Structure of an Xgridfit Program
An Xgridfit program supplies the control-value table, pre-program, functions and glyph programs for the font. It also controls several parameters relating to TrueType instructions, stored in the font's maxp table. The contents of the file must be well-formed XML, validated against the Xgridfit RELAX NG schema (xgridfit.rnc or xgridfit.rng). Perhaps the easiest way to validate your program is to include the -V option on the Linux command line when you invoke Xgridfit:
$ xgridfit -V myprog.xgf
Another worthwhile tool is nxml-mode, an Emacs mode that validates while you edit your program. To use nxml-mode, copy xgridfit.rnc to the nxml-mode schema directory and add this line to schemas.xml in the same directory:
<documentElement localName="xgridfit" uri="xgridfit.rnc"/>
The first line in any Xgridfit program file must be the XML declaration:
<?xml version="1.0" encoding="UTF-8"?>
Then comes the <xgridfit> element, the root element that contains all other information in the file. The <xgridfit> element must contain the Xgridfit namespace declaration:
<xgridfit xmlns="http://xgridfit.sourceforge.net/Xgridfit2">
The child elements of <xgridfit> can come in any order, though it makes sense to group related elements (e.g. functions, control values) together.
Following are the possible children of the <xgridfit> element.
<glyph-select>
Specifies a list of glyphs to compile, ignoring all others. The glyph names in the list may be delimited by "+" or space--but only one or the other. It is all right to keep this element in the file when you don't need it: just empty it out. These are valid:
<glyph-select>a macron amacron</glyph-select>
<glyph-select></glyph-select>
<infile>
Specifies the name of the FontForge file or TrueType font to be opened by the script generated by Xgridfit.
<outfile>
Specifies the name of the file or font to be saved by the script generated by Xgridfit.
<datafile>
When running in merge-mode, Xgridfit needs to save information about the font. It can do so in the font.persistent object of an .sfd file it is saving or in a separate file. Use this element to specify the name of a separate file.
<constant>
Unlike the <constant> elements defined in a <glyph> program, <constant>s here are visible in all <glyph> programs, in <function>, <macro>, and <pre-program> elements.
<variable>
Like the global <constant>, the <variable> defined here is visible everywhere. It may be initialized by the <pre-program> so as to provide state information to all glyph programs, or it may be used anywhere for temporary storage.
<round-state>
These elements, permitted only here, define custom round states, which can be invoked anywhere.
<default>
These elements define graphics defaults and other defaults and settings.
<control-value>
A <control-value> element defines a control value, a specialized variable used by various instructions to regulate distance on the grid. A typical TrueType font has many <control-value> elements.
<alias>
An alternate name for a control-value, constant or variable.
<function>
A <function> element contains the definition of a function--a chunk of code that can be invoked from anywhere and, like the functions (methods, procedures) of other programming languages, can accept parameters and return values.
<macro>
A <macro> element contains the definition of a macro--a chunk of code that is compiled whenever the compiler encounters a <call-macro> element.
<pre-program>
The <pre-program> contains programming that is run whenever the font is about to be rasterized at a new size. Typical tasks for the <pre-program> are to round and adjust control values and set default values in the graphics state. The <pre-program> element is required, though it may be empty.
<glyph>
The file may contain any number of <glyph> elements. Each contains the programming for a single glyph.