\iffalse meta-comment

File: ltx-talk.tex Copyright (C) 2023-2026 Joseph Wright

It may be distributed and/or modified under the conditions of the
LaTeX Project Public License (LPPL), either version 1.3c of this
license or (at your option) any later version.  The latest version
of this license is in the file

   https://www.latex-project.org/lppl.txt

This file is part of the "ltx-talk bundle" (The Work in LPPL)
and all files in that bundle must be distributed together.

The released version of this bundle is available from CTAN.

-----------------------------------------------------------------------

The development version of the bundle can be found at

   https://github.com/josephwright/ltx-talk

for those people who are interested.

-----------------------------------------------------------------------

\fi

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\begin{document}

\title{%
  \cls{ltx-talk} -- A class for typesetting presentations%
  \thanks{This file describes \fileversion,
    last revised \filedate.}%
}

\author{%
  Joseph Wright%
  \thanks{%
    E-mail: \email{joseph@texdev.net}%
  }%
}

\date{Released \filedate}

\maketitle

\tableofcontents

\begin{documentation}

\section{Introduction%
  \label{sec:intro}}

The \cls{beamer} class was first released in 2003, and rapidly became the
most popular method for producing presentations in \LaTeX{}. As detailed in
the \cls{beamer} manual
\begin{quotation}
  Till Tantau created \cls{beamer} mainly in his spare time. Many other people
  have helped by sending him emails containing suggestions for improvement or
  corrections or patches or whole new themes (by now, this amounts to over a
  thousand emails concerning \cls{beamer}). Indeed, most of the development was
  only initiated by feature requests and bug reports. Without this feedback,
  \cls{beamer} would still be what it was originally intended to be: a small
  private collection of macros that make using the \cls{seminar} class easier.
  Till created the first version of \cls{beamer} for his PhD defense
  presentation in February 2003. A month later, he put the package on
  \acro{ctan} at the request of some colleagues. After that, things somehow got
  out of hand.
\end{quotation}

Despite the very large amount of work Till (and others) put into \cls{beamer},
there are several challenges which confront us today.
\begin{enumerate}
  \item The document interface is flexible but in places deviates from normal
    \LaTeX{} conventions
  \item Internally, the code makes use of whatever methods would give the
    visual results but not necessarily the most idiomatic style
  \item Till made few comments in the code or in commit messages in the code
    history
  \item The underlying box structure of a \cls{beamer} document is very
    different from the standard \LaTeX{} model, and a lot of material is
    boxed up multiple times
  \item Engine, font and macro development over the past 20 years offers
    new approaches for some areas
\end{enumerate}
These all feed into an issue addressing a major requirement today:
accessibility. Broadly, the internal structure (and to some extent the user
interface) of \cls{beamer} means that it is not possible to \enquote{retrofit}
PDF tagging into the class.

Instead, the approach is to develop a new class, \cls{ltx-talk}, which takes
ideas from \cls{beamer} but with tagging and accessibility of structure as a
design aim from the beginning. In contrast to work by the \LaTeX{} Project Team
on making the core classes accessible, the expectation for \cls{ltx-talk} is
that users \emph{will} need to change their sources. Unlike other documents,
presentations tend to be \enquote{single use}: revised and adjusted each time
they are used. The need to edit sources should therefore not be an
insurmountable barrier.

At present, this code is experimental: only a (small) subset of \cls{beamer}
functionality is implemented, some things are being done differently, and
almost everything is still subject to discussion. Some document commands are
stubs: either doing nothing at all or dropping optional arguments. This is to
allow testing of existing \cls{beamer} sources with minimal changes.

\section{Engine support}

Creating fully tagged PDFs is only possible with suitable engine support;
Lua\TeX{} offers the best outcomes here and \pdfTeX{} is usable but with some
limitations (for example, more \LaTeX{} runs being required). As such,
\cls{ltx-talk} is \emph{only} tested with these engines. Most notably, \XeTeX{}
is \emph{not} supported: it is not suitable for fully accessible tagged PDF
production, and transparency support is problematic. Issues arising which are
\XeTeX{}-specific will therefore be closed without investigation.

\section{Submitting ideas}

As noted in Section~\ref{sec:intro}, the class is at present experimental. As
such, missing features or restrictions should be expected throughout. Over
time, the aim is to address many of the things that \pkg{beamer} can do (though
there may be some that are not included).

At present, prioritization of requests will be focussed on the need to provide
accessible content. This means that structural issues are likely to be handled
before design aspects.

\section{Simple example documents}

Using \cls{ltx-talk} \emph{absolutely requires} the use of the
\cs{DocumentMetadata} command. As such, the most basic \cls{ltx-talk} document
is
\begin{verbatim}
  \DocumentMetadata{} % likely with "tagging = on"
  \documentclass{ltx-talk}
  \begin{document}
  \begin{frame}
    Some content here
  \end{frame}
  \end{document}
\end{verbatim}

A slightly more useful version, which generates multiple slides and shows some
(current) features, is
\begin{verbatim}
  \DocumentMetadata{}
  \documentclass{ltx-talk}
  \begin{document}
  \begin{frame}
    \frametitle{An example frame}
    \begin{itemize}[action-spec = <+->]
      \item This will be on slide one onward
      \item This will be on slide two onward
      \item<.-> So will this
      \item But this will only appear on slide three
    \end{itemize}
    Back to appearing on all slides
  \end{frame}
  \end{document}
\end{verbatim}

Tagging is activated for the standard (projector) output of \cls{ltx-talk}, but
it is perhaps more useful in \opt{handout} mode, which is activated using a
class option.
\begin{verbatim}
  \DocumentMetadata{}
  \documentclass[mode = handout]{ltx-talk}
  \begin{document}
  \begin{frame}
    \frametitle{An example frame}
    \begin{itemize}[action-spec = <+->]
      \item This will be on slide one onward
      \item This will be on slide two onward
      \item<.-> So will this
      \item But this will only appear on slide three
    \end{itemize}
    Back to appearing on all slides
  \end{frame}
  \end{document}
\end{verbatim}
For convenience, this option is also available as simply \opt{handout}, to
match the \cls{beamer} syntax.

A larger set of examples which can be edited and typeset in the browser are
available at \url{https://www.texdev.net/ltx-talk/examples/}.

\section{Class structure and design decisions}

As covered in Section~\ref{sec:intro}, the \cls{ltx-talk} class is currently
highly experimental. Active discussion is ongoing around many aspects of the
way that things should work, and very little is therefore at all stable. That
said, some decisions have been made: some of this is re-stating ideas which
carry forward from \cls{beamer}.
\begin{itemize}
  \item The basic structure of a presentation is made up of \env{frame}
    environments, which are made up of one or more slides.
  \item Variable content is indicated by an \emph{overlay specification}, given
    in optional angle brackets (|< ... >|).
  \item Slides have a \emph{fixed} height of \qty{100}{\mm}.
  \item The default font will be sans-serif using established standard
    implementations (currently \pkg{sansmathfonts} with Latin Modern for
    text for \pdfTeX{} and New Computer Modern for OpenType engines).
\end{itemize}

\section{Differences from \cls{beamer}}

The following key differences between \cls{beamer} and \cls{ltx-talk} are
important to note for the user.
\begin{itemize}
  \item The default font setup in \cls{ltx-talk} is \emph{all} sans-serif, in
    particular \cs{mathrm} and \cs{textrm} will give a sans-serif font
  \item Overlay specifications can only be given as the first argument to
    commands
  \item Where a command takes a star as an option, this comes \emph{before}
    the overlay argument, e.g. |\includegraphics*<...>|: this reflects the
    fact that the star is usually described as part of the command name rather
    than as an argument
  \item There are no optional braced arguments in \cls{ltx-talk}, in particular
    frame titles are given either using a \emph{mandatory} |{...}|
    argument or using \cs{frametitle} (see below)
  \item New overlay-aware commands and environments should be defined using
    \pkg{ltcmd} (\cs{NewDocumentCommand} and so on): no changes are made to the
    behavior of \cs{newcommand} or \cs{newenvironment}
  \item There are no commands containing the class name (or \texttt{beamer}):
    rather, methods from the kernel or other standard packages should be used
    (these are documented where needed)
  \item The class features are specifically focussed on presentation and avoid
    as far as possible other changes from standard \LaTeX{}; in particular,
    the various math-focussed additions which \cls{beamer} make (such as various
    theorem environments) are not included
\end{itemize}

\section{Creating frames}

\subsection{The frame environment}

\DescribeEnv{frame}
A presentation consists of a series of frames. Each frame consists of a series
of slides. You create a frame using the \env{frame} environment. All of the text
that is not tagged by overlay specifications is shown on all slides of the
frame. (Overlay specifications are explained in more detail in later sections.
For the moment, let's just say that an overlay specification is a list of
numbers or number ranges in angle brackets that is put after certain commands
as in |\uncover<1,2>{Text}|.) If a frame contains commands that have an overlay
specification, the frame will contain multiple slides; otherwise it contains
only one slide.

\begin{syntax}
  \cs{begin}\{\texttt{frame}\}\sarg{overlay spec}\oarg{options}
  \meta{environment contents}
  \cs{end}\{\texttt{frame}\}
\end{syntax}
The \meta{overlay spec} dictates which slides of a frame are to be shown. If
left out, the number is calculated automatically. The \meta{environment
contents} can be normal \LaTeX{} text, but may not contain |\verb| commands or
|verbatim| environments or any environment that changes the character codes.

\begin{verbatim}
  \begin{frame}
    \frametitle{A title}
    Some content
  \end{frame}
\end{verbatim}

\DescribeEnv{frame*}
The \env{frame*} environment works exactly the same way as the \env{frame} one,
\emph{except} that it can contain |\verb| commands or |verbatim| environments
or any environment that changes the character codes. The \env{frame*}
environment however \emph{cannot} be used inside the argument of another
command.
\begin{verbatim}
  \begin{frame*}
    \frametitle{A title}
    Some content \verb|$%#|.
  \end{frame*}
\end{verbatim}

In contrast to \cls{beamer}'s \opt{fragile} frame option, the \env{frame*}
environment does not use external files to handle verbatim material. As such,
there is very little performance impact processing frames containing category
code changes.

\subsection{Components of a frame}

\subsubsection{The frame title}

\DescribeMacro{\frametitle}
\DescribeMacro{\framesubtitle}
\begin{syntax}
  \cs{frametitle}\sarg{overlay spec}\oarg{options}\marg{frame title}
  \cs{framesubtitle}\sarg{overlay spec}\oarg{options}\marg{frame subtitle}
\end{syntax}
The frame title is shown prominently at the top of the frame and can be
specified with the command \cs{frametitle}. You should end the \meta{frame
title} with a period, if the title is a proper sentence. Otherwise, there
should not be a period. A frame subtitle may also be given; this is typically
used where a series of related frames are given with clearly distinct content.

Currently, the \meta{frame subtitle} is not used in output: this will be
addressed in later releases. The \meta{options} for both commands are currently
unused.

\DescribeOption{frame-title-arg}
The class can be loaded with the option \opt{frame-title-arg}. When this is
active, the \env{frame} environment \emph{requires} an argument in all cases
\begin{verbatim}
  \begin{frame}{Frame title here}
\end{verbatim}
Note that this option does \emph{not} influence frame subtitles, which can
\emph{only} be given using \cs{framesubitle}.

At present, the final form of the frame title interface is not decided:
feedback on this aspect is therefore \emph{particularly welcome}.

\subsection{Frame and margin size}

The size of a frame is actually the \enquote{paper size} of a presentation, and
it is variable. The standard size of a frame is \qty{100}{\mm} height with an
aspect ratio of $16:9$. It is the job of the presentation program to display
the slides at full screen size. The main advantage of using a small
\enquote{paper size} is that you can use all your normal fonts at their natural
sizes. In particular, inserting a graphic with \qty{11}{pt} labels will result
in reasonably sized text during the presentation.

\DescribeOption{font-size}
You can change the standard font size for the frames using the \opt{font-size}
option. This is initially set to \opt{11pt}, and if a non-standard size is
requested will use the \pkg{relsize} package to produce the output requested.
To support translation from \cls{beamer}, the classical options \opt{10pt},
\opt{11pt} and \opt{12pt} are also supported.

\DescribeOption{aspect-ratio}
The aspect ratio of the slides can be adjusted by setting the
\opt{aspect-ratio} load-time option, which takes two integer values separated
by a colon. These values are the relative width and height of the slide: in
contrast to \pkg{beamer}, the absolute height is a fixed value of
\qty{100}{\mm}.

Aside from using these options, you should refrain from changing the
\enquote{paper size}. However, you \emph{can} change the size of the left and
right margins, which default to \qty{10}{\mm}. These should be set using the
interfaces from the \pkg{geometry} package, for example
\begin{verbatim}
  \geometry{lmargin = 15mm, rmargin = 15mm}
\end{verbatim}

\subsection{Restricting the slides of a frame}

The number of slides in a frame is automatically calculated. If the largest
number mentioned in any overlay specification inside the frame is~4, four
slides are introduced (despite the fact that a specification like |<4->| might
suggest that more than four slides would be possible).

\section{Creating overlays}

\subsection{The \cs{pause} command}

\DescribeMacro{\pause}
The \cs{pause} command offers an easy, but not very flexible way of creating
frames that are uncovered piecewise. If you say \cs{pause} somewhere in a
frame, only the text on the frame up to the \cs{pause} command is shown on the
first slide. On the second slide, everything is shown up to the second
\cs{pause}, and so forth. You can also use \cs{pause} inside environments; its
effect will last after the environment. However, taking this to extremes and
using \cs{pause} deeply within a nested environment may not have the desired
result.

A much more fine-grained control over what is shown on each slide can be
attained using overlay specifications, see the next sections. However, for many
simple cases the \cs{pause} command is sufficient.

The \cs{pause} command takes an optional argument
\begin{syntax}
  \cs{pause}\oarg{overlay spec}
\end{syntax}
This causes the text following it to be shown only from the next slide on, or,
if the optional \meta{overlay spec} is given, from the slide with the number
\meta{overlay spec}. If the optional \meta{overlay spec} is given, the counter
|pauses| is set to this number. This command uses the \cs{onslide} command
internally. The effect of \cs{pause} lasts till the next \cs{pause},
\cs{onslide}, or the end of the frame.
\begin{verbatim}
  \begin{frame}
    \begin{itemize}
    \item
      Shown from first slide on.
    \pause
    \item
      Shown from second slide on.
      \begin{itemize}
      \item
        Shown from second slide on.
      \pause
      \item
        Shown from third slide on.
      \end{itemize}
    \item
      Shown from third slide on.
    \pause
    \item
      Shown from fourth slide on.
    \end{itemize}

    Shown from fourth slide on.

    \begin{itemize}
    \onslide
    \item
      Shown from first slide on.
    \pause
    \item
      Shown from fifth slide on.
    \end{itemize}
  \end{frame}
\end{verbatim}

This command does \emph{not} work correctly inside math mode environments like
\env{align} and \pkg{pgf} environments like \env{tikzpicture} or
\env{tcolorbox}, since these do really wicked things.

To \enquote{unpause} some text, that is, to temporarily suspend pausing, use
the command \cs{onslide}, see below.

\subsection{The general concept of overlay specifications}

Whilst the \cs{pause} command is easy to understand, it is quite limited and so
is best suited only to simple cases. The \cls{ltx-talk} class therefore
supports a different approach. The idea is to add \emph{overlay specifications}
to commands. These specifications are always given in pointed brackets and
follow the command as the first argument. In the simplest case, the
specification contains just a number. A command with an overlay specification
following it will only have \enquote{effect} on the slide(s) mentioned in the
specification. What exactly \enquote{having an effect} means, depends on the
command. Consider the following example.
\begin{verbatim}
  \begin{frame}
    \textbf{This line is bold on all three slides.}
    \textbf<2>{This line is bold only on the second slide.}
    \textbf<3>{This line is bold only on the third slide.}
  \end{frame}
\end{verbatim}

For the command \cs{textbf}, the overlay specification causes the text to be
set in boldface only on the specified slides. On all other slides, the text is
set in a normal font.

For a second example, consider the following frame:
\begin{verbatim}
  \begin{frame}
    \only<1>{This line is inserted only on slide~1.}
    \only<2>{This line is inserted only on slide~2.}
  \end{frame}
\end{verbatim}

The command \cs{only} normally simply inserts its parameter into the current
frame. However, if an overlay specification is present, it \enquote{throws
away} its parameter on slides that are not mentioned.

Overlay specifications can only be written behind certain commands, not every
command. Which commands you can use and which effects this will have is
explained in the next section. However, it is quite easy to redefine an
existing command such that it becomes \enquote{overlay specification aware},
see also Section~\ref{sec:overlay-cmds}.

The syntax of (basic) overlay specifications is the following: they are
comma-separated lists of slides and ranges. Ranges are specified like this:
|2-5|, which means slide two through to five. The start or the end of a range
can be omitted. For example, |3-| means \enquote{slides three, four, five, and
so on} and |-5| means the same as |1-5|. A complicated example is
|-3,6-8,10,12-15|, which selects the slides 1, 2, 3, 6, 7, 8, 10, 12, 13, 14
and~15.

\subsection{Commands with overlay specifications%
  \label{sec:overlay-cmds}}

For the following commands, adding an overlay specification causes the command
to be simply ignored on slides that are not included in the specification:
\cs{textbf}, \cs{textit}, \cs{textmd}, \cs{textnormal}, \cs{textrm},
\cs{textsc}, \cs{textsf}, \cs{textsl}, \cs{texttt}, \cs{textup}, \cs{emph};
\cs{color}, \cs{mathcolor}, \cs{textcolor}; \cs{alert}; \cs{includegraphics};
\cs{label}.

If a command takes several arguments, like \cs{color}, the specification should
directly follow the command as in the following example:
\begin{verbatim}
  \begin{frame}
    \color<2-3>[rgb]{1,0,0} This text is red on slides 2 and 3, otherwise
      black.
  \end{frame}
\end{verbatim}

For the following commands, the effect of an overlay specification is special:

\DescribeMacro{\onslide}
\begin{syntax}
  \cs{onslide}\sarg{overlay spec}
\end{syntax}
All text following this command will only be shown (uncovered) on the specified
slides. On non-specified slides, the text still occupies space. If no slides
are specified, the following text is always shown. You need not call this
command in the same \TeX{} group, its effect transcends block groups.
\begin{verbatim}
  \begin{frame}
    Shown on first slide.
    \onslide<2-3>
    Shown on second and third slide.
    \begin{itemize}
    \item
      Still shown on the second and third slide.
    \onslide<4->
    \item
      Shown from slide~4 on.
    \end{itemize}
    Shown from slide~4 on.
    \onslide
    Shown on all slides.
  \end{frame}
\end{verbatim}

\DescribeMacro{\only}
\begin{syntax}
  \cs{only}\sarg{overlay spec}\marg{text}
\end{syntax}
The \meta{text} is inserted only into the specified slides. For other slides, 
the text is simply thrown away. In particular, it occupies no space.
\begin{verbatim}
  \only<3->{Text inserted from slide 3 on.}
\end{verbatim}

\DescribeMacro{\uncover}
\begin{syntax}
  \cs{uncover}\sarg{overlay spec}\marg{text}
\end{syntax}
If the \meta{overlay spec} is present, the \meta{text} is shown
(\enquote{uncovered}) only on the specified slides. On other slides, the text
still occupies space and it is still typeset, but it is not shown or only shown
as if transparent. For details on how to specify whether the text is invisible
or just transparent see Section~\ref{sec:opacity}.
\begin{verbatim}
  \uncover<3->{Text shown from slide 3 on.}
\end{verbatim}

\DescribeMacro{\visible}
\begin{syntax}
  \cs{visible}\sarg{overlay spec}\marg{text}
\end{syntax}
This command does almost the same as \cs{uncover}. The only difference is that
if the text is not shown, it is never shown in a transparent way, but rather it
is not shown at all. Thus, for this command the transparency settings have no
effect.
\begin{verbatim}
  \visible<2->{Text shown from slide 2 on.}
\end{verbatim}

\DescribeMacro{\invisible}
\begin{syntax}
  \cs{invisible}\sarg{overlay spec}\marg{text}
\end{syntax}
This command does the opposite of \cs{visible}.
\begin{verbatim}
  \invisible<2->{Text hidden from slide~2 on.}
\end{verbatim}

\DescribeMacro{\alt}
\begin{syntax}
  \cs{alt}\sarg{overlay spec}\marg{default text}\marg{alternative text}
\end{syntax}
The default text is shown on the specified slides, otherwise the alternative
text.
\begin{verbatim}
  \alt<2>{On slide~2}{Not on slide~2.}
\end{verbatim}

\DescribeMacro{\temporal}
\begin{syntax}
  \cs{temporal}\sarg{overlay spec}\marg{before slide text}\marg{default text}%
    \marg{after slide text}
\end{syntax}
This command alternates between three different texts, depending on whether the
current slide is temporally before the specified slides, is one of the
specified slides, or comes after them. If the \meta{overlay spec} is not an
interval (that is, if it has a \enquote{hole}), the \enquote{hole} is
considered to be part of the before slides.
\begin{verbatim}
  \temporal<3-4>{Shown on 1, 2}{Shown on 3, 4}{Shown 5, 6, 7, ...}
  \temporal<3,5>{Shown on 1, 2, 4}{Shown on 3, 5}{Shown 6, 7, 8, ...}
\end{verbatim}
As a possible application of the \cs{temporal} command consider the following
example:
\begin{verbatim}
  \NewDocumentCommand\colorize{D<>{all}}{%
    \temporal<#1>{\color{red!50}}{\color{black}}{\color{black!50}}}
  \begin{frame}
    \begin{itemize}
      \colorize<1> \item First item.
      \colorize<2> \item Second item.
      \colorize<3> \item Third item.
      \colorize<4> \item Fourth item.
    \end{itemize}
  \end{frame}
\end{verbatim}

\DescribeMacro{\item}
\begin{syntax}
  \cs{item}\sarg{action spec}\oarg{item label}
\end{syntax}
The effect of \meta{action spec} is described in Section~\ref{sec:action-spec};
this extends the \meta{overlay spec} to include the potential to
\enquote{alert} items.
\begin{verbatim}
  \begin{frame}
    \begin{itemize}
    \item<1-> First point, shown on all slides.
    \item<2-> Second point, shown on slide~2 and later.
    \item<2-> Third point, also shown on slide~2 and later.
    \item<3-> Fourth point, shown on slide~3.
    \end{itemize}
  \end{frame}

  \begin{frame}
    \begin{enumerate}
    \item<3-| alert@3>[0.] A zeroth point, shown at the very end.
    \item<1-| alert@1> The first and main point.
    \item<2-| alert@2> The second point.
    \end{enumerate}
  \end{frame}
\end{verbatim}

\subsection{Environments with overlay specifications}

\DescribeEnv{onlyenv}
\DescribeEnv{invisibleenv}
\DescribeEnv{uncoverenv}
\DescribeEnv{visibleenv}
For each of the basic commands \cs{only}, \cs{visible}, \cs{uncover} and
\cs{invisible} there exists \enquote{environment versions} \env{onlyenv},
\env{visibleenv}, \env{uncoverenv} and \env{invisibleenv}. Except for
\env{onlyenv}, these environments do the same as the commands.

For the \env{onlyenv} environment, the contents of the environment is inserted
into the text only on the specified slides. The difference to \cs{only} is,
that the text is actually typeset inside a box that is then thrown away,
whereas \cs{only} immediately throws away its contents. If the text is not
\enquote{typesettable}, the \env{onlyenv} may produce an error where \cs{only}
would not.
\begin{verbatim}
  \begin{frame}
    This line is always shown.
    \begin{onlyenv}<2>
      This line is inserted on slide~2.
    \end{onlyenv}
  \end{frame}
\end{verbatim}

\subsection{Dynamically changing text or images}

You may sometimes wish to have some part of a frame change dynamically from
slide to slide. On each slide of the frame, something different should be shown
inside this area. You could achieve the effect of dynamically changing text by
giving a list of \cs{only} commands like this:
\begin{verbatim}
  \only<1>{Initial text.}
  \only<2>{Replaced by this on second slide.}
  \only<3>{Replaced again by this on third slide.}
\end{verbatim}

The trouble with this approach is that it may lead to slight, but annoying
differences in the heights of the lines, which may cause the whole frame to
\enquote{wobble} from slide to slide. This problem becomes much more severe if
the replacement text is several lines long.

To solve this problem, you can use two environments: \env{overlayarea} and
\env{overprint}. The first is more flexible, but less user-friendly.

\DescribeEnv{overlayarea}
\begin{syntax}
  \cs{begin}\{overlayarea\}\marg{area width}\marg{area height}
\end{syntax}
Everything within the environment will be placed in a rectangular area of the
specified size. The area will have the same size on all slides of a frame,
regardless of its actual contents.
\begin{verbatim}
  \begin{overlayarea}{\textwidth}{3cm}
    \only<1>{Some text for the first slide.\\Possibly several lines long.}
    \only<2>{Replacement on the second slide.}
  \end{overlayarea}
\end{verbatim}

\DescribeEnv{overprint}
\begin{syntax}
  \cs{begin}\{overprint\}\oarg{area width}
\end{syntax}
The \meta{area width} defaults to the text width. Inside the environment, use
\cs{only} or \env{onlyenv} to specify different things that should be shown for
this environment on different slides. Everything within the environment will be
placed in a rectangular area of the specified width. The height and depth of
the area are chosen large enough to accommodate the largest contents of the
area. Two compilations will be needed to allow \LaTeX{} to track the tallest
version of the frame contents.
\begin{verbatim}
  \begin{overprint}
    \only<1|handout:1>{%
        Some text for the first slide.\\
        Possibly several lines long.
      }%
    \only<2|handout:0>{%
       Replacement on the second slide. Suppressed for handout.
     }%
  \end{overprint}
  Following text
\end{verbatim}

A similar need for dynamical changes arises when you have, say, a series of
pictures named \file{first}, \file{second}, and \file{third} that show
different stages of some process. To make a frame that shows these pictures on
different slides, the following code might be used:
\begin{verbatim}
  \begin{frame}
    \frametitle{The Three Process Stages}

    \includegraphics<1>{first}
    \includegraphics<2>{second}
    \includegraphics<3>{third}
  \end{frame}
\end{verbatim}

\subsection{Advanced overlay specifications}

\subsubsection{Mode specifications}

This section is only important if you use the mode mechanism to create
different versions of your presentation. If you are not familiar with modes,
please skip this section or read Section~\ref{sec:modes} first.

In certain cases you may wish to have different overlay specifications to apply
to a command in different modes. For example, you might wish a certain text to
appear only from the third slide on during your presentation, but in a handout
for the audience there should be no third slide and the text should appear
already on the second slide. In this case you could write
\begin{verbatim}
  \only<3| handout:2>{Some text}
\end{verbatim}

The vertical bar separates the two different specifications |3| and
|handout:2|. By writing a mode name before a colon, you specify that the
following specification only applies to that mode. If no mode is given, as in
|3|, the mode |projector| is automatically added. For this reason, if you write
|\only<3>{Text}| and you are in |handout| mode, the text will be shown on all
slides since there is no restriction specified for handouts and since the |3|
is the same as |projector:3|.

It is also possible to give an overlay specification that contains only a mode
name (or several, separated by vertical bars):
\begin{verbatim}
  \only<handout>{This text is shown only in handout mode.}
\end{verbatim}

An overlay specification that does not contain any slide numbers is called a
(pure) \emph{mode specification}. If a mode specification is given, all modes
that are not mentioned are automatically suppressed. Thus |<projector:1->|
means \enquote{on all slides in |projector| mode and also on all slides in all
other modes, since nothing special is specified for them}, whereas
|<projector>| means \enquote{on all slides in |projector| mode and not on any
other slide}.

You can also mix pure mode specifications and overlay specifications, although
this can get confusing:
\begin{verbatim}
  \only<article| projector:1>{Riddle}
\end{verbatim}

This will cause the text |Riddle| to be inserted in |article| mode and on the
first slide of a frame in |projector| mode, but not at all in |handout| mode.

\subsubsection{Action specifications%
  \label{sec:action-spec}}

This section also introduces a rather advanced concept. You may also wish to
skip it on first reading.

Some overlay specification-aware commands can handle not only normal overlay
specifications, but also a so called \emph{action spec}. In an action
specification, the list of slide numbers and ranges is prefixed by
\meta{action}|@|, where \meta{action} is the name of a certain action to be
taken on the specified slides:
\begin{verbatim}
  \item<3-|alert@3> Shown from slide~3 on, alerted on slide~3.
\end{verbatim}

In the above example, the \cs{item} command, which allows actions to be
specified, will uncover the item text from slide three on and it will,
additionally, alert this item exactly on slide~3.

Not all commands can take an action specification. Currently, only \cs{item},
\cs{action}, the environment \env{actionenv}, and the block environments (like
\env{block}) handle them.

By default, the following actions are available:
\begin{itemize}
  \item |alert| alters the item or block.
  \item |uncover| uncovers the item or block (this is the default, if no action
    is specified).
  \item |only| causes the whole item or block to be inserted only on the
    specified slides.
  \item |visible| causes the text to become visible only on the specified
    slides (the difference between |uncover| and |visible| is the same as
    between \cs{uncover} and \cs{visible}).
  \item |invisible| causes the text to become invisible on the specified
    slides.
\end{itemize}

The whole action mechanism is based on the following environment:
\DescribeEnv{actionenv}
\begin{syntax}
  \cs{begin}\{actionenv\}\sarg{action spec}
\end{syntax}
This environment extracts all actions from the \meta{action spec} for the
current mode. For each action of the form \meta{action}|@|\meta{slide numbers},
it inserts the equivalent internal code to:
|\begin{|\meta{action}|env}<|\meta{slide number}|>| at the beginning of the
environment and the text |\end{|\meta{action}|env}| at the end. An
\meta{overlay spec} without an action is promoted to |uncover@|\meta{overlay
spec}.
\begin{verbatim}
  \begin{frame}
    \begin{actionenv}<2-|alert@3-4,6>
      This text is shown the same way as the text below.
    \end{actionenv}

    \begin{uncoverenv}<2->
      \begin{alertenv}<3-4,6>
        This text is shown the same way as the text above.
      \end{alertenv}
    \end{uncoverenv}
  \end{frame}
\end{verbatim}

\DescribeMacro{\action}
\begin{syntax}
  \cs{action}\sarg{action spec}\marg{text}
\end{syntax}
This has the same effect as putting \meta{text} in an \env{actionenv}.

\subsubsection{Incremental specifications%
  \label{sec:incremental}}

This section is mostly important for people who have already used overlay
specifications a lot and have grown tired of writing things like |<1->|,
|<2->|, |<3->|, and so on again and again. You should skip this section on
first reading.

Often you want to have overlay specifications that follow a pattern similar to
the following:
\begin{verbatim}
  \begin{itemize}
    \item<1-> Apple
    \item<2-> Peach
    \item<3-> Plum
    \item<4-> Orange
  \end{itemize}
\end{verbatim}

The problem starts if you decide to insert a new fruit, say, at the beginning.
In this case, you would have to adjust all of the overlay specifications. Also,
if you add a \cs{pause} command before the |itemize|, you would also have to
update the overlay specifications.

The class offers a special syntax to make creating lists as the one above more
convenient. You can replace it by the following list of \emph{incremental
overlay specifications}:
\begin{verbatim}
  \begin{itemize}
    \item<+-> Apple
    \item<+-> Peach
    \item<+-> Plum
    \item<+-> Orange
  \end{itemize}
\end{verbatim}

The effect of the |+| sign is the following. You can use it in any overlay
specification at any point where you would usually use a number. If a |+| sign
is encountered, it is replaced by one more than the current value of the
\LaTeX{} counter |pauses|; that is $0$ at the beginning of the frame, so the
first possible replacement value is~$1$. At the end of the overlay
specification, the |pauses| counter is incremented. This only happens once even
if the specification contains multiple |+| signs (they are replaced by the same
number, which is also the new value of |pauses|).

In the above example, the first specification is replaced by |<1->|. Then the
second is replaced by |<2->| and so forth. We can now easily insert new
entries, without having to change anything else. We might also write the
following:
\begin{verbatim}
  \begin{itemize}
    \item<+-| alert@+> Apple
    \item<+-| alert@+> Peach
    \item<+-| alert@+> Plum
    \item<+-| alert@+> Orange
  \end{itemize}
\end{verbatim}

This will alert the current item when it is uncovered. For example, the first
specification \verb"<+-| alert@+>" is replaced by \verb"<1-| alert@1>", the
second is replaced by \verb"<2-| alert@2>", and so on. Since the \env{itemize}
environment also allows you to specify a default overlay specification, see the
documentation of that environment, the above example can be written even more
economically as follows:
\begin{verbatim}
  \begin{itemize}[<+-| alert@+>]
    \item Apple
    \item Peach
    \item Plum
    \item Orange
  \end{itemize}
\end{verbatim}

The \cs{pause} command also updates the counter |pauses|. You can change this
counter yourself using the normal \LaTeX{} commands \cs{setcounter} or
\cs{addtocounter}.

Any occurrence of a |+| sign may be followed by an \emph{offset} in round
brackets. This offset will be added to the value of the |pauses| counter. Thus,
if |pauses| is $2$, then |<+(1)->| expands to |<3->| and |<+(-1)-+>| expands to
|<1-2>|. For example
\begin{verbatim}
  \begin{itemize}
    \item<2-> Apple
    \item<3-> Peach
    \item<4-> Plum
    \item<5-> Orange
  \end{itemize}
\end{verbatim}
and
\begin{verbatim}
  \begin{itemize}[<+(1)->]
    \item Apple
    \item Peach
    \item Plum
    \item Orange
  \end{itemize}
\end{verbatim}
are equivalent.

There is another special sign you can use in an overlay specification that
behaves similarly to the |+| sign: a dot. When you write |<.->|, a similar
process occurs to that for |<+->|, \emph{except} that the counter |pauses| is
used unchanged. Thus a dot, possibly followed by an offset, just expands to the
current value of the counter |pauses|, possibly offset. This dot notation can
be useful in case like the following:
\begin{verbatim}
  \begin{itemize}[<+->]
    \item Apple
    \item<.-> Peach
    \item Plum
    \item Orange
  \end{itemize}
\end{verbatim}

In the example, the second item is shown at the same time as the first one
since it does not update the counter.

In the following example, each time an item is uncovered, the specified text is
alerted. When the next item is uncovered, this altering ends.
\begin{verbatim}
  \begin{itemize}[<+->]
    \item This is \alert<.>{important}.
    \item We want to \alert<.>{highlight} this and \alert<.>{this}.
    \item What is the \alert<.>{matrix}?
  \end{itemize}
\end{verbatim}

The replacements of the |+| sign and the |.| sign are no less than zero. This
prevents errors when encountering large negative offsets, for example
|<+(-7)->| is converted to |<0->| rather than |<-6->|.

\section{Structuring a presentation: the global structure}

This section lists the commands that are used for structuring a presentation
\enquote{globally} using commands like \cs{section} or \cs{part}. These
commands are used to create a \emph{static} structure, meaning that the
resulting presentation is normally presented one slide after the other in the
order the slides occur.

\subsection{Adding a title frame}

You can use the standard \cs{maketitle} command to insert a title frame. As
standard, it will arrange the following elements on the title page: the
document title and subtitle, the authors' names, their affiliation, and the
date.

\DescribeMacro{\maketitle}
\begin{syntax}
  \cs{maketitle}\oarg{settings}
\end{syntax}
The \cs{maketitle} command may be inserted either inside or outside of a
\env{frame} environment; the result will be a full frame in either case. The
optional argument to the command is used to specify the following
\meta{settings}
\begin{itemize}
  \item \opt{element-order} which determines which entries are printed
    and the order they appear: a comma-list
  \item \opt{frame-style} sets the frame style
  \item \opt{horizontal-alignment}
  \item \opt{vertical-alignment}
\end{itemize}

\DescribeMacro{\author}
\DescribeMacro{\date}
\DescribeMacro{\institute}
\DescribeMacro{\subtitle}
\DescribeMacro{\title}
\begin{syntax}
  \cs{author}\oarg{options}\marg{author} \\
  \cs{date}\oarg{options}\marg{date} \\
  \cs{institute}\oarg{options}\marg{institute} \\
  \cs{subtitle}\oarg{options}\marg{subtitle} \\
  \cs{title}\oarg{options}\marg{title}
\end{syntax}
As well as the standard \LaTeX{} elements \cs{author}, \cs{title} and
\cs{date}, \cls{ltx-talk} supports \cs{subtitle} and \cs{institute}. These
working the same way as the standard commands for providing metadata. The
\meta{options} may be used to set the \opt{short-} version of each item: if no
key name is given, the entire optional argument is used as the short version.
If multiple \meta{options} are given, they are interpreted as key-values.

\section{Structuring a presentation: the local structure}

\LaTeX{} provides different commands for structuring text \enquote{locally},
for example, \foreign{via} the \env{itemize} environment. These environments
are also available in the \cls{ltx-talk} class, although their appearance has
been slightly changed.

\subsection{Itemizations, enumerations and descriptions}

\DescribeEnv{description}
\DescribeEnv{enumerate}
\DescribeEnv{itemize}
There are three predefined environments for creating lists, namely
\env{enumerate}, \env{itemize}, and \env{description}. The first two can be
nested to depth three, but nesting them to this depth creates totally
unreadable slides.

\DescribeMacro{\item}
The \cs{item} command is overlay specification-aware. If an overlay
specification is provided, the item will only be shown on the specified slides,
see the following example. If the \cs{item} command is to take an optional
argument and an overlay specification, the overlay specification comes first as
in |\item<1>[Cat]|.
\begin{verbatim}
  \begin{frame}
    There are three important points:
    \begin{enumerate}
      \item<1-> A first one,
      \item<2-> a second one with a bunch of subpoints,
        \begin{itemize}
          \item first subpoint. (Only shown from second slide on!).
          \item<3-> second subpoint added on third slide.
          \item<4-> third subpoint added on fourth slide.
        \end{itemize}
      \item<5-> and a third one.
    \end{enumerate}
  \end{frame}
\end{verbatim}

The list environments have syntax
\begin{syntax}
  \cs{begin}\marg{list type}\oarg{options}
\end{syntax}

If the option \opt{action-spec} is given, in every occurrence of an \cs{item}
command that does not have an overlay specification attached to it, the
\meta{overlay specification} is used. By setting this specification to be an
incremental overlay specification, see Section~\ref{sec:incremental}, you
can implement, for example, a stepwise uncovering of the items. The
\meta{overlay specification} is inherited by sub-environments.
Naturally, in a sub-environment you can reset it locally by setting it to
|<1->| (the subitems will be shown on all slides) or |<.->| (the subitems will
be shown starting from the same slide as the parent item).
\begin{verbatim}
  \begin{itemize}[action-spec = <+->]
    \item This is shown from the first slide on.
    \item This is shown from the second slide on.
    \item This is shown from the third slide on.
    \item<1-> This is shown from the first slide on.
    \item This is shown from the fourth slide on.
  \end{itemize}
\end{verbatim}
If you do not need to give any other options for the list environment, you may
use the shortened format |[<...>]|, which matches the \cls{beamer} syntax:
\begin{verbatim}
  \begin{itemize}[<+->]
    \item This is shown from the first slide on.
    \item This is shown from the second slide on.
    \item This is shown from the third slide on.
    \item<1-> This is shown from the first slide on.
    \item This is shown from the fourth slide on.
  \end{itemize}
\end{verbatim}

\subsection{Highlighting}

\DescribeMacro{\alert}
\begin{syntax}
  \cs{alert}\sarg{overlay spec}\marg{text}
\end{syntax}
The given \meta{text} is highlighted, typically by coloring the text red. If
the \meta{overlay spec} is present, the command only has an effect on the
specified slides.
\begin{verbatim}
  This is \alert{important}.
\end{verbatim}

\DescribeEnv{alertenv}
\begin{syntax}
  \cs{begin}\{\texttt{alertenv}\}\sarg{overlay spec}
\end{syntax}
Environment version of the \cs{alert} command.

\subsection{Block environments}

\subsection{Figures and tables}

You can use the standard \LaTeX{} environments \env{figure} and \env{table}
much the same way you would normally use them. However, any placement
specification will be ignored. Figures and tables are immediately inserted
where the environments start. If there are too many of them to fit on the
frame, you must manually split them among additional frames.
\begin{verbatim}
  \begin{frame}
    \begin{figure}
      \includegraphics{myfigure}
      \caption{This caption is placed below the figure.}
    \end{figure}

    \begin{figure}
      \caption{This caption is placed above the figure.}
      \includegraphics{myotherfigure}
    \end{figure}
  \end{frame}
\end{verbatim}

Note that at present the standard setting for \cls{ltx-talk} place the float
content left-aligned. This is similar to the \LaTeX{} base classes and
contrasts with \cls{beamer}. See the example directory for the supported method
to center content in floats.

The floating environments, like other blocks, accept a first action argument.
This can be used to apply an overlay to the entire content, most obviously a
graphics and the associated caption.

\subsection{Splitting a frame into multiple columns}

The class offers several commands and environments for splitting (perhaps only
part of) a frame into multiple columns. These commands have nothing to do with
\LaTeX{}'s commands for creating columns. Columns are especially useful for
placing a graphic next to a description/explanation.

The main environment for creating columns is called \env{columns}. Inside this
environment, you can place several \env{column} environments, each of which
creates a new column.

\DescribeEnv{columns}
\begin{syntax}
  \cs{begin}\{\texttt{columns}\}\sarg{action spec}\oarg{options}
\end{syntax}
A multi-column area. If the \meta{action spec} is present, the given actions
are taken on the specified slides, see Section~\ref{sec:action-spec}. The
following \meta{options} may be given:
\begin{itemize}
  \item \opt{vertical-alignment} a choice of one of \opt{bottom}, \opt{center}
    or \opt{top}; this causes the content of the columns to be aligned as
    specified. The standard setting is \opt{center}. For compatibility with
    \cls{beamer}, the short options \opt{b}, \opt{c} and \opt{t} are available.
  \item \opt{width} will cause the columns to occupy \meta{width}: the standard
    setting is \cs{textwidth}.
\end{itemize}

\DescribeEnv{column}
\begin{syntax}
  \cs{begin}\{\texttt{column}\}\sarg{action spec}\oarg{placement}\marg{column width}
\end{syntax}
To create a column, you can use the \env{column} environment.
\begin{verbatim}
  \begin{columns}
    \begin{column}{5cm}
      Two\\lines.
    \end{column}
    \begin{column}{5cm}
      One line (but aligned).
    \end{column}
  \end{columns}
\end{verbatim}

Creates a single column of width \meta{column width}. If the \meta{action
specification} is present, the given actions are taken on the specified slides,
see Section~\ref{sec:action-spec}. The vertical placement of the enclosing
\env{columns} environment can be overruled by specifying a specific
\meta{placement}.

\subsection{Footnotes}

First a word of warning: Using footnotes is usually not a good idea. They
disrupt the flow of reading.

\DescribeMacro{\footnote}
\begin{syntax}
  \cs{footnote}\sarg{overlay spec}\oarg{mark-number}\marg{text}
\end{syntax}
Inserts a footnote into the current frame. Footnotes will always be shown on
the current frame; they will never be \enquote{moved} to other frames. As
usual, one can give a \meta{mark-number}, which will cause the footnote to use
that number. Footnotes are placed at the bottom of the content area of the
frame: unlike \cls{beamer}, this is true for footnotes in columns,
\foreign{etc.}: for \cls{beamer}, this behavior is similar to the
\opt{framefootnotes} class option or \cs{footnote[frame]}\marg{text}.

If an \meta{overlay spec} is given, this causes the footnote \meta{text} to be
shown only on the specified slides. The footnote symbol in the text is shown on
all slides.

\section{Creating alternative output formats%
  \label{sec:modes}}

\DescribeOption{mode}
The \cls{ltx-talk} class allows you to select different output formats from the
same source using the option \opt{mode}. This works with the overlay
specification to typeset some of the input selectively.

\subsection{Creating handouts using the \opt{handout} mode}
  
During a presentation it is very much desirable that the audience has a
\emph{handout} or even \emph{lecture notes} available to it. A handout allows
everyone in the audience to individually go back to things they have not
understood.

The easiest way of creating a handout for your audience is to set the
\opt{mode} option set to \meta{handout}. This will cause the document to be
typeset in |handout| mode. It will look like a presentation, but it can be
printed or annotated more easily (the overlays are \enquote{flattened}).

\section{Creating tagging PDF output}

As detailed in the introduction (Section~\ref{sec:intro}), the \cls{ltx-talk}
class has been written from the start with tagging in mind. As such, where
possible, tagging is generated automatically. This works best with predictable
frame structure, for example the classical \enquote{uncover a list one item at
a time} approach.

\subsection{Enabling tagging}

Tagging can be enabled using the standard \LaTeX{} interface
\begin{verbatim}
  \DocumentMetadata{tagging = on}
\end{verbatim}
as the first line of the source.

\subsection{Current tagging approach}

Whilst some aspects of tagging are straight-forward, there are specific
challenges in tagging presentations that mean that some parts of the approach
are experimental. In particular, the relationship between frame titles and
sectioning is still very much open. At present, frame titles are tagged as
sections at the level \texttt{H4}, \foreign{i.e.}, deliberately well below
sections and subsections. This may lead to warning from some accessibility
checkers.

Feedback here is needed from users of assistive technology tools, to allow
adjustment to the standard tagging to best aid them. It is likely that in the
future, a richer range of options will be provided in this area. At present, if
you want to adjust the tagging you could use for example
\begin{verbatim}
  \tagpdfsetup{role / new-tag = frametitle / H1}
\end{verbatim}

\subsection{Controlling tagging}

\DescribeOption{tag-slides}
To enable control tagging of slides, the frame option \opt{tag-slides} is
available. This can be used to list which slides in a frame are tagged: those
omitted are marked as artifacts. The setting should be a comma-list of slide
numbers, which an include \opt{n} as a marker to mean the last slide of the
frame. The standard setting is \opt{tag-slides = n}, meaning only the last
slide of the frame is tagged. You may use mode specifications in the
\opt{tag-slides} option, for example \opt{tag-slides = n\|handout:1,n} would
tag the last slide in the frame in \texttt{projector} mode and the first and
last for \texttt{handout} mode.

\section{Changing the way things look}

The design of frames in \cls{ltx-talk} is controlled by \emph{templates}, a
feature introduced in the \LaTeX{} kernel in the 2024-06-01 release. The
general concept is currently documented in \file{lttemplates.pdf} with some
coverage in \file{clsguide.pdf}.

Several of the templates are illustrated in the examples, available along with
this document or at \url{https://www.texdev.net/ltx-talk/examples/} as live
demonstrations.

\subsection{Font choice}

For \pdfTeX{} users, font choice will typically be determined by the available
\LaTeX{} font packages. For \LuaTeX{} users, the font choice will be partly
determined by the system fonts installed. To get consistent appearance for text
and math mode material, \LuaTeX{} users are strongly encouraged to choose a
font available with Unicode math support. A demonstration of the majority of
current options in this area is available at
\url{https://mathfonts.github.io/latex-table.html}, where you can also try out
the appearance of expressions you choose.

\subsection{Template \texttt{floatenv}}

This template is used to create environments which map to the floats in a
starndard \LaTeX{} document. Note that they do \emph{not} float in an
\cls{ltx-talk} document. The template takes two arguments: options to apply to
an individual environment, and the type of \enquote{float}. The standard
template is \texttt{talk}, which recognizes the keys
\begin{itemize}
  \item \opt{float-placement} Ignored at present
  \item \opt{horizontal-alignment} Takes a choice of \opt{left}, \opt{center}
    or \opt{right}
\end{itemize}

\subsection{Template \texttt{footer}}

This template is used to populate the footer area of the slide: this takes no
arguments. The standard template is \texttt{talk}, which recognizes the keys
\begin{itemize}
  \item \opt{background-color} The color of the background of the entire footer
    area
  \item \opt{color} The color of the text in the footer area
  \item \opt{element-order} A comma-separated list of metadata elements to
    print: this can include the \texttt{framenumber}. Where available,
    \enquote{short} versions of the metadata items are printed.
  \item \opt{font} Font for printing elements
  \item \opt{left-hspace} Margin on left of footer
  \item \opt{right-hspace} Margin on right of footer
  \item \opt{separator} Tokens inserted between elements.
\end{itemize}

The current list of supported metadata entries is
\begin{itemize}
  \item \texttt{author}
  \item \texttt{date}
  \item \texttt{framenumber}
  \item \texttt{institute}
  \item \texttt{subtitle}
  \item \texttt{title}
  \item \texttt{totalframes}
\end{itemize}

Two instances of this template are pre-defined: \texttt{std} and
\texttt{wallpaper}. The \texttt{wallpaper} version omits any elements, meaning
that it only applies a colored background to the footer.

\subsection{Template \texttt{header}}

This template is used to populate the header area of the slide: this takes no
arguments. The standard template is \texttt{talk}, which recognizes the keys
\begin{itemize}
  \item \opt{background-color} The color of the background of the entire footer
    area
  \item \opt{color} The color of the text in the footer area
  \item \opt{font} Font for printing elements
  \item \opt{height} The overall height of the header background area
  \item \opt{left-hspace} Margin on left of footer
  \item \opt{print-frame-title} A boolean to turn title printing on and off
  \item \opt{right-hspace} Margin on right of footer
\end{itemize}

Two instances of this template are pre-defined: \texttt{std} and
\texttt{wallpaper}. The \texttt{wallpaper} version sets \opt{print-frame-title}
to \opt{false}, and thus only adds the background color to the header.

\section{Guidelines for creating presentations}

In this section we sketch the guidelines that we try to stick to when we create
presentations. These guidelines either arise out of experience, out of common
sense, or out of recommendations by other people or books. These rules are
certainly not intended as commandments that, if not followed, will result in
catastrophe. The central rule of typography also applies to creating
presentations: \emph{Every rule can be broken, but no rule may be ignored.}

\subsection{Structuring a presentation}

\subsubsection{Know the time constraints}

When you start to create a presentation, the very first thing you should worry
about is the amount of time you have for your presentation. Depending on the
occasion, this can be anything between 2~minutes and two hours.
\begin{itemize}
  \item A simple rule for the number of frames is that you should have at most
    one frame per minute.
  \item In most situations, you will have less time for your presentation than
    you would like.
  \item \emph{Do not try to squeeze more into a presentation than time allows
    for.} No matter how important some detail seems to you, it is better to
    leave it out, but get the main message across, than getting neither the
    main message nor the detail across.
\end{itemize}

In many situations, a quick appraisal of how much time you have will show that
you won't be able to mention certain details. Knowing this can save you hours
of work on preparing slides that you would have to remove later anyway.

\subsubsection{Global structure}

To create the \enquote{global structure} of a presentation, with the time
constraints in mind, proceed as follows:
\begin{itemize}
  \item Make a mental inventory of the things you can reasonably talk about
    within the time available.
  \item Categorize the inventory into sections and subsections.
  \item For very long talks (like a 90~minute lecture), you might also divide
    your talk into independent parts (like a \enquote{review of the previous
    lecture part} and a \enquote{main part}) using the \cs{part} command. Note
    that each  part has its own table of contents. % TODO
  \item Do not feel afraid to change the structure later on as you work on the
    talk.
\end{itemize}

\paragraph{Parts, section, and subsections}

\begin{itemize}
  \item Do not use more than four sections and not less than two per part.
\end{itemize}

Even four sections are usually too much, unless they follow a very easy pattern.
Five and more sections are simply too hard to remember for the audience. After
all, when you present the table of contents, the audience will not yet really be
able to grasp the importance and relevance of the different sections and will
most likely have forgotten them by the time you reach them.
\begin{itemize}
  \item Ideally, a table of contents should be understandable by itself. In
    particular, it should be comprehensible \emph{before} someone has heard
    your talk.
  \item Keep section and subsection titles self-explaining.
  \item Both the sections and the subsections should follow a logical pattern.
  \item Begin with an explanation of what your talk is all about. (Do not
    assume that everyone knows this. The \emph{Ignorant Audience Law} states:
    Someone important in the audience always knows less than you think
    everyone should know, even if you take the Ignorant Audience Law into
    account.)
  \item Then explain what you or someone else has found out concerning the
    subject matter.
  \item Always conclude your talk with a summary that repeats the main message
    of the talk in a short and simple way. People pay most attention at the
    beginning and at the end of talks. The summary is your \enquote{second
    chance} to get across a message.
  \item You can also add an appendix part using the \cs{appendix} command. Put
    everything into this part that you do not actually intend to talk about,
    but that might come in handy when questions are asked. % TODO
  \item Do not use subsubsections, they are evil.
\end{itemize}

\paragraph{Giving an abstract}

In papers, the abstract gives a short summary of the whole paper in about 100
words. This summary is intend to help readers appraise whether they should
read the whole paper or not.
\begin{itemize}
  \item Since your audience is unlikely to flee after the first slide, in a
    presentation you usually do not need to present an abstract.
  \item However, if you can give a nice, succinct statement of your talk, you
    might wish to include an abstract.
  \item If you include an abstract, be sure that it is \emph{not} some long
    text but just a very short message.
  \item \emph{Never, ever} reuse a paper abstract for a presentation.
\end{itemize}

\paragraph{Numbered theorems and definitions}

A common way of globally structuring (math) articles and books is to use
consecutively numbered definitions and theorems. Unfortunately, for
presentations the situation is a bit more complicated and we would like to
discourage using numbered theorems in presentations. The audience has no chance
of remembering these numbers. \emph{Never} say things like \enquote{now, by
Theorem~2.5 that I showed you earlier, we have \dots} It would be much better
to refer to, say, Kummer's Theorem instead of Theorem~2.5. If Theorem~2.5 is
some obscure theorem that does not have its own name (unlike Kummer's Theorem
or Main Theorem or Second Main Theorem or Key Lemma), then the audience will
have forgotten about it anyway by the time you refer to it again.

In our opinion, the only situation in which numbered theorems make sense in a
presentation is in a lecture, in which the students can read lecture notes in
parallel to the lecture where the theorems are numbered in exactly the same
way.

If you do number theorems and definitions, number everything consecutively.
Thus if there are one theorem, one lemma, and one definition, you would have
Theorem~1, Lemma~2, and Definition~3. Some people prefer all three to be
numbered~1. We would \emph{strongly} like to discourage this. The problem is
that this makes it virtually impossible to find anything since Theorem~2 might
come after Definition~10 or the other way round. Papers and, worse, books that
have a Theorem~1 and a Definition~1 are a pain.
\begin{itemize}
  \item Do not inflict pain on other people.
\end{itemize}

\paragraph{Bibliographies}

You may also wish to present a bibliography at the end of your talk, so that
people can see what kind of \enquote{further reading} is possible. When adding
a bibliography to a presentation, keep the following in mind:
\begin{itemize}
  \item It is a bad idea to present a long bibliography in a presentation.
    Present only very few references. (Naturally, this applies only to the talk
    itself, not to a possible handout.)
  \item If you present more references than fit on a single slide you can be
    almost sure that none of them will be remembered.
  \item Present references only if they are intended as \enquote{further
    reading.} Do not present a list of all things you used like in a paper.
  \item You should not present a long list of all your other great papers
    \emph{except} if you are giving an application talk.
  \item Using the \cs{cite} commands can be confusing since the audience has
    little chance of remembering the citations. If you cite the references,
    always cite them with full author name and year like \enquote{[Nobacon,
    2003]} instead of something like \enquote{[2,\,4]} or
    \enquote{[Nob01,\,ND02]}.
  \item If you want to be modest, you can abbreviate your name when citing
    yourself as in \enquote{[Nickelsen and N., 2003]} or
    \enquote{[Nickelsen and N, 2003]}. However, this can be confusing for the
    audience since it is often not immediately clear who exactly \enquote{N.}
    might be. We recommend using the full name.
\end{itemize}

\subsubsection{Frame structure}

Just like your whole presentation, each frame should also be structured. A
frame that is solely filled with some long text is very hard to follow. It is
your job to structure the contents of each frame such that, ideally, the
audience immediately sees which information is important, which information is
just a detail, how the presented information is related, and so on.

\paragraph{The frame title}

\begin{itemize}
  \item Put a title on each frame. The title explains the contents of the frame
    to people who did not follow all details on the slide.
  \item The title should really \emph{explain} things, not just give a cryptic
    summary that cannot be understood unless one has understood the whole
    slide. For example, a title like \enquote{The Poset} will have everyone
    puzzled what this slide might be about. Titles like \enquote{Review of the
    Definition of Partially Ordered Sets (Posets)} or \enquote{A Partial
    Ordering on the Columns of the Genotype Matrix} are \emph{much} more
    informative.
  \item Ideally, titles on consecutive frames should \enquote{tell a story}
    all by themselves.
  \item In English, you should \emph{either} \emph{always} capitalize all words
    in a frame title except for words like \enquote{a} or \enquote{the} (as in
    a title), \emph{or} you \emph{always} use the normal lowercase letters. Do
    \emph{not} mix this; stick to one rule. The same is true for block titles.
    For example, do not use titles like \enquote{A short Review of Turing
    machines.} Either use \enquote{A Short Review of Turing Machines.} or
    \enquote{A short review of Turing machines.} (Turing is still spelled with
    a capital letter since it is a proper name).
  \item In English, the title of the whole document should be capitalized,
    regardless of whether you capitalize anything else.
  \item In German and other languages that have lots of capitalized words,
    always use the correct upper-/lowercase letters. Never capitalize anything
    in addition to what is usually capitalized.
\end{itemize}

\paragraph{How much can I put on a frame?}

\begin{itemize}
  \item A frame with too little on it is better than a frame with too much on
    it. A usual frame should have between 20 and 40 words. The maximum should be
    at about 80 words.
  \item Do not assume that everyone in the audience is an expert on the subject
    matter. Even if the people listening to you should be experts, they may
    last have heard about things you consider obvious several years ago. You
    should always have the time for a quick reminder of what exactly a
    \enquote{semantical complexity class} or an \enquote{$\omega$-complete
    partial ordering} is.
  \item Never put anything on a slide that you are not going to explain
    during the talk, not even to impress anyone with how complicated your
    subject matter really is. However, you may explain things that are not on a
    slide.
  \item Keep it simple. Typically, your audience will see a slide for less than
    50~seconds. They will not have the time to puzzle through long sentences or
    complicated formulas.
\end{itemize}

\paragraph{Structuring a frame}

\begin{itemize}
  \item Use block environments like \env{block}, \env{theorem}, \env{proof},
    \env{example}, and so on.
  \item Prefer \env{enumerate} and \env{itemize} environments over plain text.
  \item Use \env{description} when you define several things.
  \item Do not use more than two levels of \enquote{subitemizing.}
  \item Do not create endless \env{itemize} or \env{enumerate}  lists.
  \item Do not uncover lists piecewise.
  \item Emphasis is an important part of creating structure. Use \cs{alert} to
    highlight important things. This can be a single word or a whole sentence.
    However, do not overuse highlighting since this will negate the effect.
  \item Use columns.
  \item \emph{Never} use footnotes. They needlessly disrupt the flow of
    reading. Either what is said in the footnote is important and should be put
    in the normal text; or it is not important and should be omitted
    (\emph{especially} in a presentation).
  \item Use \env{quote} or \env{quotation} to typeset quoted text.
  \item Do not use long bibliographies.
\end{itemize}

\paragraph{Writing the text}

\begin{itemize}
  \item Use short sentences.
  \item Prefer phrases over complete sentences. For example, instead of
    \enquote{The figure on the left shows a Turing machine, the figure on the
    right shows a finite automaton.}\ try \enquote{Left: A Turing machine.
    Right: A finite automaton.} Even better, turn this into an itemize or a
    description.
  \item Punctuate correctly: no punctuation after phrases, complete punctuation
    in and after complete sentences.
  \item \emph{Never} use a smaller font size to \enquote{fit more on a frame.}
  \item Do not hyphenate words. If absolutely necessary, hyphenate words
  \enquote{by hand}, using the command~|\-|.
  \item Break lines \enquote{by hand} using the command~|\\|. Do not rely on
    automatic line breaking. Break where there is a logical pause. For
    example, good breaks in \enquote{the tape alphabet is larger than the input
    alphabet} are before \enquote{is} and before the second \enquote{the.} Bad
    breaks are before either \enquote{alphabet} and before \enquote{larger.}
  \item Text and numbers in figures should have the \emph{same} size as normal
    text. Illegible numbers on axes usually ruin a chart and its message.
\end{itemize}

\subsection{Using graphics}

Graphics often convey concepts or ideas much more efficiently than text: A
picture can say more than a thousand words. (Although, sometimes a word can say
more than a thousand pictures.)
\begin{itemize}
  \item Put (at least) one graphic on each slide, whenever possible.
    Visualizations help an audience enormously.
  \item Usually, place graphics to the left of the text. (Use the \env{columns}
    environment.) In a left-to-right reading culture, we look at the left
    first.
  \item Graphics should have the same typographic parameters as the text: Use
    the same fonts (at the same size) in graphics as in the main text. A small
    dot in a graphic should have exactly the same size as a small dot in a
    text. The line width should be the same as the stroke width used in
    creating the glyphs of the font. For example, an \qty{11}{pt} non-bold
      Computer Modern font has a stroke width of \qty{0.4}{pt}.
  \item While bitmap graphics, like photos, can be much more colorful than the
    rest of the text, vector graphics should follow the same \enquote{color
    logic} as the main text (like black~= normal lines, red~= highlighted
    parts, green~= examples, blue~= structure).
  \item Like text, you should explain everything that is shown on a graphic.
    Unexplained details make the audience puzzle whether this was something
    important that they have missed. Be careful when importing graphics from a
    paper or some other source. They usually have much more detail than you
    will be able to explain and should be radically simplified.
  \item Sometimes the complexity of a graphic is intentional and you are
    willing to spend much time explaining the graphic in great detail. In this
    case, you will often run into the problem that fine details of the graphic
    are hard to discern for the audience.
\end{itemize}

\subsection{Choosing appropriate colors}

\begin{itemize}
  \item Use colors sparsely. The prepared themes are already quite colorful
    (blue~= structure, red~= alert, green~= example). If you add more colors
    for things like code, math text, \foreign{etc}., you should have a \emph{very}
    good reason.
  \item Be careful when using bright colors on white background,
  \emph{especially} when using green. What looks good on your monitor may look
    bad during a presentation due to the different ways monitors, projectors,
    and printers reproduce colors. Add lots of black to pure colors when you
    use them on bright backgrounds.
  \item Maximize contrast. Normal text should be black on white or at least
    something very dark on something very bright. \emph{Never} do things like
    \enquote{light green text on not-so-light green background.}
  \item Background shadings decrease the legibility without increasing the
    information content. Do not add a background shading just because it
    \enquote{somehow looks nicer.}
  \item Inverse video (bright text on dark background) can be a problem during
    presentations in bright environments since only a small percentage of the
    presentation area is light up by the projector. Inverse video is harder to
    reproduce on printouts and on transparencies.
\end{itemize}

\end{documentation}

\PrintIndex

\end{document}