ISA -- Image Session Analyzer

Helpers for grabbing screenshots over a time period, analyzing them, and stitching them together for later playback.

We largely use ISA for checking video playback on our device tests.

Why would you use it?

We run a lot of tests that capture screenshots. Often we'll take several screenshots over the duration of a test and we wanted some way of comparing the captures over the test run in order to determine if video playback is working.

ISA performs a diff between screenshots and returns an integer representing the amount of change between screenshots. A value of zero indicates the screenshots are identical. Wheras a value in the thousands indicates a large differential between the images. We find this diff technique gives us a good indication that video is playing in our tests, and will also spot buffering problems and crashes (the diff value drops significantly).

ISA also stitches together the screenshots its taken over a session to create an animated gif that is a handy reference for checking why a particular test failed.


The gem is very simple and doesn't do any capture itself -- you'll need to have some mechanism for doing that yourself. We use the device_api gem for grabbing screenshots from physical android and ios devices.

The gem uses ImageMagick to perform the screenshot comparisons -- you will need to install this dependency first.

Usage example

You first need to define a lambda for creating a screenshot. For example, using the device_api we might do:

take_screenshot = ->(filename) {
  device.screenshot(:filename => filename)

You can then use that lambda with ISA to establish a capture session, giving it a directory where you want the screen shots.

session = :name => 'test-01', :capture => take_screenshot, :dir => 'captures' )

# Call capture for the initial screenshot
frame = session.capture

# session.diff performs a screenshot and does the diff
10.times do
  puts session.diff

# Finish the session and create the composite
file = session.end( './composite.gif' )


ISA is available to everyone under the terms of the MIT open source licence. Take a look at the LICENSE file in the code.

Copyright (c) 2015 BBC