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FastImage finds the size or type of an image given its uri by fetching as little as needed



FastImage finds the size or type of an image given its uri by fetching as little as needed

The problem

Your app needs to find the size or type of an image. This could be for adding width and height attributes to an image tag, for adjusting layouts or overlays to fit an image or any other of dozens of reasons.

But the image is not locally stored – it’s on another asset server, or in the cloud – at Amazon S3 for example.

You don’t want to download the entire image to your app server – it could be many tens of kilobytes, or even megabytes just to get this information. For most common image types (GIF, PNG, BMP etc.), the size of the image is simply stored at the start of the file. For JPEG files it’s a little bit more complex, but even so you do not need to fetch much of the image to find the size.

FastImage does this minimal fetch for image types GIF, JPEG, PNG, TIFF, BMP, ICO, CUR, PSD, SVG and WEBP. And it doesn’t rely on installing external libraries such as RMagick (which relies on ImageMagick or GraphicsMagick) or ImageScience (which relies on FreeImage).

You only need supply the uri, and FastImage will do the rest.


Fastimage can also read local (and other) files – anything that is not parseable as a URI will be
interpreted as a filename, and FastImage will attempt to open it with File#open.

FastImage will also automatically read from any object that responds to :read – for
instance an IO object if that is passed instead of a URI.

FastImage will follow up to 4 HTTP redirects to get the image.

FastImage will obey the http_proxy setting in your environment to route requests via a proxy. You can also pass a :proxy argument if you want to specify the proxy address in the call.

You can add a timeout to the request which will limit the request time by passing :timeout => number_of_seconds.

FastImage normally replies will nil if it encounters an error, but you can pass :raise_on_failure => true to get an exception.

FastImage also provides a reader for the content length header provided in HTTP. This may be useful to assess the file size of an image, but do not rely on it exclusively – it will not be present in chunked responses for instance.

FastImage accepts additional HTTP headers. This can be used to set a user agent or referrer which some servers require. Pass an :http_header argument to specify headers, e.g., :http_header => {’User-Agent’ => ‘Fake Browser’}.


As of v1.6.7 FastImage no longer uses openuri to open files, but directly calls But take care to sanitise the strings passed to FastImage; it will try to read from whatever is passed.


require 'fastimage'

=> [266, 56]  # width, height
=> :png
=> :gif
FastImage.size("", :raise_on_failure=>true, :timeout=>0.1)
=> FastImage::ImageFetchFailure: FastImage::ImageFetchFailure
FastImage.size("", :raise_on_failure=>true, :timeout=>2.0)
=> [9545, 6623]"").content_length
=> 432
FastImage.size("", :http_header => {'User-Agent' => 'Fake Browser'})
=> [266, 56]



gem install fastimage


Add fastimage to your Gemfile, and bundle.

gem 'fastimage'

Then you’re off – just use FastImage.size() and FastImage.type() in your code as in the examples.



It’s way faster than conventional methods (for example the image_size gem) for most types of file when fetching over the wire.

irb> uri = ""
irb> puts Benchmark.measure {open(uri, 'rb') {|fh| p}}
[9545, 6623]
  0.680000   0.250000   0.930000 (  7.571887)
irb> puts Benchmark.measure {p FastImage.size(uri)}
[9545, 6623]
  0.010000   0.000000   0.010000 (  0.090640)

The file is fetched in about 7.5 seconds in this test (the number in brackets is the total time taken), but as FastImage doesn’t need to fetch the whole thing, it completes in less than 0.1s.

You’ll see similar excellent results for the other file types, except for TIFF. Unfortunately TIFFs tend to have their
metadata towards the end of the file, so it makes little difference to do a minimal fetch. The result shown below is
mostly dependent on the exact internet conditions during the test, and little to do with the library used.

irb> uri = ""
irb> puts Benchmark.measure {open(uri, 'rb') {|fh| p}}
[1120, 1559]
  1.080000   0.370000   1.450000 ( 13.766962)
irb> puts Benchmark.measure {p FastImage.size(uri)}
[1120, 1559]
  3.490000   3.810000   7.300000 ( 11.754315)


You’ll need to gem install fakeweb and possibly also gem install test-unit to be able to run the tests.

$ ruby test.rb 
Run options: 

# Running tests:

Finished tests in 1.033640s, 23.2189 tests/s, 82.2337 assertions/s.             
24 tests, 85 assertions, 0 failures, 0 errors, 0 skips


FastImage in other languages




Pull requests and suggestions are always welcome. Thanks to all the contributors!

  • @felixbuenemann
  • @speedmax
  • @sebastianludwig
  • @benjaminjackson
  • @muffinista
  • @marcandre
  • @apanzerj
  • @forresty
  • kikihakiem
  • lulalalalistia
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