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Hello, Harfbuzz

Here's the simplest Harfbuzz that can possibly work. We will improve it later.

  1. Create a buffer and put your text in it.

  #include <hb.h>
  hb_buffer_t *buf;
  buf = hb_buffer_create();
  hb_buffer_add_utf8(buf, text, strlen(text), 0, strlen(text));
  1. Guess the script, language and direction of the buffer.

  hb_buffer_guess_segment_properties(buf);
  1. Create a face and a font, using FreeType for now.

  #include <hb-ft.h>
  FT_New_Face(ft_library, font_path, index, &face)
  hb_font_t *font = hb_ft_font_create(face);
  1. Shape!

  hb_shape(font, buf, NULL, 0);
  1. Get the glyph and position information.

  hb_glyph_info_t *glyph_info    = hb_buffer_get_glyph_infos(buf, &glyph_count);
  hb_glyph_position_t *glyph_pos = hb_buffer_get_glyph_positions(buf, &glyph_count);
  1. Iterate over each glyph.

  for (i = 0; i < glyph_count; ++i) {
    glyphid = glyph_info[i].codepoint;
    x_offset = glyph_pos[i].x_offset / 64.0;
    y_offset = glyph_pos[i].y_offset / 64.0;
    x_advance = glyph_pos[i].x_advance / 64.0;
    y_advance = glyph_pos[i].y_advance / 64.0;
    draw_glyph(glyphid, cursor_x + x_offset, cursor_y + y_offset);
    cursor_x += x_advance;
    cursor_y += y_advance;
  }
  1. Tidy up.

  hb_buffer_destroy(buf);
  hb_font_destroy(hb_ft_font);

What Harfbuzz doesn't do

The code above will take a UTF8 string, shape it, and give you the information required to lay it out correctly on a single horizontal (or vertical) line using the font provided. That is the extent of Harfbuzz's responsibility.

If you are implementing a text layout engine you may have other responsibilities, that Harfbuzz will not help you with:

  • Harfbuzz won't help you with bidirectionality. If you want to lay out text with mixed Hebrew and English, you will need to ensure that the buffer provided to Harfbuzz has those characters in the correct layout order. This will be different from the logical order in which the Unicode text is stored. In other words, the user will hit the keys in the following sequence:

    A B C [space] ג ב א [space] D E F
            

    but will expect to see in the output:

    ABC אבג DEF
            

    This reordering is called bidi processing ("bidi" is short for bidirectional), and there's an algorithm as an annex to the Unicode Standard which tells you how to reorder a string from logical order into presentation order. Before sending your string to Harfbuzz, you may need to apply the bidi algorithm to it. Libraries such as ICU and fribidi can do this for you.

  • Harfbuzz won't help you with text that contains different font properties. For instance, if you have the string "a huge breakfast", and you expect "huge" to be italic, you will need to send three strings to Harfbuzz: a, in your Roman font; huge using your italic font; and breakfast using your Roman font again. Similarly if you change font, font size, script, language or direction within your string, you will need to shape each run independently and then output them independently. Harfbuzz expects to shape a run of characters sharing the same properties.

  • Harfbuzz won't help you with line breaking, hyphenation or justification. As mentioned above, it lays out the string along a single line of, notionally, infinite length. If you want to find out where the potential word, sentence and line break points are in your text, you could use the ICU library's break iterator functions.

    Harfbuzz can tell you how wide a shaped piece of text is, which is useful input to a justification algorithm, but it knows nothing about paragraphs, lines or line lengths. Nor will it adjust the space between words to fit them proportionally into a line. If you want to layout text in paragraphs, you will probably want to send each word of your text to Harfbuzz to determine its shaped width after glyph substitutions, then work out how many words will fit on a line, and then finally output each word of the line separated by a space of the correct size to fully justify the paragraph.

As a layout engine implementor, Harfbuzz will help you with the interface between your text and your font, and that's something that you'll need - what you then do with the glyphs that your font returns is up to you. The example we saw above enough to get us started using Harfbuzz. Now we are going to use the remainder of Harfbuzz's API to refine that example and improve our text shaping capabilities.

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