Last week, I described how to add a dynamic watermark to your images on the JVM. I didn't find any library, so I had to develop the feature, or, more precisely, an embryo of a feature, by myself. Depending on your tech stack, you must search for an existing library or roll up your sleeves. For example, Rust offers such an out-of-the-box library. Worse, this approach might be impossible to implement if you don't have access to the source image.

Another alternative is to use ready-made components, namely imgproxy and Apache APISIX. I already combined them to resize images on-the-fly.

Here's the general sequence flow of the process:

• When APISIX receives a specific pattern, it calls imgproxy with the relevant parameters
• imgproxy fetches the original image and the watermark to apply
• It watermarks the original image and returns the result to APISIX

Let's say the pattern is /watermark/*.

We can define two routes:

routes:
  - uri: "*"                                                                     #1
    upstream:
      nodes:
        "server:3000": 1
  - uri: /watermark/*                                                            #2
    plugins:
      proxy-rewrite:                                                             #3
        regex_uri:
          - /watermark/(.*)
          - /dummy_sig/watermark:0.8:nowe:20:20:0.2/plain/http://server:3000/$1  #4
    upstream:
      nodes:
        "imgproxy:8080": 1                                                       #5

1. Catch-all route that forwards to the web server
2. Watermark images route
3. Rewrite the URL...
4. ...with an imgproxy-configured route and...
5. ...forward to imageproxy

You can find the exact rewritten URL syntax in imgproxy documentation. The watermark itself is configured via a single environment variable. You should buy imgproxy's Pro version if you need different watermarks. As a poor man's alternative, you could also set up different instances, each with its watermark, and configure APISIX to route the request to the desired instance.

In this post, we implemented a watermarking feature with the help of imgproxy. The more I think about it, the more I think they make a match made in Heaven.

The complete source code for this post can be found on GitHub:

To go further:

• Digital watermarking
• imgproxy documentation
• imgproxy interactive demo

Originally published at A Java Geek on July 7^th, 2024