GTK applications typically use [GdkPixbuf](https://gitlab.gnome.org/GNOME/gdk-pixbuf/) to load images. But `gdk-pixbuf` package only supports basic bitmap formats like JPEG, PNG or TIFF, requiring to use third-party loader modules for other formats. This is especially painful since GTK itself includes SVG icons, which cannot be rendered without a loader provided by `librsvg`.

Unlike other libraries mentioned in this section, GdkPixbuf only supports a single value in its controlling environment variable `GDK_PIXBUF_MODULE_FILE`. It is supposed to point to a cache file containing information about the available loaders. Each loader package will contain a `lib/gdk-pixbuf-2.0/2.10.0/loaders.cache` file describing the default loaders in `gdk-pixbuf` package plus the loader contained in the package itself. If you want to use multiple third-party loaders, you will need to create your own cache file manually. Fortunately, this is pretty rare as [not many loaders exist](https://gitlab.gnome.org/federico/gdk-pixbuf-survey/blob/master/src/modules.md).

`gdk-pixbuf` contains [a setup hook](#ssec-gnome-hooks-gdk-pixbuf) that sets `GDK_PIXBUF_MODULE_FILE` from dependencies but as mentioned in further section, it is pretty limited. Loaders should propagate this setup hook.

### Icons {#ssec-gnome-icons}

When an application uses icons, an icon theme should be available in `XDG_DATA_DIRS` during runtime. The package for the default, icon-less [hicolor-icon-theme](https://www.freedesktop.org/wiki/Software/icon-theme/) (should be propagated by every icon theme) contains [a setup hook](#ssec-gnome-hooks-hicolor-icon-theme) that will pick up icon themes from `buildInputs` and add their datadirs to `XDG_ICON_DIRS` environment variable (this is Nixpkgs specific, not actually a XDG standard variable). Unfortunately, relying on that would mean every user has to download the theme included in the package expression no matter their preference. For that reason, we leave the installation of icon theme on the user. If you use one of the desktop environments, you probably already have an icon theme installed.

In the rare case you need to use icons from dependencies (e.g. when an app forces an icon theme), you can use the following to pick them up:

```nix
{
  buildInputs = [
    pantheon.elementary-icon-theme
  ];
  preFixup = ''
    gappsWrapperArgs+=(
      # The icon theme is hardcoded.
      --prefix XDG_DATA_DIRS : "$XDG_ICON_DIRS"
    )
  '';
}
```

To avoid costly file system access when locating icons, GTK, [as well as Qt](https://woboq.com/blog/qicon-reads-gtk-icon-cache-in-qt57.html), can rely on `icon-theme.cache` files from the themes' top-level directories. These files are generated using `gtk-update-icon-cache`, which is expected to be run whenever an icon is added or removed to an icon theme (typically an application icon into `hicolor` theme) and some programs do indeed run this after icon installation. However, since packages are installed into their own prefix by Nix, this would lead to conflicts. For that reason, `gtk3` provides a [setup hook](#ssec-gnome-hooks-gtk-drop-icon-theme-cache) that will clean the file from installation. Since most applications only ship their own icon that will be loaded on start-up, it should not affect them too much. On the other hand, icon themes are much larger and more widely used so we need to cache them. Because we recommend installing icon themes globally, we will generate the cache files from all packages in a profile using a NixOS module. You can enable the cache generation using `gtk.iconCache.enable` option if your desktop environment does not already do that.

### Packaging icon themes {#ssec-icon-theme-packaging}

Icon themes may inherit from other icon themes. The inheritance is specified using the `Inherits` key in the `index.theme` file distributed with the icon theme. According to the [icon theme specification](https://specifications.freedesktop.org/icon-theme-spec/latest), icons not provided by the theme are looked for in its parent icon themes. Therefore the parent themes should be installed as dependencies for a more complete experience regarding the icon sets used.