PngPong Image Manipulation Library

For the recent UK Elections the folks The Guardian wanted to show big images along with their push notifications. Being bandwidth-aware they wanted a solution in which they could use a template image (which could then be cached) and then draw some stuff onto it. Only problem: Service Workers don’t have access to the Canvas API.

Enter PngPong, an basic image manipulation library, they’ve created:

PngPong is a very, very basic replacement for the Canvas API in environments that do not support it – primarily, service workers. Instead, it manually manipulates the bytes of a PNG file to copy the contents of another image, or draw basic shapes (currently only rectangles).

Their code to handle push notifications looked something like this:

self.addEventListener('push', (e) => {
        .then((res) => res.arrayBuffer())
        .then((arrayBuffer) => {
            return blobToDataURL(new Blob([arrayBuffer]))
        .then((imageURL) => {
            return self.registration.showNotification("Title", {
                image: imageURL

Inside addResultsToTemplate they heavy lifting would be done by PngPong, drawing some small rectangles onto the template pictured below, yielding a push notification as pictured at the top of this post.

The template used

Example PngPong usage:

import {
} from 'png-pong';

// Create new PngPong instance
const pngPong = new PngPong(imageArrayBuffer);

// Draw a 30px red square 10px from the top and 10px from the left
const shape = new PngPongShapeTransformer(pngPong);
shape.drawRect(10, 10, 30, 30, [255, 0, 0])

// Copy a 50x50 image 10px from the top left of the source image,
// and draw it 30px into our target image. 
const toCopyFrom = new ArrayBuffer();
const imageCopy = new PngPongImageCopyTransformer(toCopyFrom, pngPong);
imageCopy.copy(10, 10, 50, 50, 30, 30);

// Run the transforms;

PngPong (GitHub) →
PngPong Use Case: Generating Images in JavaScript Without Using the Canvas API →

Published by Bramus!

Bramus is a frontend web developer from Belgium, working as a Chrome Developer Relations Engineer at Google. From the moment he discovered view-source at the age of 14 (way back in 1997), he fell in love with the web and has been tinkering with it ever since (more …)

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