How to resize pictures from folder in Node.js with Jimp


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Recently I wrote a few posts on my personal blog with lots of pictures. To save on bandwidth and load time it’s very important to reduce the size of the images. Which until recently I did manually in Preview, MacOS. It was a rather cumbersome manual process, that I automated with the help of Node.js/ExpressJS and Jimp In this post I will presently the implementation.

The code for this nad usage instructions are open source and available on Github

How it is implemented behind the scenes?

Becasue the implementation is so small, everything happens in the router:

const path = require('path');

const express = require('express');
const router = express.Router();
const Jimp = require('jimp');

const fs = require('fs');
const util = require('util');

const readdir = util.promisify(fs.readdir);

router.post('/', async (request, response) => {
    const {width} = request.body;
    if ( !width ) {
        response.status(400).send({"validation_error": "desired width is mandatory"});
    }
    const quality = request.body.quality || 100;
    const numberPrefixOnly = request.body.numberPrefixOnly || false;

    const imgDir = request.body.imgDir || 'images';//fallback to images in input folder if no path is provided
    const outputImgDir = `${imgDir}/resized`;

    let files = await readdir(imgDir, {withFileTypes: true});

    files = files.filter(file => {
        if ( numberPrefixOnly ) {
            return file.name.match(/^\d/) && file.name.match(/\.(jpe?g|png|gif)$/)
        } else {
            return file.name.match(/\.(jpe?g|png|gif)$/)
        }
    });

    for ( const file of files ) {
        const image = await Jimp.read(`${imgDir}/${file.name}`);
        await image.resize(width, Jimp.AUTO);
        await image.quality(quality);
        let newFileName = '';
        if ( file.name.match(/^\d/) ) {
            const photoNumber = file.name.substring(0, file.name.indexOf('-'));
            newFileName = `${photoNumber}-${width}x${image.bitmap.height}-${file.name.substring(file.name.indexOf('-') + 1, file.length)}`;
        } else {
            newFileName = `${width}x${image.bitmap.height}-${file.name}`;
        }

        await image.writeAsync(`${outputImgDir}/${newFileName}`);
        console.log(`${outputImgDir}/${newFileName}`);
    }

    return response.status(201).send();
});

module.exports = router;

The API expects a POST request with the following parameters in the body, where as seen in code some defaults are set:

  • width - required desired width list (height is automatically scaled)
  • quality - (0 to 100)
  • numberPrefixOnly
    • false - default all images in the directory are considered for resizing
    • true - ONLY images with number prefixes are resized (e.g. 4-good-view-zweisimmen-north.jpeg)
  • imgDir (optional) - absolute path where the images are stored
    • the resized images are placed in the ${imgDir}/resized sub-directory
    • if this parameter is not provided the program expects that the images are placed in the images directory. Resized images are then generated in images/resized directory
    • tested only in MacOS. Should work fine in Linux OS. For Windows place the images in the input directory as mentioned above

The whole Jimp part is listed in the following lines of code:

    const image = await Jimp.read(`${imgDir}/${file.name}`);
    await image.resize(width, Jimp.AUTO);
    await image.quality(quality);
    let newFileName = '';
    if ( file.name.match(/^\d/) ) {
        const photoNumber = file.name.substring(0, file.name.indexOf('-'));
        newFileName = `${photoNumber}-${width}x${image.bitmap.height}-${file.name.substring(file.name.indexOf('-') + 1, file.length)}`;
    } else {
        newFileName = `${width}x${image.bitmap.height}-${file.name}`;
    }

    await image.writeAsync(`${outputImgDir}/${newFileName}`);
  • the image is read given its path
  • the resize method expects the width and the height is autoscaled (Jimp.AUTO)
  • the quality is via the quality method (defaults to 90, but with reasonable picture quality still)
  • in the end a new name is generated (depending on the number prefix) and written to the resized folder

Install and start the server with the following commands:

npm install
npm start

Then you can use curl or your favorite api client tool to make POST requests:

curl -0 -v -X POST localhost:9000/resize \
-H 'Content-Type: application/json; charset=utf-8' \
--data-binary @- << EOF
{
    "width": 1200,
    "quality": 90,
    "numberPrefixOnly": false,
    "imgDir": "/Users/ama/Desktop/post-zweisimmen"
}
EOF

It’s as simple as that…

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Adrian Matei

Adrian Matei
Life force expressing itself as a coding capable human being

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