Lens

Lens is a simple express.js server which hosts visual assets. It will also compress images for you using next-gen formats.

I built Lens as a way to cope with next.js loading images slowly all the time (since it doesn't persist compressed images). Maybe you'll find it useful too, though you might need some elbow grease to get it working in your setup.

Why should you use next/image instead of this?

• It's good enough
• It handles cache busting
• The initial compression is faster and uses fewer resources
• You don't want to write a custom image loader
• You don't want to set up a caching proxy

Why should you use this instead of next/image?:

• If a compressed image isn't ready, it sends the uncompressed one and doesn't block
• It persists compressed images between builds
• It re-generates missing compressed images by keeping a log of image requests
• During compression, it uses SSIM comparison to smartly select image quality levels
• Because of the SSIM comparison, AVIF images end up way smaller than WebP, which more fully realizes their potential

How to configure the SSIM levels

They're in lib/cache.js. I decided on the levels in that file through testing about 220 images, so you might not want to mess with this unless you do the same. (Using abx.html and some elbow grease...)

How to run

Clone this repo, then add a .env file with the following info:

PORT=3001
SOURCE_DIR=/path/to/source/assets/dir
CACHE_DIR=/path/to/image/cache/dir
REGEN_LOG=/path/to/regen/log.json

After that:

yarn

# If developing locally
yarn dev

# If running on a server
yarn start

# If you want it to regenerate the image cache based on previous requests
yarn regen && yarn start

How to access compressed images

Using an <img> tag, request the image you want to compress (.jpg or .png) with this src:

http://localhost:3001/path/to/image/in/SOURCE_DIR/img.jpg?d=1x&w=640

Or this one:

http://localhost:3001/path/to/image/in/SOURCE_DIR/img.jpg?q=ultra&w=640

There are two situations where you won't receive a compressed image in response.

• The compressed image isn't ready (so, your request just started that job)
• You visited the URL directly instead of loading through <img />.

When requesting with <img />, the browser will send the server a different Accept header with info about the next-gen formats it supports. This is how Lens decides on which format to send it. When you visit the image URL directly, it won't receive this header and send you the raw asset instead (without recompression or scaling).

The d query parameter (for "density") is one of 1x, 2x, or 3x. It refers to the target pixel density, optimizing the image for each one. This defaults to 1x if not supplied. It doesn't affect the scale of the final image; just the quality optimization.

The w query parameter (for "width") and determines the width of the final image in pixels. This defaults to the width of the source image if not supplied. It won't scale up.

If you don't like the result of the d parameter, you can replace it with q=ultra. Lens will then return a lightly compressed (but much larger) image to compensate.