mirror of https://github.com/texmacs/doc.git
259 lines
12 KiB
Tcl
259 lines
12 KiB
Tcl
<TeXmacs|1.99.8>
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<style|<tuple|tmdoc|english|old-spacing>>
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<\body>
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<tmdoc-title|<TeXmacs> fonts>
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<section|Classical conceptions of fonts>
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The way <TeXmacs> handles fonts is quite different from classical text
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editors and even from <TeX>. Let us first analyze some classical ways of
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conceiving fonts.
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<\itemize>
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<item>Physical fonts are just given by the name of a file, which contains
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a character set, i.e. a list of bitmaps. Usually the size of a character
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set is limited by 256 (or 65536).
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<item>True type fonts essentially work in the same way, except that the
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bitmaps can now be computed for any desired size.
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<item>In the X-window system, the name of the font is replaced by a more
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systematic name, which explicitly contains a certain number of font
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parameters, such as its size, series and shape. This makes it easier for
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applications to select an appropriate font. However, character sets are
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still limited in size.
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<item>In <TeX>, symbols are seen as commands, which select an appropriate
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physical font (which corresponds to a <verbatim|.tfm> and a
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<verbatim|.pk> file), based on symbol font declarations and environment
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variables (such as size, series and shape).
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</itemize>
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Clearly, among all these methods, <TeX> provides the largest flexibility.
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However, philosophically speaking, we think that it also has some
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drawbacks:
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<\itemize>
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<item>There is no distinction between usual commands and commands to make
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symbols: the current time might be considered as a symbol.
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<item>The encoding of the font is fixed by the names of the commands. For
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instance, for mathematical symbols, no clean general encoding scheme is
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provided, except the default naming of symbols by commands.
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<item>For beginners, it remains extremely hard to use non standard fonts.
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</itemize>
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Actually, in <TeX>, the notion of \Pthe current font\Q is ill-defined: it
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is merely the superposition of all character generating commands.
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<section|The conception of a font in TeXmacs>
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Philosophically speaking, we think that a font should be characterized by
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the following two essential properties:
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<\enumerate>
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<item>A font associates graphical meanings to
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<with|font-shape|italic|words>. The words can always be represented by
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strings.
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<item>The way this association takes place is coherent as a function of
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the word.
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</enumerate>
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By a word, we either mean a word in a natural language, or a sequence of
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mathematical, technical or artistic symbols.
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This way of viewing fonts has several advantages:
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<\enumerate>
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<item>A font may take care of kerning and ligatures.
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<item>A font may consist of several \Pphysical fonts\Q, which are somehow
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merged together.
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<item>A font might in principle automatically build very complicated
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glyphs like hieroglyphs or large delimiters from words in a well chosen
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encoding.
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<item>A font is an irreducible and persistent entity, not a bunch of
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commands whose actions may depend on some environment.
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</enumerate>
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Notice finally that the \Pgraphical meaning\Q of a word might be more than
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just a bitmap: it might also contain some information about a logical
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bounding box, appropriate places for scripts, etc. Similarly, the
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\Pcoherence of the association\Q should be interpreted in its broadest
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sense: the font might contain additional information for the global
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typesetting of the words on a page, like the recommended distance between
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lines, the height of a fraction bar, etc.
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<section|String encodings>
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All text strings in <TeXmacs> consist of sequences of either specific or
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universal symbols. A specific symbol is a character, different from
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<verbatim|'\\0'>, <verbatim|'\<less\>'> and <verbatim|'\<gtr\>'>. Its
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meaning may depend on the particular font which is being used. A universal
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symbol is a string starting with <verbatim|'\<less\>'>, followed by an
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arbitrary sequence of characters different from <verbatim|'\\0'>,
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<verbatim|'\<less\>'> and <verbatim|'\<gtr\>'>, and ending with
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<verbatim|'\<gtr\>'>. The meaning of universal characters does not depend
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on the particular font which is used, but different fonts may render them
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in a different way.
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Universal symbols can also be used to represent mathematical symbols of
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variable sizes like large brackets. The point here is that the shapes of
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such symbols depend on certain size parameters, which can not conveniently
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be thought of as font parameters. This problem is solved by letting the
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extra parameters be part of the symbol. For instance,
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<verbatim|"\<less\>left-(-1\<gtr\>"> would be usual bracket and
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<verbatim|"\<less\>left-(-2\<gtr\>"> a slightly larger one.
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<section|The abstract font class>
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The main abstract <verbatim|font> class is defined in <verbatim|font.hpp>:\
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<\verbatim>
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\ \ \ \ struct font_rep: rep\<less\>font\<gtr\> {<next-line>
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\ \ \ \ \ display \ dis; \ \ \ \ \ \ \ \ \ \ \ \ \ // underlying
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display<next-line> \ \ \ \ \ encoding enc; \ \ \ \ \ \ \ \ \ \ \ \ \ //
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underlying encoding of the font<next-line> \ \ \ \ \ SI
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\ \ \ \ \ \ design_size; \ \ \ \ \ // design size in
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points/256<next-line> \ \ \ \ \ SI \ \ \ \ \ \ display_size; \ \ \ \ //
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display size in points/PIXEL<next-line> \ \ \ \ \ double \ \ slope;
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\ \ \ \ \ \ \ \ \ \ \ // italic slope<next-line> \ \ \ \ \ space
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\ \ \ spc; \ \ \ \ \ \ \ \ \ \ \ \ \ // usual space between
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words<next-line> \ \ \ \ \ space \ \ \ extra; \ \ \ \ \ \ \ \ \ \ \ //
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extra space at end of words<next-line><next-line> \ \ \ \ \ SI
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\ \ \ \ \ \ y1; \ \ \ \ \ \ \ \ \ \ \ \ \ \ // bottom y
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position<next-line> \ \ \ \ \ SI \ \ \ \ \ \ y2;
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\ \ \ \ \ \ \ \ \ \ \ \ \ \ // top y position<next-line> \ \ \ \ \ SI
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\ \ \ \ \ \ yfrac; \ \ \ \ \ \ \ \ \ \ \ // vertical position fraction
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bar<next-line> \ \ \ \ \ SI \ \ \ \ \ \ ysub; \ \ \ \ \ \ \ \ \ \ \ \ //
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base line for subscripts<next-line> \ \ \ \ \ SI \ \ \ \ \ \ ysup;
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\ \ \ \ \ \ \ \ \ \ \ \ // base line for
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superscripts<next-line><next-line> \ \ \ \ \ SI \ \ \ \ \ \ wpt;
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\ \ \ \ \ \ \ \ \ \ \ \ \ // width of one point in font<next-line>
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\ \ \ \ \ SI \ \ \ \ \ \ wquad; \ \ \ \ \ \ \ \ \ \ \ // wpt * design
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size in points<next-line> \ \ \ \ \ SI \ \ \ \ \ \ wunit;
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\ \ \ \ \ \ \ \ \ \ \ // unit width for extendable fonts<next-line>
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\ \ \ \ \ SI \ \ \ \ \ \ wfrac; \ \ \ \ \ \ \ \ \ \ \ // width fraction
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bar<next-line> \ \ \ \ \ SI \ \ \ \ \ \ wsqrt; \ \ \ \ \ \ \ \ \ \ \ //
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width horzontal line in square root<next-line> \ \ \ \ \ SI
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\ \ \ \ \ \ wneg; \ \ \ \ \ \ \ \ \ \ \ \ // width of negation
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line<next-line><next-line> \ \ \ \ \ font_rep (display dis, string
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name);<next-line> \ \ \ \ \ font_rep (display dis, string name, font
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fn);<next-line> \ \ \ \ \ void copy_math_pars (font
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fn);<next-line><next-line> \ \ \ \ \ virtual void \ \ get_extents (string
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s, text_extents& ex) = 0;<next-line> \ \ \ \ \ virtual void \ \ draw
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(ps_device dev, string s, SI x, SI y) = 0;<next-line><next-line>
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\ \ \ \ \ virtual SI \ \ \ \ get_sub_base (string s);<next-line>
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\ \ \ \ \ virtual SI \ \ \ \ get_sup_base (string s);<next-line>
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\ \ \ \ \ virtual double get_left_slope \ (string s);<next-line>
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\ \ \ \ \ virtual double get_right_slope (string s);<next-line>
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\ \ \ \ \ virtual SI \ \ \ \ get_left_correction \ (string s);<next-line>
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\ \ \ \ \ virtual SI \ \ \ \ get_right_correction (string s);<next-line>
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\ \ \ \ \ virtual SI \ \ \ \ get_lsub_correction (string s, double
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level);<next-line> \ \ \ \ \ virtual SI \ \ \ \ get_lsup_correction
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(string s, double level);<next-line> \ \ \ \ \ virtual SI
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\ \ \ \ get_rsub_correction (string s, double level);<next-line>
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\ \ \ \ \ virtual SI \ \ \ \ get_rsup_correction (string s, double
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level);<next-line><next-line> \ \ \ \ void var_get_extents (string s,
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text_extents& ex);<next-line> \ \ \ \ void var_draw (ps_device dev,
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string s, SI x, SI y);<next-line> \ \ \ \ virtual bitmap_char get_bitmap
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(string s);<next-line> \ \ };
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</verbatim>
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The main abstract routines are <verbatim|get_extents> and <verbatim|draw>.
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The first routine determines the logical and physical bounding boxes of a
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graphical representation of a word, the second one draws the string on the
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the screen.
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The additional data are used for global typesetting using the font. The
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other virtual routines are used for determining additional properties of
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typeset strings.
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<section|Implementation of concrete fonts>
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Several types of concrete fonts have been implemented in <TeXmacs>:
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<\description>
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<item*|TeX text fonts>See <verbatim|src/Resource/Fonts/tex_font.cc>.
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<item*|TeX rubber fonts>See <verbatim|src/Resource/Fonts/tex_rubber_font.cpp>.
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<item*|X fonts>See <verbatim|src/Resource/Fonts/ps_font.cpp>.
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<item*|Mathematical fonts>See <verbatim|src/Resource/Fonts/math_font.cpp>.
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<item*|Virtual fonts>See <verbatim|src/Resource/Fonts/virtual_font.cpp>.
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</description>
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In most cases, the lowest layer of the implementation consists of a
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collection of bitmaps, together with some font metric information. The font
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is responsible for putting these bitmaps together on the screen using some
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appropriate spacing. The <verbatim|ps_device> class comes with a method to
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display bitmaps in a nice, anti-aliased way, or to print them out.
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<section|Font selection>
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After having implemented fonts themselves, an important remaining issue is
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the selection of the appropriate font as a function of a certain number of
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parameters, such as its name, series, shape and size. For optimal
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flexibility, <TeXmacs> comes with a powerful macro-based font-selection
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scheme (using the <scheme> syntax), which allows the user to decide which
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parameters should be considered meaningful.
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At the lowest level, we provide a fixed number of macros which directly
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correspond to the above types of concrete fonts. For instance, the macro\
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<\verbatim>
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\ \ \ \ (tex $name $size $dpi)
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</verbatim>
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corresponds to the constructor\
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<\verbatim>
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\ \ \ \ font tex_font (display dis, string fam, int size, int dpi, int
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dsize=10);
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</verbatim>
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of a <TeX> text font.
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At the middle level, it is possible to specify some rewriting rules like\
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<\verbatim>
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\ \ \ \ ((roman rm medium right $s $d) (ec ecrm $s $d))<next-line>
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\ \ \ ((avant-garde rm medium right $s $d) (tex rpagk $s $d
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0))<next-line> \ \ \ ((x-times rm medium right $s $d) (ps
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adobe-times-medium-r-normal $s $d))
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</verbatim>
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When a left hand pattern is matched, it is recursively substituted by the
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right hand side. The files in the directory <verbatim|progs/fonts> contain
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a large number of rewriting rules.
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At the top level, <TeXmacs> calls a macro of the form\
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<\verbatim>
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\ \ \ \ ($name $family $series $shape $size $dpi)
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</verbatim>
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as a function of the current environment in the text. In the future, the
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top level macro call might change in order to enable the user to let the
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font depend on other environment variables.
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<tmdoc-copyright|1998--2002|Joris van der Hoeven>
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<tmdoc-license|Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License, Version 1.1
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or any later version published by the Free Software Foundation; with no
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Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled "GNU Free
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Documentation License".>
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</body>
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<initial|<\collection>
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</collection>> |