Source

beamer / doc / beamerug-color.tex

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% Copyright 2003, 2004 by Till Tantau <tantau@users.sourceforge.net>.
%
% This program can be redistributed and/or modified under the terms
% of the GNU Public License, version 2.

\section{Colors}

\label{section-colors}

\beamer's color management allows you to specify the color of every
element (like, say, the color of the section entries in a table of
contents or, say, the color of the subsection entries in a mini table
of contents in a sidebar). While the system is quite powerful, it is
not trivial to use. To simplify the usage of the color system, you
should consider using a predefined color theme, which takes care of
everything for you.

In the following, color themes are explained first. The rest of the
section consists of explanations of how the color management works
internally. You will need to read these sections only if you wish to
write your own color themes; or if you are quite happy with the
predefined themes but you absolutely insist that displayed
mathematical text simply has to be typeset in a lovely pink.



\subsection{Color Themes}

In order to also show the effect of the different color themes on the
sidebar, in the following examples the color themes are used together
with the outer theme |sidebar|.

\subsubsection{Default and Special-Purpose Color Themes}

\begin{colorthemeexample}{default}
  The |default| color theme is very sober. It installs little special
  colors and even less backgrounds. The default color theme sets up
  the default parent relations between the different \beamer-colors.

  The main colors set in the |default| color theme are the following: 
  \begin{itemize}
  \item
    |normal text| is black on white.
  \item
    |alerted text| is red.
  \item
    |example text| is a dark green (green with 50\% black).
  \item
    |structure| is set to a light version of MidnightBlue
    (more precisely, 20\% red, 20\% green, and 70\% blue).  
  \end{itemize}
  Use this theme for a no-nonsense presentation. Since this theme is
  loaded by default, you cannot ``reload'' it after having loaded
  another color theme.
\end{colorthemeexample}

\begin{colorthemeexample}[\oarg{options}]{structure}
  The example was created using |\usecolortheme[named=SeaGreen]{structure}|. 
  
  This theme offers a convenient way of changing the color used for
  structural elements. More precisely, it just changes the foreground
  of the \beamer-color |structure|. You can also achieve this by
  directly invoking the function |\setbeamercolor|, but this color
  theme makes things a bit easier.

  The theme offers several \meta{options}, which can be used to
  specify the color to be used for structural elements:
  \begin{itemize}
  \item
    \declare{|rgb=|\marg{rgb tuple}} sets the |structure| foreground
    to the specified red-green-blue tuple. The numbers are given as
    decimals between 0 and 1. For example, |rgb={0.5,0,0}| yields a
    dark red.
  \item
    \declare{|RGB=|\marg{rgb tuple}} does the same as |rgb|, except
    that the numbers range between 0 and 255. For example,
    |RGB={128,0,0}|  yields a dark red.
  \item
    \declare{|cmyk=|\marg{cymk tuple}} sets the |structure| foreground
    to the specified cyan-magenta-yellow-black tuple. The numbers are
    given as decimals between 0 and 1. For example, |cymk={0,1,1,0.5}|
    yields a dark red.
  \item
    \declare{|cmy=|\marg{cym tuple}} is similar to |cmyk|, except that
    the black component is not specified.
  \item
    \declare{|hsb=|\marg{hsb tuple}}  sets the |structure| foreground
    to the specified hue-saturation-brightness tuple. The numbers are
    given as decimals between 0 and 1. For example, |hsb={0,1,.5}|
    yields a dark red.
  \item
    \declare{|named=|\marg{color name}} sets the |structure| foreground
    to a named color. This color must previously have been defined
    using the |\DefineNamedColor| command. Adding the class option
    |xcolor=dvipsnames| will install a long list of standard
    names. See the file |dvipsnam.def| for the list.
  \end{itemize}
\end{colorthemeexample}

\begin{colorthemeexample}{sidebartab}
  This theme changes the colors in a sidebar such that the current
  entry in a table of contents shown there gets hilighted by showing a
  different background behind it.
\end{colorthemeexample}



\subsubsection{Complete Color Themes}

A ``complete'' color theme is a color theme that completely specifies
all colors for all parts of a frame. It installs specific colors and
does not derive the colors from, say, the |structure| \beamer-color.
Complete complete themes happen to have names of flying animals.

\begin{colorthemeexample}{albatross}
  The color theme is a ``dark'' or ``inverted'' theme using yellow on
  blue as the main colors. The color theme also installs a slightly
  darker background color for blocks, which is necessary for
  presentation themes that use shadows, but which (in my opinion) is
  undesirable for all other presentation themes. By using the |lily|
  color theme together with this theme, the backgrounds for blocks can
  be removed.

  When using a light-on-dark theme like this one, be aware that there
  are certain disadvantages:
  \begin{itemize}
  \item
    If the room in which the talk is given has been ``darkened,''
    using such a theme makes it more difficult for the audience to
    take or read notes.
  \item
    Since the room becomes darker, the pupil becomes larger, thus
    making it harder for the eye to focus. This \emph{can} make text
    harder to  read.
  \item
    Printing such slides is difficult at best.
  \end{itemize}

  On the other hand, a light-on-dark presentation often appears to be
  more ``stylish''  than a plain black-on-white one.

  The following \meta{options} may be given:
  \begin{itemize}
  \item \declare{|overlystylish|} installs a background canvas that
    is, in my opinion, way too stylish. But then, I do not want
    to press my taste on other people. When using this option, it is
    probably a very good idea to also use the |lily| color theme.      
  \end{itemize}

  \example The |overlystylish| option together with the |lily| color theme:
  \genericthemeexample{colorthemealbatrossstylish}
\end{colorthemeexample}


\begin{colorthemeexample}{beetle}
  The main ``theme behing this theme'' is to use white and black text
  on gray background. The white text is used for special emphasis, the
  black text for normal text. The ``outer stuff'' like the headline
  and the footline use, however, a bluish color. To change this color,
  change the background of |palette primary|.
  
  Great care must be taken with this theme since both the white/gray
  and the black/gray contrasts are much lower than with other
  themes. Make sure that the contrast is high enough for the actual
  presentation.

  You can change the ``grayish'' background by changing the background
  of |normal text|.
\end{colorthemeexample}

\begin{colorthemeexample}{crane}
  This theme uses the colors of the Lufthansa, whose logo is a
  crane. It is \emph{not} an official theme by that company, however. 
\end{colorthemeexample}

\begin{colorthemeexample}{dove}
  This theme is nearly a black and white theme and useful for creating
  presentations that are easy to print on a black-and-white
  printer. The theme uses grayscale in certain unavoidable cases, but
  never color. It also changes the font of alerted text to boldface.

  When using this theme, you should consider using the class option
  |gray|, which ensures that all colors are converted to
  grayscale. Also consider using the |structurebold| font theme.
\end{colorthemeexample}


\begin{colorthemeexample}{fly}
  This theme is the ``consequent'' version  of |beetle| and uses
  white/black/gray throughout. It does not go particularly well with
  themes that use shadows.
\end{colorthemeexample}

\begin{colorthemeexample}{seagull}
  Like the |dove| color theme, this theme is useful for printing on a
  black-and-white printer. However, it uses different shades of gray
  extensively, which may or may not look good on a transparency.
\end{colorthemeexample}


\subsubsection{Block Color Themes}

Block themes only specify the colors of block environments. They can
be used together with other (color) themes. If they are used to change the
block colors installed by a presentation theme or another color theme,
they should obviously be specified \emph{after} the other theme has
been loaded. Block themes happen to have flower names.

\begin{colorthemeexample}{lily}
  This theme is mainly used to \emph{uninstall} any block colors setup
  by another theme, restoring the colors used in the |default|
  theme. In particular, using this theme will remove all background
  colors for blocks.
\end{colorthemeexample}

\begin{colorthemeexample}{orchid}
  This theme installs white-on-dark block titles. The background of
  the title of a normal block is set to the foreground of the
  structure color, the foreground is set to white. The background of
  alerted blocks are set to red and of example blocks to green. The
  body of blocks get a nearly transparent background.
\end{colorthemeexample}

\begin{colorthemeexample}{rose}
  This theme installs nearly transparent backgrounds for both block
  titles and block bodies. This theme is much less ``aggressive'' than
  the |orchid| theme. The background colors are derived from the
  foreground of the structure \beamer-color.
\end{colorthemeexample}


\subsubsection{Palette Color Themes}

A palette color theme  changes the palette colors, on which the colors
used in the headline, footline, and sidebar 
are based by default. Palette color themes do not change block
titles. They have happen to sea-animal names.

\begin{colorthemeexample}{whale}
  Installs a white-on-dark palette for the headline, footline, and
  sidebar. The backgrounds used there are set to shades between the
  structure \beamer-color and black. The foreground is set to
  white.

  While this color theme can appear to be agressive, you should note
  that a dark bar at the border of a frame will have a somewhat
  different appearance during a presentation than it has on paper:
  During a presentation the projection on the 
  wall is usually surrounded by blackness. Thus, a dark bar will
  not create a contrast as opposed to the way it does on
  paper. Indeed, using this theme will cause the main part of the
  frame to be more at the focus of attention.

  The counterpart to the theme with respect to blocks is the |orchid|
  theme. However, pairing it with the |rose| color theme is also
  interesting. 
\end{colorthemeexample}

\begin{colorthemeexample}{seahorse}
  Installs a near-transparent backgrounds for the headline, footline,
  and sidebar. Using this theme will cause navigational elements to be
  much less ``dominant'' than when using the |whale| theme (see the
  dicussion on contrast there, though).

  It goes well with the |rose| or the |lily| color theme. Pairing it
  with the |orchid| overemphasizes blocks (in my opinion).
\end{colorthemeexample}




\subsection{Changing the Colors Used for Different Elements of a Presentation}

This section explains how \beamer's color management works.



\subsubsection{Overview of Beamer's Color Management}

In \beamer's philosophy, every element of a presentation can have a
different color. Unfortunately, it turned out that simply assigning a
single color to every element of a presentation is not such a good
idea. First of all, we sometimes  want colors of elements to change
during a presentation, like the color of the item idicators when they
become alerted or inside an example block. Second, some elements
naturally have two colors, namely a foreground and a background, but
not always. Third, sometimes element somehow should not have any
special color but should simply ``run along'' with the color of their
surrounding. Finally, giving a special color to every element makes it
very hard to globally change colors (like changing all the different
kind-of-blue things into kind-of-red things) and it makes later
extensions even harder.

For all these reasons, the color of an element in \beamer\ is a
structured object, which I call a \emph{\beamer-color}. Every
\beamer-color has two parts: a foreground and a background. Either of
these may be ``empty,'' which means that whatever foreground or
background was active before should remain active when the color is
used.

\beamer-colors can \emph{inherit} from other \beamer-colors and the
default themes make extensive use of this feature. For example, their
is a \beamer-color called |structure| and all sorts of elements
inherit from this color. Thus, if someone changes |structure|, the
color of all these elements automatically change accordingly. When a
color inherits from another color, it can nevertheless still override
only the foreground or the background.

It is also possible to ``inherit'' from another \beamer-color in a more
sophisticated way, which is more like \emph{using} the other
\beamer-color in an indirect way. You can specify that, say, the
background of the title should be a 90\% of the background of normal
text and 10\% of the foreground of |structure|.

Inheritance and using of other \beamer-colors is done
dynamically. This means that if one of the parent \beamer-colors
changes during the presentation, the derived colors automatically also
change.

The default color theme, which is always loaded, installs numerous
\beamer-colors and inheritance relations between them. These colors
are explained throughout this guide. The color used for, say,
frametitles is discussed in the section on frametitles, and so on.  


\subsubsection{Using Beamer's Colors}

A \beamer-color is not a normal color as defined by the |color| and
|xcolor| packages and, accordingly, cannot be used directly as in
commands like |\color| or |\colorlet|. Instead, in order to use a
\beamer-color, you should first call the command |\usebeamercolor|,
which is explained below. This command will setup two (normal) colors
called |fg| (for foreground) and |bg| (for, well, guess what). You can
then say |\color{fg}| the install the foreground color and
|\color{bg}| to install the background color. You can also use the
colors |fg| and |bg| in any context in which you normally use a color
like, say, |red|. If a \beamer-color does not have a foreground or a
background, the colors |fg| or |bg| (or both) remain unchanged. 

Inside templates, this command will typically have already been called
for you with the option |[fg]|. 

\begin{command}{\usebeamercolor\opt{|*|}\oarg{fg or
      bg}\marg{beamer-color name}}
  This command (possibly) changes the two colors |fg| and |bg| to the
  foreground and background color of the \meta{beamer-color name}. If
  the \beamer-color does not specify a foreground, |fg| is left
  unchanged; if does not specify a background, |bg| is left
  unchanged. 

  You will often wish to directly use the color |fg| or |bg| after
  using this command. For this common situation, the optional argument
  \meta{fg or bg} is useful, which may be either |fg| or
  |bg|. Giving this option will cause the foreground |fg| or the
  background |bg| to be immediately installed after they have been
  setup. Thus, the following command
\begin{verbatim}
\usebeamercolor[fg]{normal text}
\end{verbatim}
  is a shortcut for
\begin{verbatim}
\usebeamercolor{normal text}
\color{fg}
\end{verbatim}

  If you use the starred version of this command, the \beamer-color
  |normal text| is used before the command is invoked. This ensures
  that, barring evil trickery, the colors |fg| and |bg| will be setup
  independently of whatever colors happened to be in use when the
  command is invoked.

  This command has special side-effects. First, the (normal) color
  |parent.bg| is set to the value of |bg| prior to this call. Thus you
  can access the color that was in use prior to the call of this
  command via the color |parent.bg|.

  Second, the special color \meta{beamer-color name}|.fg| is \emph{globally}
  set to the same value as |fg| and \meta{beamer-color name}|.bg| is
  globally set to the value of |bg|. This allows you to access the
  foreground or background of a certain \meta{beamer-color name} after
  another \beamer-color has been used. However, referring to these
  special global colors should be kept to the unavoidable minimum and
  should be done as locally as possible since a change of the
  \beamer-color will not reflect in a change of the colors
  \meta{beamer-color name}|.fg| and \meta{beamer-color name}|.bg|
  until the next invocation of |\usebeamercolor|. Also, if the
  \meta{beamer-color name} does not specify a foreground or a
  background color, then the values of the special colors are whatever
  happened to be the foreground or background at the time of the last
  invocation of |\usebeamercolor|.

  So, try not to get into the habit of writing |\color{structure.fg}|
  all the time, at least not without a |\usebeamercolor{structure}|
  close by.

  \example
\begin{verbatim}
  This text is {\usebeamercolor[fg]{alerted text} alerted}. The
  following box uses the fore- and background of frametitles:
  {
    \usebeamercolor[fg]{frametitle}
    \colorbox{bg}{Frame Title}
  }
\end{verbatim}
  
  \articlenote
  This command has no effect in |article| mode.
\end{command}


\begin{command}{\ifbeamercolorempty\oarg{fg or bg}\marg{beamer-color
      name}\marg{if undefined}\marg{if defined}}
  This command can be used to check whether the foreground or
  background of some \meta{beamer-color name} is non-empty. If the
  foreground or background of \meta{beamer-color name} is defined,
  \meta{if defined} will be executed, otherwise the \meta{if undefined} code.

  \example
\begin{verbatim}
\ifbeamercolorempty[bg]{frametitle}
{ % ``Transparent background''
  \usebeamercolor[fg]{frametitle}
  \insertframetitle
}
{ % Opaque background
  \usebeamercolor[fg]{frametitle}
  \colorbox{bg}{\insertframetitle}
}
\end{verbatim}
\end{command}




\subsubsection{Setting Beamer's Colors}

To set or to change a \beamer-color, you can use the command
|\setbeamercolor|.

\begin{command}{\setbeamercolor\opt{|*|}\marg{beamer-color name}\marg{options}}
  Sets or changes a \beamer-color. The \meta{beamer-color name} should
  be a reasonably simple text (do not try too much trickery and avoid
  punctuation symbols), but it may contain spaces. Thus, |normal text|
  is a valid \meta{beamer-color name} and so is |My Color Number 2|.

  In the most simple case, you just specify a foreground by giving the
  |fg=| option and, possibly, also a background using the |bg=|
  option.

  \example |\setbeamercolor{normal text}{fg=black,bg=mylightgrey}|
  \example |\setbeamercolor{alerted text}{fg=red!80!black}|
  
  The effect of this command is accumulative, thus the following two
  commands
\begin{verbatim}
\setbeamercolor{section in toc}{fg=blue}
\setbeamercolor{section in toc}{bg=white}
\end{verbatim}
  have the same effect as 
\begin{verbatim}
\setbeamercolor{section in toc}{fg=blue,bg=white}
\end{verbatim}
  Naturally, a second call with the same kind of \meta{option} set to
  a different value overrides a previous call.

  The starred version first resets everything, thereby ``switching
  off'' the accumulative effect. Use this starred version to
  completely reset the definition of some \beamer-color.

  The following \meta{options} may be given:
  \begin{itemize}
  \item \declare{|fg=|\meta{color}} sets the foreground color of
    \meta{beamer-color name} to the given (normal)   \meta{color}. The
    \meta{color} may also be a color expression like |red!50!black|,
    see the manual of the \textsc{xcolor} package. If \meta{color}
    is empty, the \meta{beamer-color name} ``has no special
    foreground'' and when the color is used, the foreground
    currently in force should not be changed.

    Specifying a foreground this way will override any inherited
    foreground color.
  \item  \declare{|bg=|\meta{color}} does the same as the |fg|
    option, but for the background.
  \item \declare{|parent=|\meta{parent beamer-color(s)}} specifies
    that \meta{beamer-color name} should inherit from the specified
    \meta{parent beamer-color(s)}. Any foreground and/or background
    color set by the parents will also be used when
    \meta{beamer-color name} is used. If multiple parents specify
    a foreground, the last one ``wins''; and likewise for the
    backgrounds.

    \example
\begin{verbatim}
\setbeamercolor{father}{fg=red}
\setbeamercolor{mother}{bg=green}
\setbeamercolor{child}{parent={father,mother}}
\begin{beamercolorbox}{child}
  Terrible red on green text.
\end{beamercolorbox}

\setbeamercolor{father}{fg=blue}
\begin{beamercolorbox}{child}
  Now terrible blue on green text, since parent was changed.
\end{beamercolorbox}
\end{verbatim}

    Note that a change of the foreground or background of a parent
    changes the corresponding foreground or background of the child
    (unless it is overruled).

    A \beamer-color cannot only have parents, but also grandparents
    and so on.
  \item \declare{|use=|\meta{another beamer-color}} is used to
    make sure that another \beamer-color is setup correctly before the
    foreground or background color specification are evaluated.

    Suppose you wish the foreground of items to be a mixture
    of 50\% of the foreground of structural elements and 50\% of the
    normal foreground color. You could try
\begin{verbatim}
\setbeamercolor{item}{fg=structure.fg!50!normal text.fg}
\end{verbatim}
    However, this will not necessarily give the desired result: If
    the \beamer-color |structure| changes, the (normal) color
    |structure.fg| is not immediately updated. In order to ensure
    that the normal color |structure.fg| is correct, use the
    following:
\begin{verbatim}
\setbeamercolor{item}{use={structure,normal text},fg=structure.fg!50!normal text.fg}
\end{verbatim}
    This will guarantee that the colors |structure.fg| and
    |normal text.fg| are setup correctly when the foreground of
    |item| is computed.

    To show the difference, consider the following example:
\begin{verbatim}
\setbeamercolor{grandfather}{fg=red}
\setbeamercolor{grandmother}{bg=white}
\setbeamercolor{father}{parent={grandfather,grandmother}}
\setbeamercolor{mother}{fg=black}
{
  \usebeamercolor{father}\usebeamercolor{mother}
  %% Defines father.fg and mother.fg globally
}
\setbeamercolor{my color A}{fg=father.fg!50!mother.fg}
\setbeamercolor{my color B}{use={father,mother},fg=father.fg!50!mother.fg}

{\usebeamercolor[fg]{my color A} dark red text}
{\usebeamercolor[fg]{my color b} also dark red text}

\setbeamercolor{grandfather}{fg=green}

{\usebeamercolor[fg]{my color A} still dark red text}
{\usebeamercolor[fg]{my color b} now dark green text}
\end{verbatim}
  \end{itemize}
\end{command}



  
\subsection{The Color of Mathematical Text}

By default, mathematical text does not have any special color---it
just inherits the ``surrounding'' color. Some people prefer
mathematical text to have some special color. Though I do not
recommend this (I think mathematical text should \emph{not} stand out
admit the normal text), \beamer\ makes it (reasonably) easy to change
the color of mathematical text. Simply change the following colors:

\begin{element}{math text}\no\yes\no
  This color is the parent of |math text inlined| and
  |math text displayed|. It is empty by default. See those colors for
  details. 
\end{element}

\begin{element}{math text inlined}\no\yes\no
  \colorparents{math text}
  If the foreground of this color is set, inlined mathematical text is
  typeset using this color. This is done via some |\everymath| hackery
  and may not work in all cases. If not, you'll have to try to find a
  way around the problem. The background is currently
  ignored. 
\end{element}

\begin{element}{math text displayed}\no\yes\no
  \colorparents{math text}
  Like |math text inlined|, only for so-called ``displayed''
  mathematical text. This is mathematical text between |\[| and |\]| or
  between |$$| and |$$| or inside environments like |equation| or
  |align|. The setup of this color is somewhat fragile, use at your
  own risk. The background is currently
  ignored.   
\end{element}

\begin{element}{normal text in math text}\no\yes\no
  If the foreground of this color is set, normal text inside
  mathematical text (which is introduced using the |\text| command)
  will be typeset using this color. The background is currently
  ignored. 
\end{element}



\subsection{The Color Palettes}

When one designs a color theme, one faces the following problem:
Suppose we want the colors in the headline to gradually change from
black to, say, blue. Whatever is at the very top of the headline
should be black, what comes right below it should be dark blue, and at
the bottom of the headline things should just be blue. Unfortunately,
different outer themes will put different things at the top. One theme
might put the author at the top, another theme might put the document
title there. This makes it impossible to directly assign one of the
three colors ``black'', ``dark blue,'' and ``blue'' to the different
elements that are typically rendered in the headline. No matter how we
assign them, things will look wrong for certain outer themes.

To circumvent this problem, \beamer\ uses a layer of \emph{palette
colors}. Color themes typically only change these palette colors. For
example, a color theme might make the \beamer-color |palette primary|
blue, make |palette secondary| a dark blue, and make |palette ternary|
black. Outer themes can now setup things such that whatever they show
at the top of the headline inherits from |palette ternary|, what comes
below inherits from |palette secondary|, and whatever is at the bottom
inherits from |palette ternary|. This way, color themes can change the
way even complicated outer themes look and they can do so
consistently.

Note that the user can still change the color of every element
individually, simply by overriding the color(s) of the elements in the
headline. In a sense, the palette colors are just a ``suggestion'' how
things should be colored by an outer theme.

In detail, the following palette colors are used by outer themes.


\begin{element}{palette primary}\no\yes\no
  Outer themes (should) base the color of navigational elements and,
  possibly, also of other elements, on the four palette colors. The
  ``primary'' palette should be used for the most important
  navigational elements, which are usually the ones that change most
  often and hence require the most attention by the audience. The
  ``secondary'' and ``ternary'' are less important, the ``fourth'' one
  is least important.

  By default, the palette colors do not have a background and the
  foreground ranges from |structure.fg| to |black|.

  For the sidebar, there is an extra set of palette colors, see
  |palette sidebar primary|. 
\end{element}

\begin{element}{palette secondary}\no\yes\no
  See |palette primary|.
\end{element}

\begin{element}{palette ternary}\no\yes\no
  See |palette primary|.
\end{element}

\begin{element}{palette fourth}\no\yes\no
  See |palette primary|.
\end{element}

\begin{element}{palette sidebar primary}\no\yes\no
  Similar to |palette primary|, only outer themes (should) base the
  colors of elements in the sidebar of the four sidebar palette
  colors.
\end{element}

\begin{element}{palette sidebar secondary}\no\yes\no
  See |palette sidebar primary|.
\end{element}

\begin{element}{palette sidebar ternary}\no\yes\no
  See |palette sidebar primary|.
\end{element}

\begin{element}{palette sidebar fourth}\no\yes\no
  See |palette sidebar primary|.
\end{element}



\subsection{Miscellaneous  Colors}

In this section some ``basic'' colors are listed that do not
``belong'' to any special commands. 

\begin{element}{normal text}\no\yes\yes
  The color is used for normal text. At the beginning of
  the document the foreground color is installed as
  |\normalcolor|. The background of this color is used by the
  default background canvas for the background of the
  presentation, see Section~\ref{section-canvas}. The background is
  also the default value of the normal color |bg|. 

  Since the color is the ``root'' of all other \beamer-colors, both a
  foreground and a background must be installed. In particular, to get
  a transparent background canvas, make the background of the
  \beamer-color |background canvas| empty, not the background of this
  color.

  The \beamer-font currently is not used. In particular, redefining this
  font will not have any effect. This is likely to change in the future.
\end{element}

\begin{element}{example text}\no\yes\yes
  The color/font is used when text is typeset inside an |example|
  block. 
\end{element}

\begin{element}{titlelike}\no\yes\yes
  This color/font is a more specialized form of the |structure|
  color/font. It is the base for all elements that are ``like
  titles.'' This includes the frame title and subtitle as well as the
  document title and subtitle.
\end{element}

\begin{element}{separation line}\no\yes\no
  The foreground of this color is used for separating lines. If the
  foreground is empty, no separation line is drawn.
\end{element}

\begin{element}{upper separation line head}\no\yes\no
  \colorparents{separation line}
  Special case for the uppermost separation line in a headline.
\end{element}

\begin{element}{middle separation line head}\no\yes\no
  \colorparents{separation line}
  Special case for the middle separation line in a headline.
\end{element}

\begin{element}{lower separation line head}\no\yes\no
  \colorparents{separation line}
  Special case for the lower separation line in a headline.
\end{element}

\begin{element}{upper separation line foot}\no\yes\no
  \colorparents{separation line}
  Special case for the uppermost separation line in a footline.
\end{element}

\begin{element}{middle separation line foot}\no\yes\no
  \colorparents{separation line}
  Special case for the middle separation line in a footline.
\end{element}

\begin{element}{lower separation line foot}\no\yes\no
  \colorparents{separation line}
  Special case for the lower separation line in a footline.
\end{element}




\subsection{Transparency Effects}
\label{section-transparent}

By default, \emph{covered} items are not shown during a
presentation. Thus if you write |\uncover<2>{Text.}|, the text
is not shown on any but the second slide. On the other slide, the text
is not simply printed using the background color -- it is not shown at
all. This effect is most useful if your background does not have a
uniform color.

Sometimes however, you might prefer that covered items are not
completely covered. Rather, you would like them to be shown already in
a very dim or shaded way. This allows your audience to get a feeling
for what is yet to come, without getting distracted by it. Also, you
might wish text that is covered ``once more'' still to be visible to
some degree.

Ideally, there would be an option to make covered text
``transparent.'' This would mean that when covered text is shown, it
would instead be mixed with the background behind it. Unfortunately,
|pgf| does not support real transparency yet. Instead, transparency is
created by mixing the color of the object you want to show with the
current background color (the color |bg|, which has hopefully been
setup such that it is the average color of the background on which the
object should be placed). To install this effect, you can use:
\begin{verbatim}
\setbeamercovered{transparent}
\end{verbatim}
This commands allows you to specify in a quite general way how a
covered item should be rendered. You can even specify different ways
of rendering the item depending on how long it will take before this
item is shown or for how long it has already been covered once
more. The transparency effect will automatically apply to all colors,
\emph{except} for the colors in images. For images there is a
workaround, see the documentation of the \pgf\ package. 

\begin{command}{\setbeamercovered\marg{options}}
  This command offers several different options, the most important of
  which is |transparent|. All options are internally mapped to the two
  options |still covered| and |again covered|.

  In detail, the following \meta{options} may be given:
  \begin{itemize}
  \item \declare{|invisible|} is the default and causes covered text
    to ``completely disappear.
  \item \declare{|transparent|}\opt{|=|\meta{opaqueness}} causes
    covered text to be typset in a ``transparent'' way. By default,
    this means that 85\% of the background color is mixed into all
    colors or that the \meta{opaqueness} of the text is 15\%. You can
    specify a different \meta{percentage}, where |0| means ``totally
    transparent'' and |100| means ``totally opaque.''

    Unfortunately, this value is kind of ``specific'' to every
    projector. What looks good on your screen need not look good
    during a presentation.
  \item \declare{|dynamic|} Makes all covered text quite transparent,
    but in a dynamic way. The longer it will take till the text is
    uncovered, the stronger the transparency. 
  \item \declare{|highly dynamic|} Has the same effect as |dynamic|,
    but the effect is stronger.
  \item \declare{|still covered=|\meta{not yet list}} specifies  how
    to render covered items that have not yet been uncovered. The
    \meta{not yet lest} should be a list of |\opaqueness| commands,
    see the description of that command, below.
    \example
\begin{verbatim}
\setbeamercovered{%
  still covered={\opaqueness<1>{15}\opaqueness<2>{10}\opaqueness<3>{5}\opaqueness<4->{2}},
  again covered={\opaqueness<1->{15}}}
\end{verbatim}
  \item \declare{|again covered=|\meta{once more list}} specifies how
    to render covered items that have once more been covered, that is,
    that had been shown before but are now covered again.
  \end{itemize}
\end{command}

\begin{command}{\opaqueness\ssarg{overlay
      specification}\marg{percentage of opaqueness}}
  The \meta{overlay specification} specifies on which slides covered
  text should have which \meta{percentage of opaqueness}. Unlike
  other overlay specifications, this \meta{overlay specification} is a
  ``relative'' overlay specification. For example, the specification
  ``3'' here means ``things that will be uncovered three slides
  ahead,'' respectively ``things that have once more been covered for
  three slides.'' More precisely, if an item is uncovered for more
  than one slide and then covered once more, only the ``first moment
  of uncovering'' is used for the calculation of how long the item has
  been covered once more.

  An opaqueness of 100 is fully opaque and 0 is fully
  transparent. Currently, since real transparency is not yet
  implemented, this command causes all colors to get a mixing of
  \meta{percentage of opaqueness} of the current
  |bg|. At some future point this command might result in real
  transparency. 

  The alternate \pgf\ extension used inside an opaque area is
  \meta{percentage of opaqueness}|opaque|. In case of nested calls,
  only the innermost opaqueness specification is used. 
  \example
\begin{verbatim}
\setbeamercovered{still covered={\opaqueness<1->{15}},again covered={\opaqueness<1->{15}}}
\pgfdeclareimage{book}{book}
\pgfdeclareimage{book.!15opaque}{filenameforbooknearlytransparent}
\end{verbatim}
  Makes everything that is uncovered in two slides only 15 percent
  opaque. 
\end{command}





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