browser-image-resizer

Introduction

This library allows for cross-browser image downscaling utilizing OffscreenCanvas.

Note

• This is browser-only utility and will not work in Node.js.
• Safari 16.4 or later is required due to the use of OffscreenCanvas.
  https://caniuse.com/offscreencanvas

Installation

NPM/Yarn/pnpm

• npm install @misskey-dev/browser-image-resizer
• yarn add @misskey-dev/browser-image-resizer
• pnpm add @misskey-dev/browser-image-resizer

Usage

In the main thread

import { readAndCompressImage } from 'browser-image-resizer';

const config = {
  quality: 0.7,
  width: 800,
  height: 600
};

// Note: A single file comes from event.target.files on <input>
async function uploadImage(file) {
  try {
    let resizedImage = await readAndCompressImage(file, config);

    const url = `http://localhost:3001/upload`;
    const formData = new FormData();
    formData.append('images', resizedImage);
    const options = {
      method: 'POST',
      body: formData
    };

    let result = await fetch(url, options);

    // TODO: Handle the result
    console.log(result);
    return result;
  } catch (error) {
    console.error(error);
    throw(error);
  }
}

In worker

Even large images can be processed in a separate thread using a worker.

worker.js

import { readAndCompressImage } from "browser-image-resizer";

onmessage = async (e) => {
    const converted = await readAndCompressImage(e.data, { maxWidth: 300 });
    postMessage(converted, [converted]);
}

Main Thread

const worker = new Worker('worker.js');

const img = document.getElementById('viewer_img');
worker.onmessage = (e) => {
  img.src = URL.createObjectURL(e.data);
};

async function convert(file: File) {
  const bmp = await createImageBitmap(file);
  worker.postMessage(bmp, [bmp]);
}

API

readAndCompressImage(file, config) => Promise<Blob | OffscreenCanvas>

Inputs

• file: An image source that createImageBitmap can read.
  See https://developer.mozilla.org/en-US/docs/Web/API/createImageBitmap
• config: See below

Property Name	Purpose	Default Value
argorithm	Algorithm used for downscaling (see below)	'null'
processByHalf	Whether to process downscaling by drawImage(source, 0, 0, source.width / 2, source.height / 2) until the size is smaller than twice the target size.	true
quality	The quality of jpeg (or webp)	0.5
maxWidth	The maximum width for the downscaled image	800
maxHeight	The maximum height for the downscaled image	600
debug	console.log image update operations	false
mimeType	specify image output type other than jpeg / If set null, function returns OffscreenCanvas	'image/jpeg'

argorithm

• null: Just resize with drawImage(). The best quality and fastest.
• bilinear: Better quality, slower. Comes from upstream (ericnogralesbrowser-image-resizer).
• hermite: Worse quality, faster. Comes from viliusle/Hermite-resize. Will dispatch workers for better performance.
• hermite_single: Worse quality, faster. Single-threaded.

Outputs

A Promise that yields an Image Blob or OffscreenCanvas

calculateSize(src, config)

calculateSize(src: { width: number; height: number; }, config) => { width: number; height: number; }

With this function you can get the pre-calculated width and height of the resulting image.

Output Image Specification

The output image is derived from OffscreenCanvas.convertToBlob.
https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas/convertToBlob

• EXIF and other metadata will be erased.
• Rotation will be automatically corrected.
  • It is based on the specifications of recent versions of modern browsers and may not work with older browsers.
  • See w3c/csswg-drafts#4666 (comment)
  • Firefox support seems to be available from version 78. by mei23
• Color profile is srgb. Firefox 97 does not attach the ICC profile, but Chrome does.
• You can specify image/webp as the mimeType but Safari does not support..