An icon theme is a set of icons that share a common look and feel. The user can then select the icon theme that they want to use, and all apps use icons from the theme. The initial user of icon themes is the icon field of the desktop file specification, but in the future it can have other uses (such as mimetype icons).

From a programmer perspective an icon theme is just a mapping. Given a set of directories to look for icons in and a theme name it maps from icon name and nominal icon size to an icon filename.

Icons and themes are looked for in a set of directories. By default, apps should look in $HOME/.icons (for backwards compatibility), in $XDG_DATA_DIRS/icons and in /usr/share/pixmaps (in that order). Applications may further add their own icon directories to this list, and users may extend or change the list (in application/desktop specific ways).In each of these directories themes are stored as subdirectories. A theme can be spread across several base directories by having subdirectories of the same name. This way users can extend and override system themes.

In order to have a place for third party applications to install their icons there should always exist a theme called "hicolor" ^[1]. The data for the hicolor theme is available for download at: http://www.freedesktop.org/software/icon-theme/. Implementations are required to look in the "hicolor" theme if an icon was not found in the current theme.

Each theme is stored as subdirectories of the base directories. The internal name of the theme is the name of the subdirectory, although the user-visible name as specified by the theme may be different. Hence, theme names are case sensitive, and are limited to ASCII characters. Theme names may also not contain comma or space.

In at least one of the theme directories there must be a file called index.theme that describes the theme. The first index.theme found while searching the base directories in order is used. This file describes the general attributes of the theme.

In the theme directory are also a set of subdirectories containing image files. Each directory contains icons designed for a certain nominal icon size and scale, as described by the index.theme file. The subdirectories are allowed to be several levels deep, e.g. the subdirectory "48x48/apps" in the theme "hicolor" would end up at $basedir/hicolor/48x48/apps.

The image files must be one of the types: PNG, XPM, or SVG, and the extension must be ".png", ".xpm", or ".svg" (lower case). The support for SVG files is optional. Implementations that do not support SVGs should just ignore any ".svg" files. In addition to this there may be an additional file with extra icon-data for each file. It should have the same basename as the image file, with the extension ".icon". e.g. if the icon file is called "mime_source_c.png" the corresponding file would be named "mime_source_c.icon".

Both the icon theme description file and the icon data files are ini-style text files, as described in the desktop file specification. They don't have any encoding field. Instead, they must always be stored in UTF-8 encoding.

The index.theme file must start with a section called Icon Theme, with contents according to table 1 below. All lists are comma-separated.

Each directory specified in the Directory key has a corresponding section with the same name as the directory. The contents of this section is listed in table 2 below.

In addition to these groups you may add extra groups to the index.theme file in order to extend it. These extensions must begin with "X-", and can be used to add desktop specific information to the theme file. Example group names would be "X-KDE Icon Theme" or "X-Gnome Icon Theme".

The optional filename.icon file contains a group called "Icon Data", with the content listed in table 3.

Extensions to the filename.icon file are allowed, but the keys must be begin with "X-" to avoid collisions with future standardized extensions to this format.

The icon lookup mechanism has two global settings, the list of base directories and the internal name of the current theme. Given these we need to specify how to look up an icon file from the icon name, the nominal size and the scale.

The lookup is done first in the current theme, and then recursively in each of the current theme's parents, and finally in the default theme called "hicolor" (implementations may add more default themes before "hicolor", but "hicolor" must be last). As soon as there is an icon of any size that matches in a theme, the search is stopped. Even if there may be an icon with a size closer to the correct one in an inherited theme, we don't want to use it. Doing so may generate an inconsistant change in an icon when you change icon sizes (e.g. zoom in).

The lookup inside a theme is done in three phases. First all the directories are scanned for an exact match, e.g. one where the allowed size of the icon files match what was looked up. Then all the directories are scanned for any icon that matches the name. If that fails we finally fall back on unthemed icons. If we fail to find any icon at all it is up to the application to pick a good fallback, as the correct choice depends on the context.

The exact algorithm (in pseudocode) for looking up an icon in a theme (if the implementation supports SVG) is:

FindIcon(icon, size, scale) {
  filename = FindIconHelper(icon, size, scale, user selected theme);
  if filename != none
    return filename

  filename = FindIconHelper(icon, size, scale, "hicolor");
  if filename != none
    return filename

  return LookupFallbackIcon (icon)
}
FindIconHelper(icon, size, scale, theme) {
  filename = LookupIcon (icon, size, scale, theme)
  if filename != none
    return filename

  if theme has parents
    parents = theme.parents

  for parent in parents {
    filename = FindIconHelper (icon, size, scale, parent)
    if filename != none
      return filename
  }
  return none
}
     

With the following helper functions:

LookupIcon (iconname, size, scale, theme) {
  for each subdir in $(theme subdir list) {
    for each directory in $(basename list) {
      for extension in ("png", "svg", "xpm") {
        if DirectoryMatchesSize(subdir, size, scale) {
          filename = directory/$(themename)/subdir/iconname.extension
          if exist filename
	    return filename
        }
      }
    }
  }
  minimal_size = MAXINT
  for each subdir in $(theme subdir list) {
    for each directory in $(basename list) {
      for extension in ("png", "svg", "xpm") {
        filename = directory/$(themename)/subdir/iconname.extension
        if exist filename and DirectorySizeDistance(subdir, size, scale) < minimal_size {
	   closest_filename = filename
	   minimal_size = DirectorySizeDistance(subdir, size, scale)
        }
      }
    }
  }
  if closest_filename set
     return closest_filename
  return none
}

LookupFallbackIcon (iconname) {
  for each directory in $(basename list) {
    for extension in ("png", "svg", "xpm") {
      if exists directory/iconname.extension
        return directory/iconname.extension
    }
  }
  return none
}

DirectoryMatchesSize(subdir, iconsize, iconscale) {
  read Type and size data from subdir
  if Scale != iconscale
     return False;
  if Type is Fixed
    return Size == iconsize
  if Type is Scaled
    return MinSize <= iconsize <= MaxSize
  if Type is Threshold
    return Size - Threshold <= iconsize <= Size + Threshold
}

DirectorySizeDistance(subdir, iconsize, iconscale) {
  read Type and size data from subdir
  if Type is Fixed
    return abs(Size*Scale - iconsize*iconscale)
  if Type is Scaled
    if iconsize*iconscale < MinSize*Scale
        return MinSize*Scale - iconsize*iconscale
    if iconsize*iconscale > MaxSize*Scale
        return iconsize*iconscale - MaxSize*Scale
    return 0
  if Type is Threshold
    if iconsize*iconscale < (Size - Threshold)*Scale
        return MinSize*Scale - iconsize*iconscale
    if iconsize*iconsize > (Size + Threshold)*Scale
        return iconsize*iconsize - MaxSize*Scale
    return 0
}

In some cases you don't always want to fall back to an icon in an inherited theme. For instance, sometimes you look for a set of icons, prefering any of them before using an icon from an inherited theme. To support such operations implementations can contain a function that finds the first of a list of icon names in the inheritance hierarchy. I.E. It would look something like this:

FindBestIcon(iconList, size, scale) {
  filename = FindBestIconHelper(iconList, size, scale, user selected theme);
  if filename != none
    return filename

  filename = FindBestIconHelper(iconList, size, scale, "hicolor");
  if filename != none
    return filename

  for icon in iconList {
    filename = LookupFallbackIcon (icon)
    if filename != none
      return filename
  }
  return none;
}
FindBestIconHelper(iconList, size, scale, theme) {
  for icon in iconList {
    filename = LookupIcon (icon, size, theme)
    if filename != none
      return filename
  }

  if theme has parents
    parents = theme.parents

  for parent in parents {
    filename = FindBestIconHelper (iconList, size, scale, parent)
    if filename != none
      return filename
  }
  return none
}

This can be very useful for example when handling mimetype icons, where there are more and less "specific" versions of icons.

Here is an example index.theme file:

[Icon Theme]
Name=Birch
Name[sv]=Björk
Comment=Icon theme with a wooden look
Comment[sv]=Träinspirerat ikontema
Inherits=wood,default
Directories=48x48/apps,48x48@2/apps48x48/mimetypes,32x32/apps,32x32@2/apps,scalable/apps,scalable/mimetypes

[scalable/apps]
Size=48
Type=Scalable
MinSize=1
MaxSize=256
Context=Applications

[scalable/mimetypes]
Size=48
Type=Scalable
MinSize=1
MaxSize=256
Context=MimeTypes

[32x32/apps]
Size=32
Type=Fixed
Context=Applications

[32x32@2/apps]
Size=32
Scale=2
Type=Fixed
Context=Applications

[48x48/apps]
Size=48
Type=Fixed
Context=Applications

[48x48@2/apps]
Size=48
Scale=2
Type=Fixed
Context=Applications

[48x48/mimetypes]
Size=48
Type=Fixed
Context=MimeTypes

The corresponding directory tree in the /usr/share/icons directory could look like this:

birch/index.theme
birch/scalable/apps/mozilla.svg
birch/scalable/mimetypes/mime_text_plain.svg
birch/scalable/mimetypes/mime_text_plain.icon
birch/48x48/apps/mozilla.png
birch/48x48@2/apps/mozilla.png
birch/32x32/apps/mozilla.png
birch/32x32@2/apps/mozilla.png
birch/48x48/mimetypes/mime_text_plain.png
birch/48x48/mimetypes/mime_text_plain.icon

Where birch/scalable/mimetypes/mime_text_plain.icon contains:

[Icon Data]
DisplayName=Mime text/plain
EmbeddedTextRectangle=100,100,900,900
AttachPoints=200,200|800,200|500,500|200,800|800,800

And birch/48x48/mimetypes/mime_text_plain.icon contains:

[Icon Data]
DisplayName=Mime text/plain
EmbeddedTextRectangle=8,8,40,40
AttachPoints=20,20|40,40|50,10|10,50

In this example a lookup of "mozilla" would get the prerendered 48x48 and 32x32 icons before the SVG icons due to the order of Directories.

So, you're an application author, and want to install application icons so that they work in the KDE and Gnome menus. Minimally you should install a 48x48 icon in the hicolor theme. This means installing a PNG file in $prefix/share/icons/hicolor/48x48/apps. Optionally you can install icons in different sizes. For example, installing a svg icon in $prefix/share/icons/hicolor/scalable/apps means most desktops will have one icon that works for all sizes. You might even want to install icons with a look that matches other well known themes so your application will fit in with some specific desktop environment.

It is recommended that the icons installed in the hicolor theme look neutral, since it is a fallback theme that will be used in combination with some very different looking themes. But if you don't have any neutral icon, please install whatever icon you have in the hicolor theme so that all applications get at least some icon in all themes.

The algorithm as described in this document works by always looking up filenames in directories (a stat in unix terminology). A good implementation is expected to read the directories once, and do all lookups in memory using that information.

This caching can make it impossible for users to add icons without having to restart applications. In order to handle this, any implementation that does caching is required to look at the mtime of the toplevel icon directories when doing a cache lookup, unless it already did so less than 5 seconds ago. This means that any icon editor or theme installation program need only to change the mtime of the the toplevel directory where it changed the theme to make sure that the new icons will eventually get used.

The icon theme specification is based on the original KDE icon theme system designed by Antonio Larossa, Geert Janssen and Torsten Rahn. The common specification mostly adds support for .icon files, renames the icon theme description files and removes a few references to kde in them.