Icon Lookup
Prev |  | Next

## Icon Lookup

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

Prev |  | Next
File Formats | Home | Example