niri: make focus ring the inverse of the average of the background color

home-manager/progs/niri.nix

@@ -5,6 +5,9 @@
   inputs,
   ...
 }:
+let
+  invert_image_avg = pkgs.callPackage ../util/inverse_color.nix;
+in
 {
   imports = [
     inputs.niri.homeModules.config
@@ -51,7 +54,9 @@
         focus-ring = {
           enable = true;
           active = {
-            color = "#bf7778";
+            color = builtins.readFile (invert_image_avg {
+              src = ../wallpaper.png;
+            });
           };
         };
       };

home-manager/util/inverse_color.nix

@@ -0,0 +1,26 @@
+{
+  stdenv,
+  imagemagick,
+  gawk,
+  src,
+}:
+stdenv.mkDerivation {
+  pname = "invert-image-avg";
+  version = "1.0";
+
+  inherit src;
+
+  buildInputs = [
+    imagemagick
+    gawk
+  ];
+
+  # input is a file, not a directory, skip unpackPhase
+  unpackPhase = "true";
+
+  # command taken from: https://old.reddit.com/r/swaywm/comments/oz3t7v/setting_a_blurred_background_with_swaylock_and/
+  buildPhase = ''
+    rm -fr $out
+    magick ${src} -channel RGB -negate -resize 1x1 txt: | awk 'FNR == 2 {print $3}' > $out
+  '';
+}