Retrieve diagonals from array of arrays in Ruby

diagonals.rb

class Array
  def diagonals
    [self, self.map(&:reverse)].inject([]) do |all_diags, matrix|
      ((-matrix.count + 1)..matrix.first.count).each do |offet_index|
        diagonal = []
        (matrix.count).times do |row_index|
          col_index = offet_index + row_index
          diagonal << matrix[row_index][col_index] if col_index >= 0
        end
        all_diags << diagonal.compact if diagonal.compact.count > 1
      end
      all_diags
    end
  end
end

yeap, it is really hard to understand. comments in the code would help