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0.1.4 • Public • Published

jest-gl npm version Build Status

jest-gl contains various helper functions allow you to easily write browserless visual tests around your webGL code. The library uses headless-gl, looks-same, and pngjs2 packages to provide a small API for saving, diffing, and comparing images of a mocked webGL context.


  • headless-gl
  • jest-gl does not have a dependency on Jest. However, it was designed with Jest testing in mind.


npm i jest-gl --save-dev
yarn add jest-gl --dev

Getting Started

jest-gl uses file name postfixes to distingush between visual testing images:

  • .spec an image representing the desired look of the webGL application
  • .head an image representing the current look of the webGL applicaton
  • .diff an image representing a comparison of a .spec and .head file with highlighted differences

Names and the highlight color are all optionally configurable to match your organization's preferences. In your test file:

let jestGl = require('../index.js')({
  specPostfix: '.spec', // postfix tag to spec images - default shown
  headPostfix: '.head', // postfix tag to head images - default shown
  diffPostfix: '.diff', // postfix tag to diff images - default shown
  highlightColor: '#ff00ff', // diff highlight color - default shown (hot-pink)

jest-gl helper functions are now available!

// create a headless-gl context (
const gl = require('gl')(500, 300, {preserveDrawingBuffer: true})
describe('gl visual tests', () => {
  beforeAll(() => {
    // mock asynchronous shader requests by reading from the file system
    http.get = jest.fn((url, responseType) =>
      jestGl.fromFile(url, (originalUrl) =>
        (originalUrl.indexOf('vs') > -1) ? './app/shaders/vs.glsl' : './app/shaders/fs.glsl'))
    canvas = jestGl.mockCanvas(gl, {
      width: {value: 300}, // does not indicate final PNG image size
      height: {value: 150}, // does not indicate final PNG image size
      clientWidth: {value: width}, // does not indicate final PNG image size
      clientHeight: {value: height}, // does not indicate final PNG image size
  it('initializes with a square and triangle', async () => {
    await app.init(canvas, gl) //start my webGL application
    let parity = await jestGl.imageParity(gl, __dirname, 'init', true, true)

When you want to update your .spec.png files, set the TEST_MODE environment variable to 'update'.

process.env.TEST_MODE = 'update'

For an example of how to set up your test runner, check out ./runner.js. Example usage:

node runner.js # runs your tests 
node runner.js -u # runs your tests but updates all .spec.png files 


fromFile(url, pathMapFn)

Mocks a network request by loading a local test fixture file instead.

  1. String url requested url

  2. Function pathMapFn function mapping url to a local resource

Returns a Promise containing a local test fixture file or error

mockCanvas(gl, defineProperties)

Creates a mock canvas element to add to the mocked gl context

  1. any gl the webGL context

  2. Object defineProperties collection to map to Object.defineProperties calls

snapshotPng(gl, imagePath, width, height)

Takes a rendered PNG image of the glCotext

  1. any gl the webGL context

  2. String imagePath path to save the screenshot

  3. Number width in px

  4. Number height in px

Returns a Promise resolving to the saved image path

imageParity(gl, dirName, imageName, strict = true, errorDiff = true)

Tests that two glContexts are the same. If found, differences are highlighted in a diff image.

  1. any gl the webGL context

  2. String dirName the image comparison directory

  3. String imageName the name of the image sans postfix and extension (test.spec.js => 'test')

  4. boolean strict do strict comparison (default = true)

  5. boolean errorDiff save a diff image on error (default = true)


  • jest-gl creates and deletes intermediate files. Best to ignore *.head.png and *.diff.png from source control.

Development Setup

npm install
npm run build
npm run test
npm run test:watch


npm i jest-gl

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