Text is an essential part of Human-computer Interaction. In the age of Instagram, YouTube, and VR we still consume information primarily through text. Computing started with text-only interfaces and every major Operating System provides libraries to render text.
Text rendering, despite being ubiquitous, has little up to date documentation, especially on Linux systems. The goal of this post is to give an overview of the modern Linux text rendering stack and to give the reader an understanding of the complexity behind it.
The stack is composed of 4 components:
Fontconfig takes a list of font properties (name, width, weight, style, etc.), then selects a font file on your system which most closely matches the properties you requested. It is configured with an XML-based language which allows you to specify default fonts, whitelist or blacklist them, create bold and italic styles for incomplete fonts, set the hinting and antialiasing levels and much more. The Arch Wiki is an excellent resource for creating or editing your configuration.
FriBidi or LibICU
FriBidi and LibICU, among other things, provide an implementation of the Unicode Bidirectional algorithm. The Unicode Bidirectional Algorithm allows you to convert between text in logical/storage format to text in visual format. The W3C has a beautiful article on it.
Consider the following example, where Arabic or Hebrew letters are represented by uppercase English letters and English text is represented by lowercase letters:
In the rendered text, the English letter h is visually followed by the Arabic letter C, but logically h is followed by the rightmost letter A. The next letter, in logical order, will be R. In other words, the logical/storage order of the same text would be:
english ARABIC text
But the rendered version also called visual format would be:
english CIBARA text
Because Arabic or Hebrew are Right-To-Left languages whereas English is Left-To-Right.
(Example taken from http://userguide.icu-project.org/transforms/bidi)
HarfBuzz is the text shaping engine: it maps codepoints from a string to their corresponding glyph indices. HarfBuzz takes a Unicode string, three properties: direction, script, language, and a font. It returns a list of glyph indices, each with a position in x, y coordinates, which incorporates kerning data from the font’s GPOS table.
Hello, world becomes
43 72 79 79 82 15 3 90 82 85 79 71 with font: Ubuntu Mono
16 33 with font: Ubuntu Mono
1603 1064 with font: Fira Code Retina
HarfBuzz is essential to correctly handle accents, emojis, ligatures and almost every language except English.
FreeType is the text rendering engine: it takes a glyph index and a font, then renders an image of that glyph index. It also provides basic kerning support. You can configure it by specifying the hinting level and which and how much antialiasing to use.