libharu：HPDFPage#set_text_rendering_mode

everyday Ruby 
#!/usr/bin/env ruby

require "hpdf"

pdf  = HPDFDoc.new
page = pdf.add_page

font = pdf.get_font("Helvetica", nil)
page.set_font_and_size(font, 40)

y = 100

[HPDFDoc::HPDF_FILL,
 HPDFDoc::HPDF_STROKE,
 HPDFDoc::HPDF_FILL_THEN_STROKE,
 HPDFDoc::HPDF_INVISIBLE,
 HPDFDoc::HPDF_FILL_CLIPPING,
 HPDFDoc::HPDF_STROKE_CLIPPING,
 HPDFDoc::HPDF_FILL_STROKE_CLIPPING,
 HPDFDoc::HPDF_CLIPPING].each do |x|
  page.begin_text
  page.set_text_rendering_mode(x)
  page.move_text_pos(100, y)
  page.show_text(x.to_s)
  page.end_text
  #page.stroke
  y += 50
end

pdf.save_to_file(File.basename($0, ".rb") + ".pdf")

描画されないものがあるのはなぜ？