Impress

Optimizes images by resizing, compressing and converting them to the best format supported by the client

Setting up the plugin

First download the libvmod_impress.so file from the latest release and place it in the vmod folder.

Below is the minimal varnish configuration to use the plugin. However you should also configure varnish to sanitize and cache based on the Accept HTTP header for your images.

vcl 4.1;

import impress;

backend default none;

sub vcl_init {
    new images = impress.new("/etc/varnish/impress.ron");
}

sub vcl_recv {
    set req.backend_hint = images.backend();
}

Create the /etc/varnish/impress.ron configuration file, the following is an example configuration file, see below for more details :

Config(
    extensions: [AVIF, WEBP],
    default_format: JPEG,
    qualities: {WEBP: 70, AVIF: 40},
    root: "/var/www/media",
    url: "/media/{size}/{path}.{ext}",
    cache_directory: "/var/cache/varnish",
    sizes: {
        "low": Size(width: 300, height: 300, qualities: {WEBP: 90, JPEG: 100}),
        "medium": Size(width: 600, height: 600),
        "high": Size(width: 1200, height: 1200),
        "product": Size(width: 546, height: 302, pattern: "^products/", pre_optimize: true),
    },
    logger: Logger(
        path: "/var/log/impress.log",
        level: WARN
    ),
)

Configuration

Config

The Config struct has the following fields :

• extensions : List of supported image formats, the order in the array defines the priority, currently only webp and avif are supported
• default_format : Default image format to use when the client does not support any of the supported formats. Currently ignored and images do not get optimized when falling back to this format, the original image format will be served
• qualities : Quality when compressing images the default value is {AVIF: 40, WEBP: 70, JPEG: 90}. Can be overriden in the size configuration
• root : Root directory where images are stored
• url : URL pattern to match and extract the image size, path and extension from
• cache_directory : Directory to store the optimized and resized images
• sizes : Map of image sizes and their configurations, see below
• logger : Logger configuration, leave empty to disable

Sizes

You can add multiple sizes to the sizes map, each size has the following fields :

• width : Maximum width to resize the image to
• height : Maximum height to resize the image to
• qualities : Quality when compressing images the default value is {AVIF: 40, WEBP: 70, JPEG: 90}. Overrides the qualities specified in the Config object
• pattern : Regex pattern to match the {path} variable in the URL pattern, if the path does not match a 404 will be returned
• pre_optimize : If set to true, a thread will be spawned to optimize all the matching images to this format. It is recommanded to also set a pattern if not all images will be served in this format to avoid generating a lot of useless files

Logger

Configures the logger, leave empty to deactivate the logger

• path : Log file path
• level : Minimum level of log, levels below will be filtered out

Running the project

First start the container which contains varnish and all the necessary tools to build the plugin :

docker compose up -d

Each time a change is made either to the rust code or to the varnish configuration file and you want to recompile the plugin, run this command :

docker exec vmod-impress /build.sh

Running this will also copy the compiled plugin and documentations into the out directory.

Todo

• Add support for AVIF and JPEG
• Support fetching images from another backend ?
• Tests
• Don't start pre optimizer thread if no image is set to pre optimize