Create placeholder images with sharp Node.js image processing library

post, Mar 27, 2020 on Mitja Felicijan's blog

I have been searching for a solution to pre-generate some placeholder images for +image server I needed to develop that resizes images on S3. I though this would +be a 15min job and quickly found out how very mistaken I was.

Even though Node.js is not really the best way to do this kind of things (surely +something written in C or Rust or even Golang would be the correct way to do +this but we didn't need the speed in our case) I found an excellent library +sharp - High performance Node.js image +processing.

Getting things running was a breeze.

Fetch image from S3 and save resized

const sharp = require('sharp');
+const aws = require('aws-sdk');
+
+const x,y = 100;
+const s3 = new aws.S3({});
+
+aws.config.update({
+  secretAccessKey: 'secretAccessKey',
+  accessKeyId: 'accessKeyId',
+  region: 'region'
+});
+
+const originalImage = await s3.getObject({
+  Bucket: 'some-bucket-name',
+  Key: 'image.jpg',
+}).promise();
+
+const resizedImage = await sharp(originalImage.Body)
+  .resize(x, y)
+  .jpeg({ progressive: true })
+  .toBuffer();
+
+s3.putObject({
+  Bucket: 'some-bucket-name',
+  Key: `optimized/${x}x${y}/image.jpg`,
+  Body: resizedImage,
+  ContentType: 'image/jpeg',
+  ACL: 'public-read'
+}).promise();
+

All this code was wrapped inside a web service with some additional security +checks and defensive coding to detect if key is missing on S3.

And at that point I needed to return placeholder images as a response in case +key is missing or x,y are not allowed by the server etc. I could have created +PNG in Gimp and just serve them but I wanted to respect aspect ratio and I +didn't want to return some mangled images.

Main problem with finding a clean solution I could copy and paste and change a +bit was a task. API is changing constantly and there weren't clear examples or +I was unable to find them.

Generating placeholder images using SVG

What I ended up was using SVG to generate text and created image with sharp and +used composition to combine both layers. Response returned by this function is a +buffer you can use to either upload to S3 or save to local file.

const generatePlaceholderImageWithText = async (width, height, message) => {
+  const overlay = `<svg width="${width - 20}" height="${height - 20}">
+    <text x="50%" y="50%" font-family="sans-serif" font-size="16" text-anchor="middle">${message}</text>
+  </svg>`;
+
+  return await sharp({
+    create: {
+      width: width,
+      height: height,
+      channels: 4,
+      background: { r: 230, g: 230, b: 230, alpha: 1 }
+    }
+  })
+    .composite([{
+      input: Buffer.from(overlay),
+      gravity: 'center',
+    }])
+    .jpeg()
+    .toBuffer();
+}
+

That is about it. Nothing more to it. You can change the color of the image by +changing background and if you want to change text styling you can adapt SVG +to your needs.

Also be careful about the length of the text. This function positions text at +the center and adds 20px padding on all sides. If text is longer than the +image it will get cut.