Lava Frame Compare

Helpers for analysing video frames and screenshots over a testing session, analyzing them, and stitching them together for later playback.

We use our lava libs for checking video playback on our device tests.

Why would you use it?

We run a lot of tests that capture screenshots or look at video playback. Often we'll take several screenshots or video samples 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.

Lava performs a diff between frames and returns an integer representing the amount of change between screenshots. A value of zero indicates the frames are identical. Whereas a value in the thousands indicates a large differential between the frames. 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).

Lava also stitches together the frames captured over a session to create an animated gif that is a handy reference for checking why a particular test failed.

Dependencies

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

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

Usage example

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

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

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

session = Lava::FrameCompare.start_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
end

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

There are a number of options for generating the composit file:

# Create a significantly smaller gif file by skipping every other frame
session.end( :step => 2 )

# Speed up the gif playback
session.end( :speed => 5) # 5x speed

# Rotate the image
session.end( :rotate => 90 ) # 90 degrees

# Set the max dimensions of the image
session.end( :max_width => 100, :max_height => 100 )

License

Lava 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) 2016 BBC