[Docs] Usermanual: integration chapter; add CoreText/Mac section.
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Nathan Willis
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@ -303,13 +303,14 @@
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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 can
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integrate with the Uniscribe engine.
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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 the shaping
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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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@ -321,8 +322,9 @@
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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 url="https://docs.microsoft.com/en-us/windows/desktop/api/Usp10/nf-usp10-scriptplace"><function>ScriptPlace()</function></ulink> functions from
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Uniscribe.
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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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@ -363,10 +365,14 @@
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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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SCRIPT_CACHE holds context, including LOGFONT https://docs.microsoft.com/en-us/windows/desktop/Intl/script-cache
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https://docs.microsoft.com/en-us/windows/desktop/api/wingdi/ns-wingdi-logfontw
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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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@ -399,30 +405,95 @@ SCRIPT_CACHE holds context, including LOGFONT https://docs.microsoft.com/en
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</section>
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<section id="integration-coretext">
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<title>CoreText integration</title>
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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>
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On a macOS or iOS system, the primary integration points offered
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by HarfBuzz are for face objects and font objects.
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</para>
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<para>
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The Apple APIs offer a pair of data structures that map well to
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HarfBuzz's face and font objects. The Core Graphics API, which
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is slightly lower-level than Core Text, provides
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<ulink url="https://developer.apple.com/documentation/coregraphics/cgfontref"><type>CGFontRef</type></ulink>, which enables access to typeface
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properties, but does not include size information. Core Text's
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<ulink url="https://developer.apple.com/documentation/coretext/ctfont-q6r"><type>CTFontRef</type></ulink> is analagous to a HarfBuzz font object,
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with all of the properties required to render text at a specific
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size and configuration.
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Consequently, a HarfBuzz <type>hb_font_t</type> font object can
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be hooked up to a Core Text <type>CTFontRef</type>, and a HarfBuzz
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<type>hb_face_t</type> face object can be hooked up to a
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<type>CGFontRef</type>.
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</para>
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<para>
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You can create a <type>hb_face_t</type> from a
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<type>CGFontRef</type> by using the
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<function>hb_coretext_face_create()</function>. Subsequently,
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you can retrieve the <type>CGFontRef</type> from a
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<type>hb_face_t</type> with <function>hb_coretext_face_get_cg_font()</function>.
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</para>
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<para>
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Likewise, you create a <type>hb_font_t</type> from a
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<type>CTFontRef</type> by calling
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<function>hb_coretext_font_create()</function>, and you can
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fetch the associated <type>CTFontRef</type> from a
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<type>hb_font_t</type> font object with
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<function>hb_coretext_face_get_ct_font()</function>.
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</para>
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<para>
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HarfBuzz also offers a <function>hb_font_set_ptem()</function>
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that you an use to set the nominal point size on any
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<type>hb_font_t</type> font object. Core Text uses this value to
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implement optical scaling.
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</para>
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<para>
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When integrating your client code with Core Text, it is
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important to recognize that Core Text <literal>points</literal>
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are not typographic points (standardized at 72 per inch) as the
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term is used elsewhere in OpenType. Instead, Core Text points
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are CSS points, which are standardized at 96 per inch.
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</para>
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<para>
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HarfBuzz's font functions take this distinction into account,
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but it can be an easy detail to miss in cross-platform
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code.
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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>coretext</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 CoreText-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 CoreText entirely, when run on a
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macOS system. That allows testing HarfBuzz's native output
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against the CoreText engine, for tracking compatibility and 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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