mini_magick_resize_and_crop.rb

module MiniMagick
  class Image
    # Size is in 'WidthxHeight' string format.
    def resize_and_crop(target_size)
      target_width, target_height = *target_size.split('x').collect{|x| x.to_f}
      width, height = self[:width].to_f, self[:height].to_f

      # If the width or height are already less than the targets
      # just take the extent -- no resizing nessessary.
      if width >= target_width && height >= target_height
        width_ratio, height_ratio = target_width/width, target_height/height

        # There are only two possible transformations: 
        # * dimensions scaled by the width_ratio
        # * scaled by the height_ratio.
        #
        # Reject the transformation if either dimension is
        # smaller than the respective target dimension.
        # 
        # Sort the transformations by pixels.
        #
        # Select the first remaining, sorted transformation.
        transformation = [
          [target_width,  (height * width_ratio).ceil],
          [(width * height_ratio).ceil, target_height]
        ].reject{|t| t[0] < target_width || t[1] < target_height}.
          sort{|a,b| (a[0]*a[1]) <=> (b[0]*b[1])}.
          first.collect{|dim| dim.to_i}

        # Resize it
        self.resize("#{transformation[0]}x#{transformation[1]})")
      end

      self.combine_options do |c|
        c.gravity 'North'
        c.background 'White'
        c.extent(target_size)
      end
    end
  end
end