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 and the nominal size.
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) {
theme = user selected theme
do {
filename = LookupIcon (icon, size, theme)
if filename != none
return filename
theme = theme.parentTheme()
} while (theme)
return LookupFallbackIcon (iconname)
}
With the following helper functions:
LookupIcon (iconname, size, theme) {
for each subdir in $(theme subdir list) {
for each directory in $(basename list) {
for extension in ("png", "svg", "xpm") {
if DirectoryMatchesSize(subdir, size) {
filename = directory/$(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) < minimal_size {
closest_filename = filename
minimal_size = DirectorySizeDistance(subdir, size)
}
}
}
}
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) {
read Type and size data from subdir
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, size) {
read Type and size data from subdir
if Type is Fixed
return abs(Size - iconsize)
if Type is Scaled
if iconsize < MinSize
return MinSize - iconsize
if iconsize > MaxSize
return iconsize - MaxSize
return 0
if Type is Threshold
if iconsize < Size - Threshold
return MinSize - iconsize
if iconsize > Size + Threshold
return iconsize - MaxSize
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) {
theme = user selected theme
do {
for icon in iconList {
filename = LookupIcon (icon, size, theme)
if filename != none
return filename
}
theme = theme.parentTheme()
} while (theme)
return LookupFallbackIcon (iconname)
}
This can be very useful for example when handling mimetype icons, where there are more and less "specific" versions of icons.