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Helper for resemble.js, used for Image comparison in Tests with WebdriverIO

codeceptjs-resemblehelper is CodeceptJS helper which can be used to compare screenshots and make the tests fail/pass based on the tolerance allowed

If two screenshot comparisons have difference greater then the tolerance provided, the test will fail.

NPM package:

To install the package, just run npm install codeceptjs-resemblehelper


This helper should be added in codecept.json/codecept.conf.js


   "helpers": {
     "ResembleHelper" : {
       "require": "codeceptjs-resemblehelper",
       "screenshotFolder" : "./tests/output/",
       "baseFolder": "./tests/screenshots/base/",
       "diffFolder": "./tests/screenshots/diff/"

To use the Helper, users must provide the three parameters:

screenshotFolder : This will always have the same value as output in Codecept configuration, this is the folder where webdriverIO saves a screenshot when using I.saveScreenshot method

baseFolder: This is the folder for base images, which will be used with screenshot for comparison

diffFolder: This will the folder where resemble would try to store the difference image, which can be viewed later,

Usage, these are major functions that help in visual testing

First one is the seeVisualDiff which basically takes two parameters

  1. baseImage Name of the base image, this will be the image used for comparison with the screenshot image, it is mandatory to have the same image file names for base and screenshot image
  2. options options can be passed which include prepaseBaseImage and tolerance
     * Check Visual Difference for Base and Screenshot Image
     * @param baseImage         Name of the Base Image (Base Image path is taken from Configuration)
     * @param options           Options ex {prepareBaseImage: true, tolerance: 5} along with Resemble JS Options, read more here:
     * @returns {Promise<void>}
    async seeVisualDiff(baseImage, options) {}

Second one is the seeVisualDiffForElement which basically compares elements on the screenshot, Selector for element must be provided

It is exactly same as seeVisualDiff function, only an additional selector CSS|XPath|ID locators is provided

     * See Visual Diff for an Element on a Page
     * @param selector   Selector which has to be compared, CSS|XPath|ID
     * @param baseImage  Base Image for comparison
     * @param options    Options ex {prepareBaseImage: true, tolerance: 5} along with Resemble JS Options, read more here:
     * @returns {Promise<void>}
    async seeVisualDiffForElement(selector, baseImage, options){}

Note: seeVisualDiffForElement only works when the page for baseImage is open in the browser, so that webdriver can fetch coordinates of the provided selector

Finally to use the helper in your test, you can write something like this:

Feature('to verify monitoried Remote Db instances');

Scenario('Open the System Overview Dashboard', async (I, adminPage, loginPage) => {
    adminPage.navigateToDashboard("OS", "System Overview");
    adminPage.viewMetric("CPU Usage");

Scenario('Compare CPU Usage Images', async (I) => {

    // setting tolerance and prepareBaseImage in the options array
    I.seeVisualDiff("Complete_Metric_Image.png", {prepareBaseImage: false, tolerance: 5});

    // passing a selector, to only compare that element on both the images now

    // We need to navigate to that page first, so that webdriver can fetch coordinates for the selector
    adminPage.navigateToDashboard("OS", "System Overview");
    I.seeVisualDiffForElement("//div[@class='panel-container']", "Complete_Dashboard_Image.png", {prepareBaseImage: false, tolerance: 3});
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