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

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
+  '';
+}