183 lines
6.8 KiB
XML
183 lines
6.8 KiB
XML
<chapter id="hello-harfbuzz">
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<title>Hello, HarfBuzz</title>
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<para>
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Here's the simplest HarfBuzz that can possibly work. We will improve
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it later.
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</para>
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<orderedlist numeration="arabic">
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<listitem>
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<para>
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Create a buffer and put your text in it.
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</para>
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</listitem>
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</orderedlist>
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<programlisting language="C">
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#include <hb.h>
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hb_buffer_t *buf;
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buf = hb_buffer_create();
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hb_buffer_add_utf8(buf, text, strlen(text), 0, strlen(text));
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</programlisting>
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<orderedlist numeration="arabic">
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<listitem override="2">
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<para>
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Guess the script, language and direction of the buffer.
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</para>
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</listitem>
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</orderedlist>
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<programlisting language="C">
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hb_buffer_guess_segment_properties(buf);
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</programlisting>
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<orderedlist numeration="arabic">
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<listitem override="3">
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<para>
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Create a face and a font, using FreeType for now.
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</para>
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</listitem>
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</orderedlist>
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<programlisting language="C">
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#include <hb-ft.h>
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FT_New_Face(ft_library, font_path, index, &face)
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hb_font_t *font = hb_ft_font_create(face);
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</programlisting>
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<orderedlist numeration="arabic">
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<listitem override="4">
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<para>
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Shape!
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</para>
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</listitem>
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</orderedlist>
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<programlisting>
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hb_shape(font, buf, NULL, 0);
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</programlisting>
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<orderedlist numeration="arabic">
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<listitem override="5">
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<para>
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Get the glyph and position information.
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</para>
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</listitem>
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</orderedlist>
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<programlisting language="C">
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hb_glyph_info_t *glyph_info = hb_buffer_get_glyph_infos(buf, &glyph_count);
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hb_glyph_position_t *glyph_pos = hb_buffer_get_glyph_positions(buf, &glyph_count);
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</programlisting>
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<orderedlist numeration="arabic">
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<listitem override="6">
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<para>
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Iterate over each glyph.
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</para>
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</listitem>
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</orderedlist>
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<programlisting language="C">
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for (i = 0; i < glyph_count; ++i) {
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glyphid = glyph_info[i].codepoint;
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x_offset = glyph_pos[i].x_offset / 64.0;
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y_offset = glyph_pos[i].y_offset / 64.0;
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x_advance = glyph_pos[i].x_advance / 64.0;
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y_advance = glyph_pos[i].y_advance / 64.0;
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draw_glyph(glyphid, cursor_x + x_offset, cursor_y + y_offset);
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cursor_x += x_advance;
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cursor_y += y_advance;
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}
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</programlisting>
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<orderedlist numeration="arabic">
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<listitem override="7">
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<para>
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Tidy up.
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</para>
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</listitem>
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</orderedlist>
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<programlisting language="C">
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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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</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> |