smaller wallpaper size

modules/gnome-wallpaper.nix

@@ -11,6 +11,6 @@
             "picture-uri-dark" = "/home/yoda/.config/background";
         };
     };
-    home.file.".config/background".source = "${../assets/img/wallpaper.jpg}";
+    home.file.".config/background".source = "${(pkgs.callPackage ./wallpaper.nix { })}/img.jpg";
   };
 }

modules/wallpaper-test.nix

@@ -0,0 +1,22 @@
+# To inspect ./wallpaper.nix, adjust the `expr` of the two tests below and then run the following command:
+#   nix --extra-experimental-features nix-command eval --show-trace --impure --expr 'import ./modules/wallpaper-test.nix {}'
+#
+# Source: https://www.tweag.io/blog/2022-09-01-unit-test-your-nix-code/
+
+{ pkgs ? import <nixpkgs> {} }:
+let
+  inherit (pkgs) lib;
+  inherit (lib) runTests;
+  math = import ./wallpaper.nix {inherit lib pkgs;};
+in
+  runTests {
+    test1 = {
+      expr = pkgs.lib.attrNames math;
+      expected = true;
+    };
+
+    test2 = {
+      expr = "${math.out}/img.jpg";
+      expected = true;
+    };
+  }

modules/wallpaper.nix

@@ -0,0 +1,19 @@
+{ pkgs, lib }:
+let
+  # Width of our display.
+  max-width = "1080";
+in
+  pkgs.stdenv.mkDerivation rec{
+    name = "yoda-wallpaper";
+    src = "${../assets/img/wallpaper.jpg}";
+    nativeBuildInputs = with pkgs; [ imagemagick ];
+    phases = "installPhase";
+    # magick limit width: https://stackoverflow.com/a/38415211/6334421
+    # magick jpg options: quality, progressive, strip metadata. https://stackoverflow.com/a/7262050/6334421
+    installPhase = ''
+      set -eu -o pipefail
+      mkdir -p $out
+      ${pkgs.imagemagick}/bin/magick $src -resize '${max-width}>' -quality 85% -interlace Plane -strip      $out/img.jpg
+      ${pkgs.imagemagick}/bin/magick $src -resize '${max-width}>'                                           $out/img.png
+    '';
+  }