Finding Photos on Flickr

Problem

You want to use Ruby code to find freely reusable photos: perhaps to automatically illustrate a piece of text.

Solution

The Flickr photo-sharing web site has a huge number of photos and provides web services for searching them. Many of the photos are licensed under Creative Commons licenses, which give you permission to reuse the photos under various restrictions.

There are several Ruby bindings to Flickrs various web service APIs, but its REST API is so simple that Im just going to use it directly. Given a tag name (like "elephants"), this code will find an appropriate picture, and return the URL to a thumbnail version of the picture.

First, a bit of setup. As with Amazon and Google, to use the Flickr API at all youll need to sign up for an API key (see below for details).

require open-uri require exml/document require cgi FLICKR_API_KEY = Your API key here

The first method, flickr_call, sends a generic query to Flickrs REST web service. It doesn do anything special: it just makes an HTTP GET request and parses the XML response.[5]

[5] Some of Flickrs APIs let you do things like upload photos and add comments. Youll need to use POST requests to make these calls, since they modify the state of the site. More importantly, youll also need to authenticate against your Flickr account.

def flickr_call(method_name, arg_map={}.freeze) args = arg_map.collect {|k,v| CGI.escape(k) + = + CGI.escape(v)}.join(&) url = "http://www.flickr.com/services/rest/?api_key=%s&method=%s&%s" % [FLICKR_API_KEY, method_name, args] doc = REXML::Document.new(open(url).read) end

Now comes pick_a_photo, a method that uses flickr_call to invoke the flickr.photos.search web service method. That method returns a REXML Document object containing a

element for each photo that matched the search criteria. I use XPath to grab the first

element, and pass it into small_photo_url (defined below) to turn it into an image URL.

def pick_a_photo(tag) doc = flickr_call(flickr.photos.search, ags => tag, license => 4, per_page => 1) photo = REXML::XPath.first(doc, //photo) small_photo_url(photo) if photo end

Finally, Ill define the method, small_photo_url. Given a

element, it returns the URL to a smallish version of the appropriate Flickr photo.

def small_photo_url(photo) server, id, secret = [server, id, secret].collect do |field| photo.attribute(field) end "http://static. flickr.com/#{server}/#{id}_#{secret}_m.jpg" end

Now I can find an appropriate photo for any common word (Figure 16-1):

pick_a_photo(elephants) # => http://static. flickr.com/32/102580480_506d5865d0_m.jpg pick_a_photo(what-will-happen-tomorrow) # => nil

Figure 16-1. A photo of elephants by Nick Scott-Smith

Discussion

Its nice if theres a predefined Ruby binding available for a particular REST-style web service, but its usually pretty easy to roll your own. All you need to do is to craft an HTTP request and figure out how to process the response document. Its usually an XML document, and a well-crafted XPath statement should be enough to grab the data you want.

Note the clause license=4 in pick_a_photos arguments to flickr_call. I wanted to find a picture that I could publish in this book, so I limited my search to pictures made available under a Creative Commons "Attribution" license. I can reproduce that picture of the elephants so long as I credit the person who took the photo. (Nick Scott-Smith of London. Hi, Nick!)

Flickr has a separate API call that lists the available licenses (flickr.licenses.getInfo), but once I looked them up and found that "Creative Commons Attribution" was number four, it was easier to hardcode the number than to look it up every time.

See Also

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