MLT XML Schema Documentation

Copyright (C) 2004-2009 Ushodaya Enterprised Limited
Authors: Charles Yates <charles.yates@pandora.be>
Dan Dennedy <dan@dennedy.org>
Last Revision: 2009-05-08


MLT XML
-------

Preamble:

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

   If you just want to go straight to the DTD, then see 
   mlt/src/modules/xml/mlt-xml.dtd, which gets installed at 
   $(prefix)/share/mlt/modules/xml/mlt-xml.dtd. Currently, the XML parser is
   non-validating.


Introduction:

   A MLT XML 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 XML serialising
   consumer itself (the latter doesn't receive frames - it merely
   interrogates the connected producer for its configuration).

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


Rules:

   As shall become apparent through the remainder of this document, the basic
   tenet of MLT XML 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 MLT XML exist, the approach taken here is to maximise
   efficiency for complex projects. 


Basic Producers:

   The simplest MLT XML document is:

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

   The XML 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 MLT XML document to
   ensure that the second audio track is loaded:

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

   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 MLT XML doc can be specified as a resource, so XML docs can naturally
   encapsulate other XML docs.


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:

   <mlt>
     <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>
   </mlt>

   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) MLT XML 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 melt 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).


Tractors:

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

   <mlt>
     <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>
     <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>
   </mlt>
   
   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:

   <mlt>
     <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>
     <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>
   </mlt>
   
   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).

   Tracks act like "layers" in an image processing program like the GIMP. The 
   bottom-most track takes highest priority and higher layers are overlays 
   and do not appear unless there are gaps in the lower layers or unless
   a transition is applied that merges the tracks on the specifed region.
   Practically speaking, for A/B video editing it does not mean too much, 
   and it will work as expected; however, as a general rule apply any CGI
   (graphic overlays with pixbuf or titles with pango) on tracks higher than
   your video tracks. Also, this means that any audio-only tracks that are
   lower than your video tracks will play rather than the audio from the video
   clip. Remember, nothing is affected like mixing or compositing until one
   applies a transition or appropriate filter.
   
   <mlt>
     <producer id="producer0">
       <property name="resource">clip1.dv</property>
     </producer>
     <playlist id="playlist0">
       <entry producer="producer0"/>
     </playlist>
     <producer id="producer1">
       <property name="resource">clip2.mpeg</property>
     </producer>
     <playlist id="playlist1">
       <blank length="50"/>
       <entry producer="producer1"/>
     </playlist>
     <tractor id="tractor0" in="0" out="315">
       <multitrack id="multitrack0">
         <track producer="playlist0"/>
         <track producer="playlist1"/>
       </multitrack>
       <transition id="transition0" in="50" out="74">
         <property name="a_track">0</property>
         <property name="b_track">1</property>
         <property name="mlt_service">luma</property>
       </transition>
       <transition id="transition1" in="50" out="74">
         <property name="a_track">0</property>
         <property name="b_track">1</property>
         <property name="mlt_service">mix</property>
         <property name="start">0.0</property>
         <property name="end">1.0</property>
       </transition>
     </tractor>
   </mlt>

   A "luma" transition is a video wipe processor that takes a greyscale bitmap
   for the wipe definition. When one does not specify a bitmap, luma performs
   a dissolve. The "mix" transition does an audio mix, but it interpolates
   between the gain scaling factors between the start and end properties - 
   in this example, from 0.0 (none of track B) to 1.0 (all of track B). 
   Because the bottom track starts out with a gap specified using the <blank>
   element, the upper track appears during the blank segment. See the demos and
   services.txt to get an idea of the capabilities of the included transitions.

Flexibility:

   The information presented above is considered the MLT XML "normal"
   form. This is the output generated by the xml consumer, for example,
   when used with melt. It is the output generated when you use the
   "XML to File" consumer in the demo script, which beginners will find
   most useful for learning to use MLT XML. This section describes 
   alternative forms the xml producer accepts.

   First of all, the normal form is more of a linear format with producers
   and playlists defined prior to their usage in a multitrack. The producer
   also accepts a hierarchical format with producers as children of tracks
   or playlist entries and with playlists as children of tracks:

   <mlt>
     <tractor>
       <multitrack>
         <track>
           <playlist>
             <entry>
               <producer>
                 <property name="resource">clip1.dv</property>
               </producer>
             </entry>
           </playlist>
         </track>
       </multitrack>
     </tractor>
   </mlt>

   Obviously, this example is meant to demonstrate hierarchy and not effective
   use of playlist or multitrack!

   Secondly, as part of error handling, the producer is forgiving if you fail
   to supply <tractor>, <track>, and <entry> where one can be understood.
   This affords an abbreviated syntax that is less verbose and perhaps less 
   intimidating for a human to read and understand. One can simplify the
   above example as:

   <mlt>
     <multitrack>
       <playlist>
         <producer>
           <property name="resource">clip1.dv</property>
         </producer>
       </playlist>
     </multitrack>
   </mlt>

   Yes, filters and transitions can be added to the above example after the
   closing multitrack tag (</multitrack>) because it is still enclosed within
   the mlt body tags.

   If you specify in and out on a producer and it has been enclosed within
   an <entry> or <playlist>, then the edit points apply to the playlist
   entry and not to the producer itself. This facilitates re-use of media:

   <playlist>
     <producer id="clip1" in="25" out="78">
       <property name="resource">clip1.dv</property>
     </producer>
     <entry producer="clip1" in="119" out="347"/>
   </playlist>

   In the above example, the producer attribute of the entry element is 
   a reference to the preceding producer. All references must follow the 
   definition. The edit points supplied on the producer above will not affect
   the entry that references it below because the producer knows the clip is a 
   playlist entry and optimises this situation. The advantage is that one
   does not need to determine every clip to be included ahead of time 
   and specify them outside the context of the mutlitrack timeline.

   This form of authoring will be easier for many to visualise as a non-linear 
   editor's timeline. Here is a more complex example:

   <mlt>
     <multitrack>
       <playlist>
         <producer id="foo" in="100" out="149">
           <property name="resource">clip2.mpeg</property>
         </producer>
         <blank length="25"/>
         <entry producer="foo" in="10" out="59"/>
       </playlist>
       <playlist>
         <blank length="25"/>
         <producer id="bar" in="100" out="199">
           <property name="resource">clip3.mpeg</property>
         </producer>
         <entry out="99" producer="bar"/>
       </playlist>
     </multitrack>
     <filter mlt_service="greyscale" track="0"/>
     <transition mlt_service="luma" in="25" out="49" a_track="0" b_track="1"/>
     <transition mlt_service="luma" in="75" out="99" a_track="0" b_track="1">
       <property name="reverse" value="1"/>
     </transition>
   </mlt>

   Did you notice something different in the last example? Properties can be 
   expressed using XML attributes on the element as well. However, only 
   non-service-specific properties are supported in this way. For example,
   "mlt_service" is available to any producer, filter, or transition. However,
   "resource" is actually service-specific. Notice the syntax of the last 
   property, on the last transition. The producer accepts property values using 
   the "value" attribute as well as using element text.

   We have seen a few different ways of expressing property values. There are
   a couple more for properties that can accept XML data. For example, the
   GDK pixbuf producer with librsvg can handle embedded SVG, and the Pango
   producer can handle embedded Pango markup. You can enclose the embedded XML
   using a CDATA section:

   <property name="resource"><![CDATA[ <svg>...</svg> ]]></property>

   Please ensure the opening CDATA tag immediately follows the opening
   property tag and that the section closing tag immediately precedes the
   closing property tag.

   However, the producer can also accept inline embedded XML:

   <property name="resource">
     <svg>
     </svg>
   </property>

   Currently, there is no namespace handling so a conflict will occur only on 
   any embedded XML that contains an element named "property" because 
   the producer collects embedded XML until it reaches a closing property tag.
   
Tips and Technique:

   If one finds the above hierarchical, abbreviated format intuitive,
   start with a simple template and fill and extend as needed:
       <mlt>
         <multitrack>
           <playlist>
           </playlist>
           ...add a playlist for each track...
         </multitrack>
         ...add filters and transitions...
       </mlt>
       
   By using a playlist for each track, it is easier to iteratively add new 
   clips and blank regions as you develop the project. You will not have to
   use <track> or later add <playlist> when necessary.
   
   A more advanced template that allows sequencing multitracks is:
       <playlist>
         <entry>
           <multitrack>
             <playlist>
             </playlist>
             ...add a playlist for each track...
           </multitrack>
           ...add filters and transitions...
         </entry>
         
         <entry>
           <multitrack>
             <playlist>
             </playlist>
             ...add a playlist for each track...
           </multitrack>
           ...add filters and transitions...
         </entry>
       </playlist>

   If you end up making a collection of templates for various situations, then
   consider using XML Entities to make the template more effective by moving
   anything that should parameterised into an entity.

   If you want to have a silent, black background for audio and video fades,
   then make the top track simply <producer mlt_service="colour"/>. Then,
   use composite and volume effects. See the "Fade from/to black/silence"
   demo for an example (demo/mlt_fade_black).
   
   If you apply the reverse=1 property to a transition like "luma," then
   be careful because it also inherently swaps the roles of A and B tracks.
   Therefore, you need to might need to swap the a_track and b_track values
   if it did not turn out the way you expected. See the "Clock in and out"
   for an example (demo/mlt_clock_in_and_out).
Topic revision: r9 - 2013-11-24 - DanDennedy
 
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