e.g) GET /{filename}/fit/300/300

imagemagick.rb

module MrubyMiniMagick
  class Convert
    attr_accessor :opts
    def initialize(src, dst)
      @opts = []
      @src = src
      @dst = dst
    end
    %i(resize define gravity background extent thumbnail).each do |name|
      define_method(name) do |value|
        opts << "-#{name} #{value}"
      end
    end
    def call(output = false)
      cmd = "convert #{opts.join(' ')} #{@src} #{@dst}"
      if output
        `#{cmd}`
      else
        system(cmd)
      end
    end
  end
end

class Processor
  def initialize(root)
    @root = root
  end
  def fill(filename, method, width, height)
    file_path = "#{@root}/#{filename}"
    size = "#{width}x#{height}"
    dst = "#{file_path}_#{method}_#{width}_#{height}"
    cmd = MrubyMiniMagick::Convert.new(file_path, dst)
    cmd.define "jpeg:size=#{size}"
    cmd.thumbnail size
    cmd.call
    ret = File.read(dst)
    File.unlink dst
    ret
  end
  def fit(filename, method, width, height)
    file_path = "#{@root}/#{filename}"
    size = "#{width}x#{height}"
    dst = "#{file_path}_#{method}_#{width}_#{height}"
    cmd = MrubyMiniMagick::Convert.new(file_path, dst)
    cmd.define "jpeg:size=#{size}"
    cmd.thumbnail "#{size}^"
    cmd.gravity 'center'
    cmd.extent size
    cmd.call
    ret = File.read(dst)
    File.unlink dst
    ret
  end
end

r = Nginx::Request.new
uris = r.uri.split("/")
uris.shift

Nginx.return Nginx::HTTP_BAD_REQUEST if uris.length < 1

case uris[1]
when 'fit'
  Nginx.rputs Processor.new(Nginx::Server.new.document_root).fit(*uris)
when 'fill'
  Nginx.rputs Processor.new(Nginx::Server.new.document_root).fill(*uris)
end
Nginx.return Nginx::HTTP_OK

nginx.conf

server {
  listen 80;
  server_name img.localhost;

  root /path/to/doc/root;

  location / {
    mruby_content_handler /etc/nginx/conf.d/imagemagick.rb;
  }
}