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

$3

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

Usage

$3

``typescript
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
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);
}
}
`

$3

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

#### worker.js

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

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

#### Main Thread

`typescript
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

$3

#### 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

$3


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.

$3

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..