Docs: Move What-HarfBuzz-doesnt-do to Usermanual-what-is-HarfBuzz.
This commit is contained in:
Nathan Willis
Khaled Hosny
parent
fd270beedb
commit
0956ab4185
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@ -90,94 +90,10 @@
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hb_buffer_destroy(buf);
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hb_font_destroy(hb_ft_font);
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</programlisting>
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<section id="what-harfbuzz-doesnt-do">
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<title>What HarfBuzz doesn't do</title>
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<para>
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The code above will take a UTF8 string, shape it, and give you the
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information required to lay it out correctly on a single
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horizontal (or vertical) line using the font provided. That is the
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extent of HarfBuzz's responsibility.
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This example shows enough to get us started using HarfBuzz. Now we
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are going to use the remainder of HarfBuzz's API to refine that
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example and improve our text shaping capabilities.
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</para>
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<para>
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If you are implementing a text layout engine you may have other
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responsibilities, that HarfBuzz will not help you with:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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HarfBuzz won't help you with bidirectionality. If you want to
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lay out text with mixed Hebrew and English, you will need to
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ensure that the buffer provided to HarfBuzz has those
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characters in the correct layout order. This will be different
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from the logical order in which the Unicode text is stored. In
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other words, the user will hit the keys in the following
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sequence:
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</para>
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<programlisting>
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A B C [space] ג ב א [space] D E F
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</programlisting>
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<para>
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but will expect to see in the output:
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</para>
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<programlisting>
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ABC אבג DEF
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</programlisting>
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<para>
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This reordering is called <emphasis>bidi processing</emphasis>
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("bidi" is short for bidirectional), and there's an
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algorithm as an annex to the Unicode Standard which tells you how
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to reorder a string from logical order into presentation order.
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Before sending your string to HarfBuzz, you may need to apply the
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bidi algorithm to it. Libraries such as ICU and fribidi can do
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this for you.
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</para>
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</listitem>
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<listitem>
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<para>
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HarfBuzz won't help you with text that contains different font
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properties. For instance, if you have the string "a
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<emphasis>huge</emphasis> breakfast", and you expect
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"huge" to be italic, you will need to send three
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strings to HarfBuzz: <literal>a</literal>, in your Roman font;
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<literal>huge</literal> using your italic font; and
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<literal>breakfast</literal> using your Roman font again.
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Similarly if you change font, font size, script, language or
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direction within your string, you will need to shape each run
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independently and then output them independently. HarfBuzz
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expects to shape a run of characters sharing the same
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properties.
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</para>
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</listitem>
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<listitem>
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<para>
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HarfBuzz won't help you with line breaking, hyphenation or
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justification. As mentioned above, it lays out the string
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along a <emphasis>single line</emphasis> of, notionally,
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infinite length. If you want to find out where the potential
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word, sentence and line break points are in your text, you
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could use the ICU library's break iterator functions.
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</para>
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<para>
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HarfBuzz can tell you how wide a shaped piece of text is, which is
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useful input to a justification algorithm, but it knows nothing
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about paragraphs, lines or line lengths. Nor will it adjust the
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space between words to fit them proportionally into a line. If you
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want to layout text in paragraphs, you will probably want to send
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each word of your text to HarfBuzz to determine its shaped width
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after glyph substitutions, then work out how many words will fit
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on a line, and then finally output each word of the line separated
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by a space of the correct size to fully justify the paragraph.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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As a layout engine implementor, HarfBuzz will help you with the
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interface between your text and your font, and that's something
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that you'll need - what you then do with the glyphs that your font
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returns is up to you. The example we saw above enough to get us
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started using HarfBuzz. Now we are going to use the remainder of
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HarfBuzz's API to refine that example and improve our text shaping
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capabilities.
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</para>
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</section>
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</chapter>
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@ -1,7 +1,7 @@
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<chapter id="what-is-harfbuzz">
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<title>What is HarfBuzz?</title>
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<para>
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HarfBuzz is a <emphasis>text shaping engine</emphasis>. If you
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HarfBuzz is a <emphasis>text-shaping engine</emphasis>. If you
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give HarfBuzz a font and a string containing a sequence of Unicode
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codepoints, HarfBuzz selects and positions the corresponding
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glyphs from the font, applying all of the necessary layout rules
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@ -21,7 +21,7 @@
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Text shaping is the process of translating a string of character
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codes (such as Unicode codepoints) into a properly arranged
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sequence of glyphs that can be rendered onto a screen or into
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final output form for a document.
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final output form for inclusion in a document.
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</para>
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<para>
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The shaping process is dependent on the input string, the active
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@ -66,6 +66,7 @@
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tags are defined by OpenType.
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</para>
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</section>
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<section id="why-do-i-need-a-shaping-engine">
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<title>Why do I need a shaping engine?</title>
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<para>
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@ -212,6 +213,98 @@
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implementor of a text-layout engine.
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</para>
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</section>
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<section id="what-harfbuzz-doesnt-do">
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<title>What HarfBuzz doesn't do</title>
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<para>
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HarfBuzz will take a Unicode string, shape it, and give you the
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information required to lay it out correctly on a single
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horizontal (or vertical) line using the font provided. That is the
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extent of HarfBuzz's responsibility.
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</para>
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<para>
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It is important to note that if you are implementing a
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text-layout engine you may have other responsibilities that
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HarfBuzz will <emphasis>not</emphasis> help you with. For example:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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HarfBuzz won't help you with bidirectionality. If you want to
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lay out text that includes a mix of Hebrew and English, you
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will need to ensure that each buffer provided to HarfBuzz has its
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characters in the correct layout order. This will be different
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from the logical order in which the Unicode text is stored. In
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other words, the user will hit the keys in the following
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sequence:
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</para>
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<programlisting>
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A B C [space] ג ב א [space] D E F
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</programlisting>
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<para>
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but will expect to see in the output:
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</para>
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<programlisting>
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ABC אבג DEF
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</programlisting>
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<para>
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This reordering is called <emphasis>bidi processing</emphasis>
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("bidi" is short for bidirectional), and there's an
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algorithm as an annex to the Unicode Standard which tells you how
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to reorder a string from logical order into presentation order.
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Before sending your string to HarfBuzz, you may need to apply the
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bidi algorithm to it. Libraries such as ICU and fribidi can do
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this for you.
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</para>
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</listitem>
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<listitem>
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<para>
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HarfBuzz won't help you with text that contains different font
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properties. For instance, if you have the string "a
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<emphasis>huge</emphasis> breakfast", and you expect
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"huge" to be italic, then you will need to send three
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strings to HarfBuzz: <literal>a</literal>, in your Roman font;
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<literal>huge</literal> using your italic font; and
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<literal>breakfast</literal> using your Roman font again.
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</para>
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<para>
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Similarly, if you change the font, font size, script,
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language, or direction within your string, then you will
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need to shape each run independently and output them
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independently. HarfBuzz expects to shape a run of characters
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that all share the same properties.
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</para>
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</listitem>
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<listitem>
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<para>
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HarfBuzz won't help you with line breaking, hyphenation, or
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justification. As mentioned above, HarfBuzz lays out the string
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along a <emphasis>single line</emphasis> of, notionally,
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infinite length. If you want to find out where the potential
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word, sentence and line break points are in your text, you
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could use the ICU library's break iterator functions.
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</para>
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<para>
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HarfBuzz can tell you how wide a shaped piece of text is, which is
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useful input to a justification algorithm, but it knows nothing
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about paragraphs, lines or line lengths. Nor will it adjust the
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space between words to fit them proportionally into a line. If you
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want to layout text in paragraphs, you will probably want to send
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each word of your text to HarfBuzz to determine its shaped width
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after glyph substitutions, then work out how many words will fit
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on a line, and then finally output each word of the line separated
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by a space of the correct size to fully justify the paragraph.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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As a layout-engine implementor, HarfBuzz will help you with the
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interface between your text and your font, and that's something
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that you'll need—what you then do with the glyphs that your font
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returns is up to you.
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</para>
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</section>
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<section id="why-is-it-called-harfbuzz">
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<title>Why is it called HarfBuzz?</title>
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<para>
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@ -220,7 +313,7 @@
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within the source code copyright declarations), but was then
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extracted out to its own project. This project is maintained by
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Behdad Esfahbod, and named HarfBuzz. Originally, it was a shaping
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engine for OpenType fonts - "HarfBuzz" is the Persian
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engine for OpenType fonts—"HarfBuzz" is the Persian
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for "open type".
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</para>
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</section>
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