\section{Specifying fonts}			\label{sec:font}

\product{}'s font specification is a two-stage process.  In the first stage,
\product{} physical fonts are mapped to fonts of the underlying windowing
system. In this stage, fonts are identified by their {\em family},
{\em style} and {\em size}.  For example

\begin{code}
font(screen, roman, 13)
\end{code}

Refers to a fixed-width font designed for use on the screen that has
normal weight, not slanted and 13-pixels high characters.

In the second stage, logical font-names are mapped to their physical
implementation.  At this level, fonts are identified by a single name
from an extensible, but preferably small set.

See \secref{winfont} for a description of Windows specific font issues.


\subsection{Physical fonts}

The default physical font set is built when the first font object is
opened (i.e.\ its window counterpart is located and made available to
the text-drawing functions).  This set is created from class-variables on
the display object.  The first class-variable is display.font_families, which
defines a chain with names of the font-families.  The default value is:%
    \footnote{See \secref{classvar} for the default syntax.}

\begin{code}
display.font_families: \
	[ screen_fonts, \
	  courier_fonts, \
	  helvetica_fonts, \
	  times_fonts, \
	  symbol_fonts
	]
\end{code}

Each of these names refers to the name of another resource of class
\class{display}, enumerating the members of this font family.  The
default value can be examined using the online manual.  Below is the
default value for the \const{screen_fonts} font-set for X11:

\begin{code}
display.screen_fonts: \
	[ font(screen, roman, 10, "6x10"), \
	  font(screen, roman, 12, "6x12"), \
	  font(screen, roman, 13, "8x13"), \
	  font(screen, roman, 14, "7x14"), \
	  font(screen, roman, 15, "9x15"), \
	  font(screen, bold, 13, "8x13bold"), \
	  font(screen, bold, 14, "7x14bold"), \
	  font(screen, bold, 15, "9x15bold") \
	]
\end{code}

The set of predefined physical fonts can be examined using the
FontViewer demo application accessible through the online manual tools.


\subsubsection{Defining additional fonts}

If an application needs additional fonts, such fonts can be declared
using directives. The fourth initialisation argument of class
\class{font} determines the window-system font that will be mapped. The
syntax for this argument depends on the window-system used. For this
Unix/X11 version it is a string consisting of 15 `-' separated fields. A
font can be searched using \manref{xfontsel}{1} or the much better
GNOME-project \manref{gfontsel}{1}.

For example, the 14-points `courier new' TrueType font can be registered
using:

\begin{code}
:- initialization
   new(_, font(courier, roman, 14,
	       '-winfonts-courier new-medium-r-normal-*-*-140-*-*-m-*-iso8859-1')).
\end{code}

This specification has various drawbacks. For example, another library
or application loaded on top of the same \product{} process may be using
the symbol,roman,14 specification, but bound to another window-system
font. A user may try to run your application on an environment that does
not have this font. Part of these problems can be eliminated by binding
the font to a logical font name. See also \secref{fontalias}.

\begin{code}
:- initialization
   send(@display, font_alias,
	adobesymbol,
	font(symbol, roman, 14,
	     '-*-symbol-*-*-*-*-14-*-*-*-*-*-adobe-*')).
\end{code}

The application will refer to this font using the font-alias.
user has other preferences or the font is not available, the user may
specify the font using the display.user_fonts class-variable described
in \secref{fontalias}.


\subsection{Logical fonts}		\label{sec:fontalias}.

It is not wise let your application code speak about physical fonts as
the user or interface guidelines may prefer using a different
font-palette. For this reason the display defines a mapping between
logical font names and physical fonts. Applications are encouraged to
use logical font names as much as possible and leave the assignment to
physical fonts to the users preferences.  \product{} predefines the following
logical font-names.  The value gives the default assignment for these
fonts.

\begin{itemize}
\newcommand{\fontalias}[2]{\tick{\makebox[1in][l]{#1}#2}}
    \fontalias{normal}{font(helvetica, roman, 12)}
The default font. Normally a proportional roman font. Should be easy to
read.
    \fontalias{bold}{font(helvetica, bold, 12)}
Bold version of the normal font.
    \fontalias{italic}{font(helvetica, oblique, 12)}
Slanted version of the normal font.  Note that italic fonts should not
be used for long text as italics is generally not easy to read on most
displays.
    \fontalias{small}{font(helvetica, roman, 10)}
Small version of the normal font.  To be used in notes, subscripts, etc.
May not be so easy to read, so avoid using it for long texts.
    \fontalias{large}{font(helvetica, roman, 14)}
Slightly larger version of the normal font.
    \fontalias{boldlarge}{font(helvetica, bold, 14)}
Bold version of large.
    \fontalias{huge}{font(helvetica, roman, 18)}
Even larger font.  To be used for titles, etc.
    \fontalias{boldhuge}{font(helvetica, bold, 18)}
Bold version of huge.
    \fontalias{fixed}{font(screen, roman, 13)}
Terminal font. To be used for code fragments, code editors, etc. Should
be easy to read.
    \fontalias{tt}{font(screen, roman, 13)}
Same as \const{fixed}.
    \fontalias{boldtt}{font(screen, bold, 13)}
Bold terminal font.
    \fontalias{symbol}{font(symbol, roman, 12)}
Symbol font using the adobe symbol-font encoding.  This font provides
many \idx{mathematical symbols}.
\end{itemize}

The end-user of an \product{} application can define the class-variable
display.user_fonts to overrule fonts.  The example below
re-binds the most commonly used fonts to be slightly larger and
choose from the Times font family rather than the Helvetica
fonts.

\begin{code}
display.user_fonts: \
	[ normal := font(times, roman, 14), \
	  bold   := font(times, bold, 14), \
	  italic := font(times, italic, 14) \
	]
\end{code}

The mapping between logical font names and physical fonts is realised
by the methods `display <->font_alias' additional font aliases may
be loaded using `display ->load_font_aliases'.

Class \class{font}'s predefined conversion will translate names to font
objects.  This implies that for any method expecting a font object the
programmer can specify the font-name instead.  In those (rare) cases
where a font needs to be passed, but the type-specification does not
require this, the conversion must be done explicitly.  The preferred
way to make the conversion is using the font type object:

\begin{code}
	...,
	get(type(font), check, bold, BoldFont),
	...,
\end{code}
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