The Thumbnailers Cache Format
=============================

Date:		2006-08-08
Author:		Benedikt Meurer <benny@xfce.org>
Revision:	$Rev: 22685 $


Motivation
----------

In order to avoid a dependency of libthunar-vfs-1.so on GConf and in order to
stay flexible wrt thumbnailers w/o adding more overhead to libthunar-vfs-1.so
(and thereby to all applications using the library), the available thumbnailers
for the ThunarVfsThumbFactory are determined by a separate utility program,
named thunar-vfs-update-thumbnailers-cache-1, which is installed to the 
$(libexecdir).

This program generates a thumbnailers.cache file in the users home directory,
in $XDG_CACHE_HOME/Thunar/ (usually ~/.cache/Thunar/), which is a binary file
that lists all mime types for a thumbnailer is available. For each thumbnailer
the command required to run it is specified, where the command can contain
various special parameters that are substituted when the command is run.

This thumbnailers.cache is generated initially when the thumbnail factory is
loaded and no cache file is present, and afterwards the utility is run every
5 minutes to check if new thumbnailers are available (or existing thumbnailers
have been removed).


Special Thumbnailer Parameters
------------------------------

The following special parameters are supported in thumbnailer commands and
will be substituted when ThunarVfsThumbFactory runs the command:

	%u	The URL of the input file for which to create a thumbnail.
	%i	The absolute path to the input file.
	%o	The absolute path to the output file where to save the generated
		thumbnail to (as PNG image).
	%s	The desired size of the thumbnail in pixel (usually 128 or 256).
	%%	Will be substituted with a single '%'.


The thunar-vfs-update-thumbnailers-cache-1 Utility
--------------------------------------------------

When compiled with support for GConf, the utility will first query all available
thumbnailers that have been registered with GConf. Otherwise, the utility checks
for several well-known thumbnailers (i.e. evince-thumbnailer, ...) and adds them
if the programs were found on the system.

Afterwards the available gdk-pixbuf formats will be queried and for each of the
returned mime types, the thunar-vfs-pixbuf-thumbnailer-1 will be registered. If
any such mime type was already registered by a thumbnailer queried from the
GConf database, the pixbuf thumbnailer will overwrite the previously registered
one.

The last step is to add all internal thumbnails (again overwriting previously
registered thumbnailers).


The thumbnailers.cache File Format
----------------------------------

The thumbnailers.cache file is a binary file, that maps mime types to thumbnai-
ler commands. The file format is designed to be mmap()ed into processes using
the thumbnailers.cache. All numbers are 32bit integers in big endian. All
strings are zero-terminated ASCII strings.

The current version of the file format is 1.0. Newer versions will either be
backward compatible with the current format or use a new file name.

The file starts with a header that contains the file format version, the number
of mime types stored in the file and the offset of the thumbnailers table:

	Bits	Meaning
	----	-------

	32	FORMAT MAJOR VERSION (currently 1)
	32	FORMAT MINOR VERSION (currently 0)
	32	NUMER OF MIME TYPES (and thereby NUMBER OF THUMBNAILERS)
	32	OFFSET OF THE THUMBNAILERS TABLE

Next comes the table of mime types, where each entry contains the length of the
mime type in bytes (excluding the trailing zero) and the offset (relative to the
cache file start) where the name of the mime type is stored. The mime types are
sorted by strlen() and strcmp(), which means that binary search can be used to
find a mime type, trying the length first (so string comparisons are reduced to
a minimum to make the search cache-friendly) and performing a string comparison
when the lengths are equal (see thunar_vfs_thumb_factory_cache_lookup() for an
example implementation of the search).

	Bits	Meaning
	----	-------

	32	LENGTH OF MIME TYPE STRING
	32	OFFSET OF MIME TYPE STRING

Once you successfully detected a mime type, you can use the index of the mime
type in the table above to locate the thumbnailer command for this mime type.
Therefore determine the OFFSET OF THE THUMBNAILERS TABLE (at offset 12 in the
cache) and look at that offset plus the index * 4 for the offset of the thumb-
nailer command string.

	Bits	Meaning
	----	-------
	32	OFFSET OF THUMBNAILER STRING

Afterwards substitute the special parameters as mentioned in "Special Thumbnai-
ler Parameters" above.
