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 an implementation that supports SVG is:

LookupIcon (iconname, size):
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
   
for each directory in $(basename list) {
    for extension in ("png", "svg", "xpm") {
	if exists directory/iconname.extension
	   return directory/iconname.extension
    }
}

return failed icon lookup
     

With the following helper functions:

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