How to respond_to PNG or JPG in Rails and generate image from HTML?

I am looking for a gem or solution to generate image in controller response.

It would be nice if it's possible to do in controller like that:

respond_to :html, :png

def show
  ...
  respond_to do |format|
    format.html
    format.png { ??? }  # some html to png converter 
  end
end

When the png format is requested the response handles with template:

#show.png.haml
%h1
  Some title
%p
  Some content

The result should be an image.

I know about pdf generation solutions PDFKit, prawn and am looking for image generation.

Does anybody know working solution/example? Any starting point would be very appreciated.