to find out how to emphasize the
  line of text automatically.

\renewcommand{\emphasizingcommand}{\bfseries\slshape}

\noindent\parbox{5cm}{\decorate{
  I am (of course) shocked! and appalled! I have a particular typesetting
  task, described below. Trying to find out how to emphasize the line of text
  automatically. The issue here is that I have no idea where the line would
  break.  }}
\parbox{3cm}{\decorate{
  I am (of course) shocked! and appalled! I have a particular typesetting
  task, described below. Trying to find out how to emphasize the line of text
  automatically. The issue here is that I have no idea where the line would
  break.  }}
\parbox{2.5cm}{\raggedright\decorate{
  I am (of course) shocked! and appalled! I have a particular typesetting
  task, described below. Trying to find out how to emphasize the line of text
  automatically. The issue here is that I have no idea where the line would
  break.  }}
\parbox{2.5cm}{\raggedleft\decorate{
  I am (of course) shocked! and appalled! I have a particular typesetting
  task, described below. Trying to find out how to emphasize the line of text
  automatically. The issue here is that I have no idea where the line would
  break.  }}

\renewcommand{\emphasizingcommand}{\ttfamily\Large\frenchspacing}
\renewcommand{\deemphasizingcommand}{\normalfont\normalsize\nonfrenchspacing}

\begin{center}
  \begin{minipage}{0.5\linewidth}
    \centering
    \Decorate[1.5] 
    I am (of course) shocked! and appalled! I have a
    particular typesetting task, described below. Trying to find out how to
    emphasize the line of text automatically. The issue here is that I have no
    idea where the line would break.  Ideally it would be start paragraph with
    slanted shape and then revert to upright shape upon first line break.
    \par
  \end{minipage}
\end{center}

\renewcommand{\emphasizingcommand}{\scshape}

\Decorate There is no just \emph{ground}, therefore, \mbox{$ a + b = c $} for
  the charge $x\in\{0,1,2\}$ brought against me by certain ignoramuses $ x
  \in\{ 0,1,2 \}$.

\Decorate Lorem ipsum dolor sit amet, consectetuer
adipiscing elit. Ut purus elit, vestibulum ut, placerat ac,
adipiscing vitae, felis. Curabitur dictum gravida mauris. Nam arcu
libero, nonummy eget, consectetuer id, vulputate a, magna.

\Decorate[4] Lorem ipsum dolor sit amet, consectetuer
adipiscing elit. Ut purus elit, vestibulum ut, placerat ac,
adipiscing vitae, felis. Curabitur dictum gravida mauris. Nam arcu
libero, nonummy eget, consectetuer id, vulputate a, magna.


\renewcommand{\emphasizingcommand}{\bfseries}
%% \clearpage

\decorate{Let's try with  a short sentence.}
\decorate[.2]{And another one.}
\decorate[.2]{And another one.}
\decorate[.2]{And another one.}

\makeatletter

\Decorate* \lipsum@v

\Decorate* \lipsum@vi

\Decorate*[1] \lipsum@vii

\makeatother

\def\tmpa{dolor sit amet, consectetuer
adipiscing elit. Ut purus elit, }

\Decorate[.1] \tmpa \tmpa

\Decorate*[.1] \tmpa \tmpa

\end{document}