サンプル

 #!/usr/bin/env ruby
 require "GD"
 # create a new image
 im = GD::Image.new(100,100)
 # allocate some colors
 white = im.colorAllocate(255,255,255)
 black = im.colorAllocate(0,0,0)
 red = im.colorAllocate(255,0,0)
 blue = im.colorAllocate(0,0,255)
 # make the background transparent and interlaced
 im.transparent(white)
 im.interlace = true
 # Put a black frame around the picture
 im.rectangle(0,0,99,99,black)
 # Draw a blue oval
 im.arc(50,50,95,75,0,360,blue)
 # And fill it with red
 im.fill(50,50,red)
 # Convert the image to PNG and print it on standard output
 im.png STDOUT

以上でpngが標準出力に生成される。