#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.

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.

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.

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 )

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