Capybara::Screenshot::Diff

Ever wondered what your project looked like two years ago? To answer that, you start taking screen shots during your tests. Capybara provides the save_screenshot method for this. Very good.

Ever introduced a graphical change unintended? Never want it to happen again? Then this gem is for you! Use this gem to detect changes in your pages by taking screen shots and comparing them to the previous revision.

Installation

Add these lines to your application's Gemfile:

gem 'capybara-screenshot-diff'
gem 'oily_png', platform: :ruby

And then execute:

$ bundle

Or install it yourself as:

$ gem install capybara-screenshot-diff

Requirements

• [for :vips driver] libvips 8.9 or later, see the libvips install instructions

Usage

Minitest

In your test class, include the Capybara::Screenshot::Diff module:

class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
  include Capybara::Screenshot::Diff
  # ...
end

RSpec

describe 'Permissions admin', type: :feature, js: true do

  include Capybara::Screenshot::Diff

  it 'works with permissions' do
    visit('/')
    screenshot 'home_page'
  end

end

But it's better to include it within your *_helper.rb file so that it can used anywhere in your feature specs.

# spec/feature_helper.rb
require 'capybara/screenshot/diff'

RSpec.configure do |config|
  config.include Capybara::Screenshot::Diff
end

Cucumber

Load Cucumber support by adding the following line (typically to your features/support/env.rb file):

require 'capybara/screenshot/diff/cucumber'

And in the steps you can use:

Then('I should not see any visual difference') do
  screenshot 'homepage'
end

Taking screenshots

Add screenshot '<my_feature>' to your tests. The screenshot will be saved in the doc/screenshots directory.

Change your existing save_screenshot calls to screenshot

test 'my useful feature' do
  visit '/'
  screenshot 'welcome_index'
  click_button 'Useful feature'
  screenshot 'feature_index'
  click_button 'Perform action'
  screenshot 'action_performed'
end

This will produce a sequence of images like this

doc
  screenshots
    action_performed
    feature_index
    welcome_index

To store the screen shot history, add the doc/screenshots directory to your version control system (git, svn, etc).

Screen shots are compared to the previously COMMITTED version of the same screen shot.

Screenshot groups

Commonly it is useful to group screenshots around a feature, and record them as a sequence. To do this, add a screenshot_group call to the start of your test.

test 'my useful feature' do
  screenshot_group 'useful_feature'
  visit '/'
  screenshot 'welcome_index'
  click_button 'Useful feature'
  screenshot 'feature_index'
  click_button 'Perform action'
  screenshot 'action_performed'
end

This will produce a sequence of images like this

doc
  screenshots
    useful_feature
      00-welcome_index
      01-feature_index
      02-action_performed

All files in the screenshot group directory will be deleted when screenshot_group is called.

Screenshot sections

You can introduce another level above the screenshot group called a screenshot_section. The section name is inserted just before the group name in the save path. If called in the setup of the test, all screenshots in that test will get the same prefix:

setup do
  screenshot_section 'my_feature'
end

test 'my subfeature' do
  screenshot_group 'subfeature'
  visit '/feature'
  click_button 'Interesting button'
  screenshot 'subfeature_index'
  click_button 'Perform action'
  screenshot 'action_performed'
end

This will produce a sequence of images like this

doc
  screenshots
    my_feature
      subfeature
        00-subfeature_index
        01-action_performed

Setting screenshot_section and/or screenshot_group for all tests

Setting the screenshot_section and/or screenshot_group for all tests can be done in the super class setup:

class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
  setup do
    screenshot_section class_name.underscore.sub(/(_feature|_system)?_test$/, '')
    screenshot_group name[5..-1]
  end
end

screenshot_section and/or screenshot_group can still be overridden in each test.

Capturing one area instead of the whole page

You can crop images before comparison to be run, by providing region to crop as [left, top, right, bottom] or by css selector like body .tag

test 'the cool' do
  visit '/feature'
  screenshot 'cool_element', crop: '#my_element'
end

Note: When using a retina device screenshots dimensions might be off. If you are using (headless) chrome you can prevent this by setting the force-device-scale-factor argument to 1.

For Rails system specs using selenium you can do so for example by using the following snippet:

driven_by :selenium, using: :chrome_headless do |options|
  options.args << '--force-device-scale-factor=1'
end

Multiple Capybara drivers

Often it is useful to test your app using different browsers. To avoid the screenshots for different Capybara drivers to overwrite each other, set

Capybara::Screenshot.add_driver_path = true

The example above will then save your screenshots like this (for poltergeist and selenium):

doc
  screenshots
    poltergeist
      useful_feature
        00-welcome_index
        01-feature_index
        02-action_performed
    selenium
      useful_feature
        00-welcome_index
        01-feature_index
        02-action_performed

Multiple OSs

If you run your tests on multiple operating systems, you will most likely find the screen shots differ. To avoid the screenshots for different OSs to overwrite each other, set

Capybara::Screenshot.add_os_path = true

The example above will then save your screenshots like this (for Linux and Windows):

doc
  screenshots
    linux
      useful_feature
        00-welcome_index
        01-feature_index
        02-action_performed
    windows
      useful_feature
        00-welcome_index
        01-feature_index
        02-action_performed

If you combine this config with the add_driver_path config, the driver will be put in front of the OS name.

Screen size

You can specify the desired screen size using

Capybara::Screenshot.window_size = [1024, 768]

This will force the screen shots to the given size, and skip taking screen shots unless the desired window size can be achieved.

Disabling screen shots

If you want to skip taking screen shots, set

Capybara::Screenshot.enabled = false

You can of course set this by an environment variable

Capybara::Screenshot.enabled = ENV['TAKE_SCREENSHOTS']

Disabling diff

If you want to skip the assertion for change in the screen shot, set

Capybara::Screenshot::Diff.enabled = false

Using an environment variable

Capybara::Screenshot::Diff.enabled = ENV['COMPARE_SCREENSHOTS']

Screen shot save path

By default, Capybara::Screenshot::Diff saves screenshots to a doc/screenshots folder, relative to either Rails.root (if you're in Rails), or your current directory otherwise.

If you want to change where screenshots are saved to, then there are two configuration options that that are relevant.

The most likely one you'll want to modify is ...

Capybara::Screenshot.save_path = "other/path"

The save_path option is relative to Capybara::Screenshot.root.

Capybara::Screenshot.root defaults to either Rails.root (if you're in Rails) or your current directory. You can change it to something entirely different if necessary, such as when using an alternative web framework.

Capybara::Screenshot.root = Hanami.root

Screen shot stability

To ensure that animations are finished before saving a screen shot, you can add a stability time limit. If the stability time limit is set, a second screen shot will be taken and compared to the first. This is repeated until two subsequent screen shots are identical.

Capybara::Screenshot.stability_time_limit = 0.1

This can be overridden on a single screenshot:

test 'stability_time_limit' do
  visit '/'
  screenshot 'index', stability_time_limit: 0.5
end

Maximum wait limit

When the stability_time_limit is set, but no stable screenshot can be taken, a timeout occurs. The timeout occurs after Capybara.default_max_wait_time, but can be overridden by an option.

test 'max wait time' do
  visit '/'
  screenshot 'index', wait: 20.seconds
end

Hiding the caret for active input elements

In Chrome the screenshot includes the blinking input cursor. This can make it impossible to get a stable screenshot. To get around this you can set the hide caret option:

Capybara::Screenshot.hide_caret = true

This will make the cursor (caret) transparent (invisible), so the blinking does not delay the screen shot.

Removing focus from the active element

Another way to avoid the cursor blinking is to set the blur_active_element option:

Capybara::Screenshot.blur_active_element = true

This will remove the focus from the active element, removing the blinking cursor.

Allowed color distance

Sometimes you want to allow small differences in the images. For example, Chrome renders the same page slightly differently sometimes. You can set set the color difference threshold for the comparison using the color_distance_limit option to the screenshot method:

test 'color threshold' do
  visit '/'
  screenshot 'index', color_distance_limit: 30
end

The difference is calculated as the eucledian distance. You can also set this globally:

Capybara::Screenshot::Diff.color_distance_limit = 42

Allowed shift distance

Sometimes you want to allow small movements in the images. For example, jquer-tablesorter renders the same table slightly differently sometimes. You can set set the shift distance threshold for the comparison using the shift_distance_limit option to the screenshot method:

test 'color threshold' do
  visit '/'
  screenshot 'index', shift_distance_limit: 2
end

The difference is calculated as maximum distance in either the X or the Y axis. You can also set this globally:

Capybara::Screenshot::Diff.shift_distance_limit = 1

Note: For each increase in shift_distance_limit more pixels are searched for a matching color value, and this will impact performance severely if a match cannot be found.

If shift_distance_limit is nil shift distance is not measured. If shift_distance_limit is set, even to 0, shift distance is measured and reported on image differences.

Allowed difference size

You can set set a threshold for the differing area size for the comparison using the area_size_limit option to the screenshot method:

test 'area threshold' do
  visit '/'
  screenshot 'index', area_size_limit: 17
end

The difference is calculated as width * height. You can also set this globally:

Capybara::Screenshot::Diff.area_size_limit = 42

Skipping an area

Sometimes you have expected change that you want to ignore. You can use the skip_area option with [left, top, right, bottom] or css selector like '#footer' or '.container .skipped_element' to the screenshot method to ignore an area:

test 'unstable area' do
  visit '/'
  screenshot 'index', skip_area: [[17, 6, 27, 16], '.container .skipped_element', '#footer']
end

The arguments are [left, top, right, bottom] for the area you want to ignore. You can also set this globally:

Capybara::Screenshot::Diff.skip_area = [0, 0, 64, 48]

If you need to ignore multiple areas:

screenshot 'index', skip_area: [[0, 0, 64, 48], [17, 6, 27, 16], 'css_selector .element']

Available Image Processing Drivers

There are several image processing supported by this gem. There are several options to setup active driver: :auto, :chunky_png and :vips.

• :auto - will try to load :vips if there is gem ruby-vips, in other cases will load :chunky_png
• :chunky_png and :vips will load correspondent driver

Enable VIPS image processing

Vips driver provides a faster comparison, and could be enabled by adding ruby-vips to Gemfile.

If need to setup explicitly Vips driver, there are several ways to do this:

• Globally: Capybara::Screenshot::Diff.driver = :vips
• Per screenshot option: screenshot 'index', driver: :vips

With enabled VIPS there are new alternatives to process differences, which easier to find and support. For example, shift_distance_limit is very heavy operation. Instead better to use median_filter_window_size.

Tolerance level (vips only)

You can set a “tolerance” anywhere from 0% to 100%. This is the amount of change that's allowable. If the screenshot has changed by more than that amount, it'll flag it as a failure.

This is alternative to "Allowed difference size", only the difference that area calculates including valid pixels. But "tolerance" compares only different pixels.

You can use the tolerance option to the screenshot method to set level:

test 'unstable area' do
  visit '/'
  screenshot 'index', tolerance: 0.3
end

You can also set this globally:

Capybara::Screenshot::Diff.tolerance = 0.3

Median filter size (vips only)

This is an alternative to "Allowed shift distance", but much faster. You can find more about this strategy on Median Filter. Think about this like smoothing of the image, before comparison.

You can use the median_filter_window_size option to the screenshot method to set level:

test 'unstable area' do
  visit '/'
  screenshot 'index', median_filter_window_size: 2
end

Skipping stack frames in the error output

If you would like to override the screenshot method or for some other reason would like to skip stack frames when reporting image differences, you can use the skip_stack_frames option:

test 'test visiting the index' do
  visit root_path
  screenshot :index
end

private

def screenshot(name, **options)
  super(name, skip_stack_frames: 1, **options)
end

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install.

To release a new version, update the version number in lib/capybara/screenshot/diff/version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/donv/capybara-screenshot-diff. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

License

The gem is available as open source under the terms of the MIT License.