CH + select best candidate based on defined buckets.

nginx.conf

worker_processes 1;
 
events {
  worker_connections 1024;
}
 
http {
  include mime.types;
  default_type  application/octet-stream;
 
  rewrite_log on;
  error_log logs/errog_log debug;
 
  # - capture the DPR resolution from CH header
  # - if no CH header is present, default to 1.0
  map $http_ch $dpr {
    ~*dpr=(?P<val>.*) $val;
    default "1.0";
  }
 
  server {
    listen       *:8080;
 
    # there is probably a cleaner way (TODO).. but for now, it works:
    # check pre-defined buckets to provide "best match" for any given resolution
    if ($dpr = 3.0) { set $c0 "3.0"; set $c1 "2.5"; set $c2 "2.0"; set $c3 "1.5"; set $c4 "1.0"; }
    if ($dpr = 2.5) { set $c0 "2.5"; set $c1 "2.0"; set $c2 "1.5"; set $c3 "1.0"; }
    if ($dpr = 2.0) { set $c0 "2.0"; set $c1 "1.5"; set $c2 "1.0"; }
    if ($dpr = 1.5) { set $c0 "1.5"; set $c1 "1.0"; }
    if ($dpr = 1.0) { set $c0 "1.0"; }
 
    # - check for appropriate DPR asset
    # - if appropriate DPR is not available, fallback to original
    location ~* /photos/(?<name>.*)\.(?<ext>.*)$ {
      set $dir "/photos/";
      try_files $dir$name-$c0.$ext $dir$name-$c1.$ext $dir$name-$c2.$ext
                $dir$name-$c3.$ext $dir$name-$c4.$ext $uri =500;
    }
 
    location / {
      root html;
    }
  }
}