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 https://github.com/w3c/csswg-drafts/issues/4666#issuecomment-610962845
  • 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..