Fetches static image from the City of Palo Alto "Creek Camera - San Francisquito Creek at West Bayshore Road", respecting response e-tag.

creek.rb

require 'open-uri'
require 'digest/sha1'

class CreekImage

  attr_reader :etag, :data, :uri, :sha1

  def initialize
    @uri = 'http://archive.cityofpaloalto.org/earlywarning/creekcam/creekcam.jpg'
    open(@uri) do |img|
      @etag = img.meta['etag']
      @data = img.read
      @sha1 = Digest::SHA1.hexdigest @data
    end
    write
  end

  def write
    raise "data is empty" unless @data
    filename = Time.now.strftime("%Y%m%dT%H%M%S%z") + ".jpg"
    File.open(filename, 'wb') {|fo| fo.write @data}
    puts "#{Time.now} Created #{filename} with SHA-1 #{@sha1}"
  end

  def update
    begin
      open(@uri, 'If-None-Match' => @etag) do |img|
        @etag = img.meta['etag']
        @data = img.read
        @sha1 = Digest::SHA1.hexdigest @data
      end
      write
    rescue OpenURI::HTTPError
      puts "#{Time.now} No file available or file has not changed."
    end
  end

end

# Reads image into memory, sets etag, writes file
@img = CreekImage.new

# Update every 5-minutes
20.times do
  @img.update
  sleep 300
end

# If you have ImageMagick installed you can animate:
# $ convert *.jpg -loop 0 creek.gif