CH + DPR negotiation example for Nginx.

nginx.conf

worker_processes 1;

events {
  worker_connections 1024;
}

http {
  include mime.types;
  default_type  application/octet-stream;

  # - capture the DPR resolution from CH header
  # - if no CH header is present, default to 1
  map $http_ch $dpr {
    ~*dpr=(?<val>.*) $val;
    default "1";
  }

  server {
    listen       *:8080;

    # - check for appropriate DPR asset
    # - if appropriate DPR is not available, fallback to original
    location ~* /photos/(?<name>.*)\.(?<ext>.*)$ {
      try_files /photos/$name-$dpr.$ext $uri =404;
    }

    location / {
      root html;
    }
  }
}

test.bash

$> curl -s  http://localhost:8080/photos/awesome.jpg | wc -c
  252680
$> curl -s -H"CH: dpr=1.5" http://localhost:8080/photos/awesome.jpg | wc -c
  381135
$> curl -s -H"CH: dpr=2" http://localhost:8080/photos/awesome.jpg | wc -c
  710175