require 'cairo'
format = Cairo::FORMAT_ARGB32
 width = 300
 height = 200
 radius = height / 3 # 半径

 surface = Cairo::ImageSurface.new(format, width, height)
 context = Cairo::Context.new(surface)

 # 背景
 context.set_source_rgb(1, 1, 1) # 白
 context.rectangle(0, 0, width, height)
 context.fill

 # 赤丸
 context.set_source_rgb(1, 0, 0) # 赤
 context.arc(width / 2, height / 2, radius, 0, 2 * Math::PI)
 context.fill

 surface.write_to_png("hinomaru3.png")