537 lines
24 KiB
XML
537 lines
24 KiB
XML
<?xml version="1.0"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
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"http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
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<!ENTITY % local.common.attrib "xmlns:xi CDATA #FIXED 'http://www.w3.org/2003/XInclude'">
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<!ENTITY version SYSTEM "version.xml">
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]>
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<chapter id="integration">
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<title>Platform Integration Guide</title>
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<para>
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HarfBuzz was first developed for use with the GNOME and GTK
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software stack commonly found in desktop Linux
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distributions. Nevertheless, it can be used on other operating
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systems and platforms, from iOS and macOS to Windows. It can also
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be used with other application frameworks and components, such as
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Android, Qt, or application-specific widget libraries.
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</para>
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<para>
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This chapter will look at how HarfBuzz fits into a typical
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text-rendering pipeline, and will discuss the APIs available to
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integrate HarfBuzz with contemporary Linux, Mac, and Windows
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software. It will also show how HarfBuzz integrates with popular
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external libraries like FreeType and International Components for
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Unicode (ICU) and describe the HarfBuzz language bindings for
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Python.
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</para>
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<para>
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On a GNOME system, HarfBuzz is designed to tie in with several
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other common system libraries. The most common architecture uses
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Pango at the layer directly "above" HarfBuzz; Pango is responsible
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for text segmentation and for ensuring that each input
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<type>hb_buffer_t</type> passed to HarfBuzz for shaping contains
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Unicode code points that share the same segment properties
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(namely, direction, language, and script, but also higher-level
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properties like the active font, font style, color, and so on).
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</para>
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<para>
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The layer directly "below" HarfBuzz is typically FreeType, which
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is used to rasterize glyph outlines at the necessary optical size,
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hinting settings, and pixel resolution. FreeType provides APIs for
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accessing font and face information, so HarfBuzz includes
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functions to create <type>hb_face_t</type> and
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<type>hb_font_t</type> objects directly from FreeType
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objects. HarfBuzz will can use FreeType's built-in functions for
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<structfield>font_funcs</structfield> vtable in an <type>hb_font_t</type>.
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</para>
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<para>
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FreeType's output is bitmaps of the rasterized glyphs; on a
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typical Linux system these will then be drawn by a graphics
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library like Cairo, but those details are beyond HarfBuzz's
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control. On the other hand, at the top end of the stack, Pango is
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part of the larger GNOME framework, and HarfBuzz does include APIs
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for working with key components of GNOME's higher-level libraries
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— most notably GLib.
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</para>
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<para>
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For other operating systems or application frameworks, the
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critical integration points are where HarfBuzz gets font and face
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information about the font used for shaping and where HarfBuzz
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gets Unicode data about the input-buffer code points.
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</para>
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<para>
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The font and face information is necessary for text shaping
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because HarfBuzz needs to retrieve the glyph indices for
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particular code points, and to know the extents and advances of
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glyphs. Note that, in an OpenType variable font, both of those
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types of information can change with different variation-axis
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settings.
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</para>
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<para>
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The Unicode information is necessary for shaping because the
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properties of a code point (such as it's General Category (gc),
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Canonical Combining Class (ccc), and decomposition) can directly
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impact the shaping moves that HarfBuzz performs.
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</para>
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<section id="integration-glib">
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<title>GNOME integration, GLib, and GObject</title>
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<para>
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As mentioned in the preceding section, HarfBuzz offers
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integration APIs to help client programs using the
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GNOME and GTK framework commonly found in desktop Linux
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distributions.
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</para>
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<para>
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GLib is the main utility library for GNOME applications. It
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provides basic data types and conversions, file abstractions,
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string manipulation, and macros, as well as facilities like
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memory allocation and the main event loop.
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</para>
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<para>
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Where text shaping is concerned, GLib provides several utilities
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that HarfBuzz can take advantage of, including a set of
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Unicode-data functions and a data type for script
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information. Both are useful when working with HarfBuzz
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buffers. To make use of them, you will need to include the
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<filename>hb-glib.h</filename> header file.
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</para>
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<para>
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GLib's <ulink
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url="https://developer.gnome.org/glib/stable/glib-Unicode-Manipulation.html">Unicode
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manipulation API</ulink> includes all the functionality
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necessary to retrieve Unicode data for the
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<structfield>unicode_funcs</structfield> structure of a HarfBuzz
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<type>hb_buffer_t</type>.
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</para>
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<para>
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The function <function>hb_glib_get_unicode_funcs()</function>
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sets up a <type>hb_unicode_funcs_t</type> structure configured
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with the GLib Unicode functions and returns a pointer to it.
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</para>
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<para>
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You can attach this Unicode-functions structure to your buffer,
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and it will be ready for use with GLib:
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</para>
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<programlisting language="C">
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#include <hb-glib.h>
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...
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hb_unicode_funcs_t *glibufunctions;
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glibufunctions = hb_glib_get_unicode_funcs();
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hb_buffer_set_unicode_funcs(buf, glibufunctions);
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</programlisting>
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<para>
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For script information, GLib uses the
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<type>GUnicodeScript</type> type. Like HarfBuzz's own
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<type>hb_script_t</type>, this data type is an enumeration
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of Unicode scripts, but text segments passed in from GLib code
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will be tagged with a <type>GUnicodeScript</type>. Therefore,
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when setting the script property on a <type>hb_buffer_t</type>,
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you will need to convert between the <type>GUnicodeScript</type>
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of the input provided by GLib and HarfBuzz's
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<type>hb_script_t</type> type.
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</para>
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<para>
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The <function>hb_glib_script_to_script()</function> function
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takes an <type>GUnicodeScript</type> script identifier as its
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sole argument and returns the corresponding <type>hb_script_t</type>.
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The <function>hb_glib_script_from_script()</function> does the
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reverse, taking an <type>hb_script_t</type> and returning the
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<type>GUnicodeScript</type> identifier for GLib.
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</para>
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<para>
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Finally, GLib also provides a reference-counted object type called <ulink
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url="https://developer.gnome.org/glib/stable/glib-Byte-Arrays.html#GBytes"><type>GBytes</type></ulink>
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that is used for accessing raw memory segments with the benefits
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of GLib's lifecycle management. HarfBuzz provides a
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<function>hb_glib_blob_create()</function> function that lets
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you create an <type>hb_blob_t</type> directly from a
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<type>GBytes</type> object. This function takes only the
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<type>GBytes</type> object as its input; HarfBuzz registers the
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GLib <function>destroy</function> callback automatically.
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</para>
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<para>
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The GNOME platform also features an object system called
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GObject. For HarfBuzz, the main advantage of GObject is a
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feature called <ulink
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url="https://gi.readthedocs.io/en/latest/">GObject
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Introspection</ulink>. This is a middleware facility that can be
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used to generate language bindings for C libraries. HarfBuzz uses it
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to build its Python bindings, which we will look at in a separate section.
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</para>
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</section>
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<section id="integration-freetype">
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<title>FreeType integration</title>
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<para>
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FreeType is the free-software font-rendering engine included in
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desktop Linux distributions, Android, ChromeOS, iOS, and multiple Unix
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operating systems, and used by cross-platform programs like
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Chrome, Java, and GhostScript. Used together, HarfBuzz can
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perform shaping on Unicode text segments, outputting the glyph
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IDs that FreeType should rasterize from the active font as well
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as the positions at which those glyphs should be drawn.
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</para>
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<para>
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HarfBuzz provides integration points with FreeType at the
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face-object and font-object level and for the font-functions
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virtual-method structure of a font object. To use the
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FreeType-integration API, include the
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<filename>hb-ft.h</filename> header.
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</para>
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<para>
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In a typical client program, you will create your
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<type>hb_face_t</type> face object and <type>hb_font_t</type>
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font object from a FreeType <type>FT_Face</type>. HarfBuzz
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provides a suite of functions for doing this.
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</para>
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<para>
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In the most common case, you will want to use
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<function>hb_ft_font_create_referenced()</function>, which
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creates both an <type>hb_face_t</type> face object and
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<type>hb_font_t</type> font object (linked to that face object),
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and provides lifecycle management.
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</para>
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<para>
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It is important to note,
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though, that while HarfBuzz makes a distinction between its face and
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font objects, FreeType's <type>FT_Face</type> does not. After
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you create your <type>FT_Face</type>, you must set its size
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parameter using <function>FT_Set_Char_Size()</function>, because
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an <type>hb_font_t</type> is defined as an instance of an
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<type>hb_face_t</type> with size specified.
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</para>
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<programlisting language="C">
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#include <hb-ft.h>
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...
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FT_New_Face(ft_library, font_path, index, &face);
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FT_Set_Char_Size(face, 0, 1000, 0, 0);
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hb_font_t *font = hb_ft_font_create(face);
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</programlisting>
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<para>
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Although <function>hb_ft_font_create_referenced()</function> is
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the recommended function, there is another variant. The simpler
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version of the function is
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<function>hb_ft_font_create()</function>, which takes an
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<type>FT_Face</type> and an optional destroy callback as its
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arguments. The critical difference between the two is that
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<function>hb_ft_font_create()</function> does not offer the
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lifecycle-management feature. Your client code will be
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responsible for tracking references to the <type>FT_Face</type> objects and
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destroying them when they are no longer needed. If you do not
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have a valid reason for doing this, user
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<function>hb_ft_font_create_referenced()</function>.
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</para>
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<para>
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After you have created your font object from your
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<type>FT_Face</type>, you can set or retrieve the
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<structfield>load_flags</structfield> of the
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<type>FT_Face</type> through the <type>hb_font_t</type>
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object. HarfBuzz provides
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<function>hb_ft_font_set_load_flags()</function> and
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<function>hb_ft_font_get_load_flags()</function> for this
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purpose. The ability to set the
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<structfield>load_flags</structfield> through the font object
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could be useful for enabling or disabling hinting, for example,
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or to activate vertical layout.
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</para>
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<para>
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HarfBuzz also provides a utility function called
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<function>hb_ft_font_has_changed()</function> that you should
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call whenever you have altered the properties of your underlying
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<type>FT_Face</type>, as well as a
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<function>hb_ft_get_face()</function> that you can call on an
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<type>hb_font_t</type> font object to fetch its underlying <type>FT_Face</type>.
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</para>
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<para>
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With an <type>hb_face_t</type> and <type>hb_font_t</type> both linked
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to your <type>FT_Face</type>, you will typically also want to
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use FreeType for the <structfield>font_funcs</structfield>
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vtable of your <type>hb_font_t</type>. As a reminder, this
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font-functions structure is the set of methods that HarfBuzz
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will use to fetch important information from the font, such as
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the advances and extents of individual glyphs.
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</para>
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<para>
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All you need to do is call
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</para>
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<programlisting language="C">
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hb_ft_font_set_funcs(font);
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</programlisting>
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<para>
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and HarfBuzz will use FreeType for the font-functions in
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<literal>font</literal>.
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</para>
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<para>
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As we noted above, an <type>hb_font_t</type> is derived from an
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<type>hb_face_t</type> with size (and, perhaps, other
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parameters, such as variation-axis coordinates)
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specified. Consequently, you can reuse an <type>hb_face_t</type>
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with several <type>hb_font_t</type> objects, and HarfBuzz
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provides functions to simplify this.
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</para>
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<para>
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The <function>hb_ft_face_create_referenced()</function>
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function creates just an <type>hb_face_t</type> from a FreeType
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<type>FT_Face</type> and, as with
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<function>hb_ft_font_create_referenced()</function> above,
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provides lifecycle management for the <type>FT_Face</type>.
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</para>
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<para>
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Similarly, there is an <function>hb_ft_face_create()</function>
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function variant that does not provide the lifecycle-management
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feature. As with the font-object case, if you use this version
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of the function, it will be your client code's respsonsibility
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to track usage of the <type>FT_Face</type> objects.
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</para>
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<para>
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A third variant of this function is
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<function>hb_ft_face_create_cached()</function>, which is the
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same as <function>hb_ft_face_create()</function> except that it
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also uses the <structfield>generic</structfield> field of the
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<type>FT_Face</type> structure to save a pointer to the newly
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created <type>hb_face_t</type>. Subsequently, function calls
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that pass the same <type>FT_Face</type> will get the same
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<type>hb_face_t</type> returned — and the
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<type>hb_face_t</type> will be correctly reference
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counted. Still, as with
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<function>hb_ft_face_create()</function>, your client code must
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track references to the <type>FT_Face</type> itself, and destroy
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it when it is unneeded.
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</para>
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</section>
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<section id="integration-uniscribe">
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<title>Uniscribe integration</title>
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<para>
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If your client program is running on Windows, HarfBuzz offers
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an additional API that can help integrate with Microsoft's
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Uniscribe engine and the Windows GDI.
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</para>
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<para>
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Overall, the Uniscribe API covers a broader set of typographic
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layout functions than HarfBuzz implements, but HarfBuzz's
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shaping API can serve as a drop-in replacement for Uniscribe's shaping
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functionality. In fact, one of HarfBuzz's design goals is to
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accurately reproduce the same output for shaping a given text
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segment that Uniscribe produces — even to the point of
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duplicating known shaping bugs or deviations from the
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specification — so you can be sure that your users'
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documents with their existing fonts will not be affected by
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switching to HarfBuzz.
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</para>
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<para>
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At a basic level, HarfBuzz's <function>hb_shape()</function>
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function replaces both the <ulink url=""><function>ScriptShape()</function></ulink>
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and <ulink
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url="https://docs.microsoft.com/en-us/windows/desktop/api/Usp10/nf-usp10-scriptplace"><function>ScriptPlace()</function></ulink>
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functions from Uniscribe.
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</para>
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<para>
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However, whereas <function>ScriptShape()</function> returns the
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glyphs and clusters for a shaped sequence and
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<function>ScriptPlace()</function> returns the advances and
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offsets for those glyphs, <function>hb_shape()</function>
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handles both. After <function>hb_shape()</function> shapes a
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buffer, the output glyph IDs and cluster IDs are returned as
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an array of <structname>hb_glyph_info_t</structname> structures, and the
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glyph advances and offsets are returned as an array of
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<structname>hb_glyph_position_t</structname> structures.
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</para>
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<para>
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Your client program only needs to ensure that it coverts
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correctly between HarfBuzz's low-level data types (such as
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<type>hb_position_t</type>) and Windows's corresponding types
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(such as <type>GOFFSET</type> and <type>ABC</type>). Be sure you
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read the <xref linkend="buffers-language-script-and-direction"
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/>
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chapter for a full explanation of how HarfBuzz input buffers are
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used, and see <xref linkend="shaping-buffer-output" /> for the
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details of what <function>hb_shape()</function> returns in the
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output buffer when shaping is complete.
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</para>
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<para>
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Although <function>hb_shape()</function> itself is functionally
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equivalent to Uniscribe's shaping routines, there are two
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additional HarfBuzz functions you may want to use to integrate
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the libraries in your code. Both are used to link HarfBuzz font
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objects to the equivalent Windows structures.
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</para>
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<para>
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The <function>hb_uniscribe_font_get_logfontw()</function>
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function takes a <type>hb_font_t</type> font object and returns
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a pointer to the <ulink
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url="https://docs.microsoft.com/en-us/windows/desktop/api/wingdi/ns-wingdi-logfontw"><type>LOGFONTW</type></ulink>
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"logical font" that corresponds to it. A <type>LOGFONTW</type>
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structure holds font-wide attributes, including metrics, size,
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and style information.
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</para>
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<!--
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<para>
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In Uniscribe's model, the <type>SCRIPT_CACHE</type> holds the
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device context, including the logical font that the shaping
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functions apply.
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https://docs.microsoft.com/en-us/windows/desktop/Intl/script-cache
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</para>
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-->
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<para>
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The <function>hb_uniscribe_font_get_hfont()</function> function
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also takes a <type>hb_font_t</type> font object, but it returns
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an <type>HFONT</type> — a handle to the underlying logical
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font — instead.
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</para>
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<para>
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<type>LOGFONTW</type>s and <type>HFONT</type>s are both needed
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by other Uniscribe functions.
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</para>
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<para>
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As a final note, you may notice a reference to an optional
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<literal>uniscribe</literal> shaper back-end in the <xref
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linkend="configuration" /> section of the HarfBuzz manual. This
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option is not a Uniscribe-integration facility.
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</para>
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<para>
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Instead, it is a internal code path used in the
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<command>hb-shape</command> command-line utility, which hands
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shaping functionality over to Uniscribe entirely, when run on a
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Windows system. That allows testing HarfBuzz's native output
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against the Uniscribe engine, for tracking compatibility and
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debugging.
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</para>
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<para>
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Because this back-end is only used when testing HarfBuzz
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functionality, it is disabled by default when building the
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HarfBuzz binaries.
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</para>
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</section>
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<section id="integration-coretext">
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<title>Core Text integration</title>
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<para>
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If your client program is running on macOS or iOS, HarfBuzz offers
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an additional API that can help integrate with Apple's
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Core Text engine and the underlying Core Graphics
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framework. HarfBuzz does not attempt to offer the same
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drop-in-replacement functionality for Core Text that it strives
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for with Uniscribe on Windows, but you can still user HarfBuzz
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to perform text shaping in native macOS and iOS applications.
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</para>
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<para>
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Note, though, that if your interest is just in using fonts that
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contain Apple Advanced Typography (AAT) features, then you do
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not need to add Core Text integration. HarfBuzz natively
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supports AAT features and will shape AAT fonts (on any platform)
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automatically, without requiring additional work on your
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part. This includes support for AAT-specific TrueType tables
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such as <literal>mort</literal>, <literal>morx</literal>, and
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<literal>kerx</literal>, which AAT fonts use instead of
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<literal>GSUB</literal> and <literal>GPOS</literal>.
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</para>
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<para>
|
|
On a macOS or iOS system, the primary integration points offered
|
|
by HarfBuzz are for face objects and font objects.
|
|
</para>
|
|
<para>
|
|
The Apple APIs offer a pair of data structures that map well to
|
|
HarfBuzz's face and font objects. The Core Graphics API, which
|
|
is slightly lower-level than Core Text, provides
|
|
<ulink url="https://developer.apple.com/documentation/coregraphics/cgfontref"><type>CGFontRef</type></ulink>, which enables access to typeface
|
|
properties, but does not include size information. Core Text's
|
|
<ulink url="https://developer.apple.com/documentation/coretext/ctfont-q6r"><type>CTFontRef</type></ulink> is analagous to a HarfBuzz font object,
|
|
with all of the properties required to render text at a specific
|
|
size and configuration.
|
|
Consequently, a HarfBuzz <type>hb_font_t</type> font object can
|
|
be hooked up to a Core Text <type>CTFontRef</type>, and a HarfBuzz
|
|
<type>hb_face_t</type> face object can be hooked up to a
|
|
<type>CGFontRef</type>.
|
|
</para>
|
|
<para>
|
|
You can create a <type>hb_face_t</type> from a
|
|
<type>CGFontRef</type> by using the
|
|
<function>hb_coretext_face_create()</function>. Subsequently,
|
|
you can retrieve the <type>CGFontRef</type> from a
|
|
<type>hb_face_t</type> with <function>hb_coretext_face_get_cg_font()</function>.
|
|
</para>
|
|
<para>
|
|
Likewise, you create a <type>hb_font_t</type> from a
|
|
<type>CTFontRef</type> by calling
|
|
<function>hb_coretext_font_create()</function>, and you can
|
|
fetch the associated <type>CTFontRef</type> from a
|
|
<type>hb_font_t</type> font object with
|
|
<function>hb_coretext_face_get_ct_font()</function>.
|
|
</para>
|
|
<para>
|
|
HarfBuzz also offers a <function>hb_font_set_ptem()</function>
|
|
that you an use to set the nominal point size on any
|
|
<type>hb_font_t</type> font object. Core Text uses this value to
|
|
implement optical scaling.
|
|
</para>
|
|
<para>
|
|
When integrating your client code with Core Text, it is
|
|
important to recognize that Core Text <literal>points</literal>
|
|
are not typographic points (standardized at 72 per inch) as the
|
|
term is used elsewhere in OpenType. Instead, Core Text points
|
|
are CSS points, which are standardized at 96 per inch.
|
|
</para>
|
|
<para>
|
|
HarfBuzz's font functions take this distinction into account,
|
|
but it can be an easy detail to miss in cross-platform
|
|
code.
|
|
</para>
|
|
<para>
|
|
As a final note, you may notice a reference to an optional
|
|
<literal>coretext</literal> shaper back-end in the <xref
|
|
linkend="configuration" /> section of the HarfBuzz manual. This
|
|
option is not a CoreText-integration facility.
|
|
</para>
|
|
<para>
|
|
Instead, it is a internal code path used in the
|
|
<command>hb-shape</command> command-line utility, which hands
|
|
shaping functionality over to CoreText entirely, when run on a
|
|
macOS system. That allows testing HarfBuzz's native output
|
|
against the CoreText engine, for tracking compatibility and debugging.
|
|
</para>
|
|
<para>
|
|
Because this back-end is only used when testing HarfBuzz
|
|
functionality, it is disabled by default when building the
|
|
HarfBuzz binaries.
|
|
</para>
|
|
</section>
|
|
|
|
<section id="integration-icu">
|
|
<title>ICU integration</title>
|
|
<para>
|
|
|
|
</para>
|
|
<para>
|
|
|
|
</para>
|
|
<para>
|
|
|
|
</para>
|
|
</section>
|
|
|
|
<section id="integration-python">
|
|
<title>Python bindings</title>
|
|
<para>
|
|
|
|
</para>
|
|
<para>
|
|
|
|
</para>
|
|
<para>
|
|
|
|
</para>
|
|
</section>
|
|
|
|
<section id="integration-">
|
|
<title></title>
|
|
<para>
|
|
|
|
</para>
|
|
<para>
|
|
|
|
</para>
|
|
</section>
|
|
|
|
</chapter>
|