Text Shaping

The built-in text shaping functionality is very limited. However, it is possible to switch to a HarfBuzz based implementation (using the harfbuzz-ruby gem) that supports many languages and scripts as well as OpenType features. Note that this only works with TrueType fonts!

In general it is advised to use TrueType font files and enable HarfBuzz as the result is more correct and visually better. The disadvantage is that the additional processing requirements have a slight performance impact.

By using the ‘font_features’ style property it is possible to selectively enable/disable OpenType features like discretionary ligatures. Note that fonts determine which features are on by default (like ‘calt’ for the Inter font used in this example).

Usage:

ruby text_shaping.rb

Resulting PDF:

text_shaping.pdf

Preview:

Code

require 'hexapdf'

sample_text = <<EOT.tr("\n", " ")
This sample shows some OpenType features, like contextual alternatives (2*3 and
-->), discretionary ligatures (difficult?!), fractions (15/157) or compositions
(5⃝ ×⃞ 3⃝ =⃞ 1⃝5⃝).
EOT

HexaPDF::Composer.create('text_shaping.pdf') do |composer|
  composer.style(:base, font: 'Inter')
  composer.style(:header, font_bold: true, margin: [10, 0, 0])
  composer.text('Without HarfBuzz', style: :header)
  composer.text(sample_text)
  composer.text('With HarfBuzz', style: :header)
  composer.text(sample_text, shaping_engine: :harfbuzz,
                font_features: {dlig: true, frac: true})
end