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timesnap is a Node.js program that records screenshots of web pages that use JavaScript animations. It uses timeweb and puppeteer to open a web page, overwrite its time-handling functions, and record snapshots at virtual times. For some web pages, this allows frames to be recorded slower than real time, while appearing smooth and consistent when recreated into a video.

You can use timesnap from the command line or as a Node.js library. It requires Node v8.9.0 or higher and npm.

To record screenshots and compile them into a video using only one command, see timecut. For using virtual time in browser, see timeweb.

<a name="limitations" href="#limitations">#</a> timeweb and timesnap Limitations

timeweb (and timesnap by extension) only overwrites JavaScript functions and video playback, so pages where changes occur via other means (e.g. through transitions/animations from CSS rules) will likely not render as intended.

Read Me Contents

<a name="from-cli" href="#from-cli">#</a> From the Command Line

<a name="cli-global-install" href="#cli-global-install">#</a> Global Install and Use

To install:

Due to an issue in puppeteer with permissions, timesnap is not supported for global installation for root. You can configure npm to install global packages for a specific user by following this guide: https://docs.npmjs.com/getting-started/fixing-npm-permissions#option-two-change-npms-default-directory

After configuring, run:

npm install -g timesnap

To use:

timesnap "url" [options]

<a name="cli-local-install" href="#cli-local-install">#</a> Local Install and Use

To install:

cd /path/to/installation/directory
npm install timesnap

To use:

node /path/to/installation/directory/node_modules/timesnap/cli.js "url" [options]

Alternatively:

To install:

cd /path/to/installation/directory
git clone https://github.com/tungs/timesnap.git
cd timesnap
npm install

To use:

node /path/to/installation/directory/timesnap/cli.js "url" [options]

<a name="cli-url-use" href="#cli-url-use">#</a> Command Line url

The url can be a web url (e.g. https://github.com) or a file path, with relative paths resolving in the current working directory. If no url is specified, defaults to index.html. Remember to enclose urls that contain special characters (like # and &) with quotes.

<a name="cli-examples" href="#cli-examples">#</a> Command Line Examples

<a name="cli-example-default" href="#cli-example-default">#</a> Default behavior:

timesnap

Opens index.html in the current working directory, sets the viewport to 800x600, captures at 60 frames per second for 5 virtual seconds, and saves the frames to 001.png to 300.png in the current working directory. The defaults may change in the future, so for long-term scripting, it's a good idea to explicitly pass these options, like in the following example.

<a name="cli-example-viewport-fps-duration-output" href="#cli-example-viewport-fps-duration-output">#</a> Setting viewport size, frames per second, duration, and output pattern:

timesnap index.html --viewport="800,600" --fps=60 --duration=5 --output-pattern="%03d.png"

Equivalent to the current default timesnap invocation, but with explicit options. Opens index.html in the current working directory, sets the viewport to 800x600, captures at 60 frames per second for 5 virtual seconds, and saves the frames to 001.png to 300.png in the current working directory.

<a name="cli-example-selector" href="#cli-example-selector">#</a> Using a selector:

timesnap drawing.html -S "canvas,svg" --output-pattern="frames/%03d.png"

Opens drawing.html in the current working directory, crops each frame to the bounding box of the first canvas or svg element, and captures frames using default settings (5 seconds @ 60fps), saving to frames/001.png... frames/300.png in the current working directory, making the directory frames if needed.

<a name="cli-example-offsets" href="#cli-example-offsets">#</a> Using offsets:

timesnap "https://tungs.github.io/amuse/truchet-tiles/#autoplay=true&switchStyle=random" \
  -S "#container" \
  --left=20 --top=40 --right=6 --bottom=30 \
  --duration=20 --output-directory=frames

Opens https://tungs.github.io/amuse/truchet-tiles/#autoplay=true&switchStyle=random (note the quotes in the url are necessary because of the # and &). Crops each frame to the #container element, with an additional crop of 20px, 40px, 6px, and 30px for the left, top, right, and bottom, respectively. Captures frames for 20 virtual seconds at 60fps to frames/0001.png... frames/1200.png in the current working directory, making the directory frames if needed.

<a name="cli-example-piping" href="#cli-example-piping">#</a> Piping:

timesnap https://breathejs.org/examples/Drawing-US-Counties.html \
  -V "1920,1080" -S "#draw-canvas" --fps=60 --duration=10 \
  --round-to-even-width --round-to-even-height \
  --output-stdout | ffmpeg -framerate 60 -i pipe:0 -y -pix_fmt yuv420p video.mp4

Opens https://breathejs.org/examples/Drawing-US-Counties.html, sets the viewport size to 1920x1080, crops each frame to the bounding box of #draw-canvas, records at 60 frames per second for ten virtual seconds, and pipes the output to ffmpeg, which reads in the data from stdin, encodes the frames using pixel format yuv420p, and saves the result as video.mp4 in the current working directory. It does not save individual frames to disk. It uses the --round-to-even-width and --round-to-even-height options to ensure the dimensions of the frames are even numbers, which ffmpeg requires for certain encodings.

<a name="cli-options" href="#cli-options">#</a> Command Line options

<a name="from-node" href="#from-node">#</a> From Node.js

timesnap can also be included as a library inside Node.js programs.

<a name="node-install" href="#node-install">#</a> Node Install

npm install timesnap --save

<a name="node-examples" href="#node-examples">#</a> Node Examples

<a name="node-example-basic" href="#node-example-basic">#</a> Basic Use:

const timesnap = require('timesnap');
timesnap({
  url: 'https://tungs.github.io/amuse/truchet-tiles/#autoplay=true&switchStyle=random',
  viewport: {
    width: 800,               // sets the viewport (window size) to 800x600
    height: 600
  },
  selector: '#container',     // crops each frame to the bounding box of '#container'
  left: 20, top: 40,          // further crops the left by 20px, and the top by 40px
  right: 6, bottom: 30,       // and the right by 6px, and the bottom by 30px
  fps: 30,                    // saves 30 frames for each virtual second
  duration: 20,               // for 20 virtual seconds
  outputDirectory: 'frames'   // to frames/001.png... frames/600.png
                              // of the current working directory
}).then(function () {
  console.log('Done!');
});

<a name="node-example-multiple" href="#node-example-multiple">#</a> Multiple pages:

const timesnap = require('timesnap');
var pages = [
  {
    url: 'https://tungs.github.io/amuse/truchet-tiles/#autoplay=true',
    outputDirectory: 'truchet-tiles'
  }, {
    url: 'https://breathejs.org/examples/Drawing-US-Counties.html',
    outputDirectory: 'counties'
  }
];
(async () => {
  for (let page of pages) {
    await timesnap({
      url: page.url,
      outputDirectory: page.outputDirectory,
      viewport: {
        width: 800,
        height: 600
      },
      duration: 20
    });
  }
})();

<a name="node-api" href="#node-api">#</a> Node API

The Node API is structured similarly to the command line options, but there are a few options for the Node API that are not accessible through the command line interface: config.logToStdErr, config.frameProcessor, config.navigatePageToURL, config.preparePage, config.preparePageForScreenshot, config.logger, and config.shouldSkipFrame.

timesnap(config)

<a name="modes" href="#modes">#</a> timesnap Modes

timesnap can capture frames using one of two modes:

<a name="how-it-works" href="#how-it-works">#</a> How it works

timesnap uses puppeteer's page.evaluateOnNewDocument feature to automatically overwrite a page's native time-handling JavaScript functions and objects (new Date(), Date.now, performance.now, requestAnimationFrame, setTimeout, setInterval, cancelAnimationFrame, cancelTimeout, and cancelInterval) to custom ones that use a virtual timeline, allowing for JavaScript computation to complete before taking a screenshot.

This work was inspired by a talk by Noah Veltman, who described altering a document's Date.now and performance.now functions to refer to a virtual time and using puppeteer to change that virtual time and take snapshots.