WESTLEY
-------

Preamble:

	Westley is the MLT projects XML serialisation/deserialisation format -
	as such, it closely mirrors the internal structure of the MLT API. 


Introduction:

	A westley document is essentially a list of 'producers' - a producer is
	an mlt object which generates mlt frames (images and associated audio
	samples). 
	
	There are 3 types of producer:
	
	* Basic Producers - these are typically file or device oriented feeds; 
	* Playlists - these are arrangements of multiple producers;
	* Multitracks - these are the fx encapsulators.

	In the mlt model, producers are created and attached to 'consumers' -
	consumers are software playback components (such as SDL), or wrappers for
	hardware drivers (such as bluefish) or even the westley serialising
	consumer itself (the latter doesn't receive frames - it merely
	interrogates the connected producer for its configuration).

	Although westley was defined as a serialisation mechanism for instantiated 
	MLT components, this document will concentrate on the hand authoring of 
	westley documents.


Rules:

	As shall become apparent through the remainder of this document, the basic
	tenet of westley authoring is to organise the document in the following
	manner:

	1) create producer elements for each unique media clip in the project;
	2) create playlists for each track;
	3) create a multitrack and specify filters and transitions;
	4) adding global filters.

	While other uses of westley exist, the approach taken here is to maximise
	efficiency for complex projects. 


Basic Producers:

	The simplest westley document is:

	<westley>
	  <producer id="producer0">
	    <property name="resource">clip1.dv</property>
	  </producer>
	</westley>

	The westley wrapping is of course superfluous here - loading this document
	with MLT is identical to loading the clip directly. 

	Of course, you can specify additional properties. For example, consider an
	MPEG file with multiple soundtracks - you could define a westley document to
	ensure that the second audio track is loaded:

	<westley>
	  <producer id="producer0">
	    <property name="resource">clip1.mpeg</property>
	    <property name="audio_track">1</property>
	  </producer>
	</westley>

	NB: This relies on the mpeg being handled by the avformat producer, rather
	than the mcmpeg one. See services.txt for more details.

	A more useful example comes with the pango producer for a text producer.

	TODO: pango example...

	Notes:

	1) It is better not to specify in/out points when defining basic producers
	as these can be specified in the playlists. The reasoning is that in/out
	restricts the amount of the clip available, and could lead to the same clip
	being loaded multiple times if you need different regions of the clip
	elsewhere;
	2) A westley can be specified as a resource, so westleys can naturally
	encapsulate other westleys.


Playlists:

	Playlists provide a 'collection' structure for producers. These can be used
	to define 'tracks' in the multitrack approach, or simple playlists for
	sequential, single track playout.

	As an example, the following defines two basic producers and a playlist with 3
	items:

	<westley>
	  <producer id="producer0">
	    <property name="resource">clip1.dv</property>
	  </producer>
	  <producer id="producer1">
	    <property name="resource">clip2.dv</property>
	  </producer>
	  <playlist id="playlist0">
	    <entry producer="producer0" in="0" out="2999"/>
	    <entry producer="producer1" in="0" out="999"/>
	    <entry producer="producer0" in="3000" out="6999"/>
	  </playlist>
	</westley>

	Here we see how the playlist defines the in/out points of the basic
	producers.

	Notes:

	1) All in/out points are absolute frame positions relative to the producer
	being appended to the playlist;
	2) Westley documents are currently authored for a specific normalisation;
	3) The last 'producer' in the document is the default for play out;
	4) Playlists can reference the same producer multiple times. In/out regions
	do not need to be contiguous - duplication and skipping is acceptable.


Interlude - Introducing Multitracks:

	So far we've defined basic producers and playlists/tracks - the tractor is
	the element that allows us to arrange our tracks and specify filters and
	transitions. Similarly to a playlist, a tractor is a container.

	Note that MLT doesn't see a filter or a transition as a producer in the
	normal sense - filters and transitions are passive when it comes to seeking.
	Internally, seeks are carried out on the producers. This is an important
	point - MLT does not follow a traditional graph oriented model.

	Visualising an MLT tractor and it's interaction with the consumer will
	assist here:

	+----------------------------------------------+
	|tractor                                       |
	| +----------+    +-+    +-+    +-+    +-+     |
	| |multitrack|    |f|    |f|    |t|    |t|     |
	| | +------+ |    |i|    |i|    |r|    |r|     |
	| | |track0|-|--->|l|- ->|l|- ->|a|--->|a|\    |
	| | +------+ |    |t|    |t|    |n|    |n| \   |
	| |          |    |e|    |e|    |s|    |s|  \  |
	| | +------+ |    |r|    |r|    |i|    |i|   \ |    +--------+
	| | |track1|-|- ->|0|--->|1|--->|t|--->|t|-----|--->|consumer|
	| | +------+ |    | |    | |    |i|    |i|   / |    +--------+
	| |          |    | |    | |    |o|    |o|  /  |         ^
	| | +------+ |    | |    | |    |n|    |n| /   |         |
	| | |track2|-|- ->| |- ->| |--->|0|- ->|1|/    |         |
	| | +------+ |    | |    | |    | |    | |     |         |
	| +----------+    +-+    +-+    +-+    +-+     |         |
	+----------------------------------------------+         |
	       ^                                                 |
	       |                                                 |
	+-----------+                                            |
	|APPLICATION|--------------------------------------------+
	+-----------+

	Internally, all frames from all tracks pass through all the filters and
	transitions - these are told which tracks to deal and which regions of the
	tracks to work on. 

	Note that the application communicates with the producer - it can alter
	playback speed, position, or even which producer is connected to which
	consumer. 
	
	The consumer receives the first non-blank frame (see below). It has no say
	in the order in which gets them (the sdl consumer when used with inigo might 
	appear to be an exception - it isn't - it simply has a route back to the 
	application to allow the application to interpret key presses).

	Whether this is better or worse than a traditional graph approach is a moot
	point. The author of this document likes it anyway :-).


Tractors:

	To create a multitrack westley, we can use two playlists and introduce a
	tractor. For the purposes of demonstration, I'll add a filter here too:

	<westley>
	  <producer id="producer0">
	    <property name="resource">clip1.dv</property>
	  </producer>
	  <producer id="producer1">
	    <property name="resource">clip2.dv</property>
	  </producer>
	  <playlist id="playlist0">
	    <entry producer="producer0" in="0" out="2999"/>
	    <blank length="1000"/>
	    <entry producer="producer0" in="3000" out="6999"/>
	  <playlist id="playlist1">
	    <blank length="3000"/>
	    <entry producer="producer1" in="0" out="999"/>
	  </playlist>
	  <tractor id="tractor0">
	    <multitrack>
	      <track producer="playlist0"/>
	      <track producer="playlist1"/>
	    </multitrack>
	    <filter>
	      <property name="track">0</property>
	      <property name="mlt_service">greyscale</property>
	    </filter>
	  </tractor>
	</westley>
	
	Here we see that blank frames are inserted into the first playlist and a
	blank is provided at the beginning of the second - this can be visualised in
	the traditional timeline widget as follows:

	+-------+   +-------------+
	|a      |   |a            |
	+-------+---+-------------+
	        |b  |
	        +---+
	
	Adding the filter on the top track, gives us:

	+-------+   +-------------+
	|a      |   |a            |
	+-------+---+-------------+
	|greyscale                |
	--------+---+-------------+
	        |b  |
	        +---+
	
	Note that it's only applied to the visible parts of the top track.

	The requirement to apply a filter to the output, as opposed to a specific track 
	leads us to the final item in the Rules section above. As an example, let's
	assume we wish to watermark all output, then we could use the following:

	<westley>
	  <producer id="producer0">
	    <property name="resource">clip1.dv</property>
	  </producer>
	  <producer id="producer1">
	    <property name="resource">clip2.dv</property>
	  </producer>
	  <playlist id="playlist0">
	    <entry producer="producer0" in="0" out="2999"/>
	    <blank length="1000"/>
	    <entry producer="producer0" in="3000" out="6999"/>
	  <playlist id="playlist1">
	    <blank length="3000"/>
	    <entry producer="producer1" in="0" out="999"/>
	  </playlist>
	  <tractor id="tractor0">
	    <multitrack>
	      <track producer="playlist0"/>
	      <track producer="playlist1"/>
	    </multitrack>
	    <filter>
	      <property name="track">0</property>
	      <property name="mlt_service">greyscale</property>
	    </filter>
	  </tractor>
	  <tractor id="tractor1">
	    <multitrack>
	      <track producer="tractor0"/>
	    </multitrack>
	    <filter>
	      <property name="mlt_service">watermark</property>
	      <property name="resource">watermark1.png</property>
	    </filter>
	  </tractor>
	</westley>
	
	Here we employ another tractor and we define a single track (being the
	tractor we previously defined) and apply a watermarking filter there.

	This is simply provided as an example - the watermarking functionality could
	be better handled at the playout stage itself (ie: as a filter automatically
	placed between all producers and the consumer).

	TODO: transition example