context_free

… is a library for Ruby-Processing that allows you to draw context-free sketches using a small declarative DSL. You can define rules which call one another probabilistically. The following block of code is the complete definition for drawing a fractal fern [samples/fern.rb] :

@fern = ContextFree.define do
    
  rule :start do
    fern :rotation => 50, :hue => 0.8
  end
    
  rule :fern do
    circle :size => 0.75, :rotation => -10
    split do
      fern :size => 0.92, :y => -2, :rotation => -5, :hue => 0.85
      rewind
      fern :size => 0.5, :y => -2, :rotation => 90
      rewind                                  
      fern :size => 0.5, :y => -2, :rotation => -90
    end
  end
    
end

With a little processing glue, produces:

There are several other samples — tree.rb, city.rb, and vine.rb — if you have one to contribute, please do.