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 and PSD. 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, and uses the Addressable library to do so.

FastImage will 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 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.


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]



gem install fastimage


Add fastimage to your Gemfile, and bundle.

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: </code>

<code># Running tests:</code>

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





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

  • @felixbuenemann
  • @speedmax
  • @sebastianludwig
  • @benjaminjackson
  • @muffinista
  • @marcandre
  • @apanzerj