libharu:HPDFPage#set_rgb_stroke
#!/usr/bin/env ruby require "hpdf" pdf = HPDFDoc.new page = pdf.add_page page.set_rgb_stroke(1, 0, 0) page.move_to(100, 100) page.line_to(150, 100) page.line_to(150, 50) page.stroke pdf.save_to_file(File.basename($0, ".rb") + ".pdf")