6 Westley is the MLT projects XML serialisation/deserialisation format -
7 as such, it closely mirrors the internal structure of the MLT API.
12 A westley document is essentially a list of 'producers' - a producer is
13 an mlt object which generates mlt frames (images and associated audio
16 There are 3 types of producer:
18 * Basic Producers - these are typically file or device oriented feeds;
19 * Playlists - these are arrangements of multiple producers;
20 * Multitracks - these are the fx encapsulators.
22 In the mlt model, producers are created and attached to 'consumers' -
23 consumers are software playback components (such as SDL), or wrappers for
24 hardware drivers (such as bluefish) or even the westley serialising
25 consumer itself (the latter doesn't receive frames - it merely
26 interrogates the connected producer for its configuration).
28 Although westley was defined as a serialisation mechanism for instantiated
29 MLT components, this document will concentrate on the hand authoring of
35 As shall become apparent through the remainder of this document, the basic
36 tenet of westley authoring is to organise the document in the following
39 1) create producer elements for each unique media clip in the project;
40 2) create playlists for each track;
41 3) create a multitrack and specify filters and transitions;
42 4) adding global filters.
44 While other uses of westley exist, the approach taken here is to maximise
45 efficiency for complex projects.
50 The simplest westley document is:
53 <producer id="producer0">
54 <property name="resource">clip1.dv</property>
58 The westley wrapping is of course superfluous here - loading this document
59 with MLT is identical to loading the clip directly.
61 Of course, you can specify additional properties. For example, consider an
62 MPEG file with multiple soundtracks - you could define a westley document to
63 ensure that the second audio track is loaded:
66 <producer id="producer0">
67 <property name="resource">clip1.mpeg</property>
68 <property name="audio_track">1</property>
72 NB: This relies on the mpeg being handled by the avformat producer, rather
73 than the mcmpeg one. See services.txt for more details.
75 A more useful example comes with the pango producer for a text producer.
77 TODO: pango example...
81 1) It is better not to specify in/out points when defining basic producers
82 as these can be specified in the playlists. The reasoning is that in/out
83 restricts the amount of the clip available, and could lead to the same clip
84 being loaded multiple times if you need different regions of the clip
86 2) A westley can be specified as a resource, so westleys can naturally
87 encapsulate other westleys.
92 Playlists provide a 'collection' structure for producers. These can be used
93 to define 'tracks' in the multitrack approach, or simple playlists for
94 sequential, single track playout.
96 As an example, the following defines two basic producers and a playlist with 3
100 <producer id="producer0">
101 <property name="resource">clip1.dv</property>
103 <producer id="producer1">
104 <property name="resource">clip2.dv</property>
106 <playlist id="playlist0">
107 <entry producer="producer0" in="0" out="2999"/>
108 <entry producer="producer1" in="0" out="999"/>
109 <entry producer="producer0" in="3000" out="6999"/>
113 Here we see how the playlist defines the in/out points of the basic
118 1) All in/out points are absolute frame positions relative to the producer
119 being appended to the playlist;
120 2) Westley documents are currently authored for a specific normalisation;
121 3) The last 'producer' in the document is the default for play out;
122 4) Playlists can reference the same producer multiple times. In/out regions
123 do not need to be contiguous - duplication and skipping is acceptable.
126 Interlude - Introducing Multitracks:
128 So far we've defined basic producers and playlists/tracks - the tractor is
129 the element that allows us to arrange our tracks and specify filters and
130 transitions. Similarly to a playlist, a tractor is a container.
132 Note that MLT doesn't see a filter or a transition as a producer in the
133 normal sense - filters and transitions are passive when it comes to seeking.
134 Internally, seeks are carried out on the producers. This is an important
135 point - MLT does not follow a traditional graph oriented model.
137 Visualising an MLT tractor and it's interaction with the consumer will
140 +----------------------------------------------+
142 | +----------+ +-+ +-+ +-+ +-+ |
143 | |multitrack| |f| |f| |t| |t| |
144 | | +------+ | |i| |i| |r| |r| |
145 | | |track0|-|--->|l|- ->|l|- ->|a|--->|a|\ |
146 | | +------+ | |t| |t| |n| |n| \ |
147 | | | |e| |e| |s| |s| \ |
148 | | +------+ | |r| |r| |i| |i| \ | +--------+
149 | | |track1|-|- ->|0|--->|1|--->|t|--->|t|-----|--->|consumer|
150 | | +------+ | | | | | |i| |i| / | +--------+
151 | | | | | | | |o| |o| / | ^
152 | | +------+ | | | | | |n| |n| / | |
153 | | |track2|-|- ->| |- ->| |--->|0|- ->|1|/ | |
154 | | +------+ | | | | | | | | | | |
155 | +----------+ +-+ +-+ +-+ +-+ | |
156 +----------------------------------------------+ |
160 |APPLICATION|--------------------------------------------+
163 Internally, all frames from all tracks pass through all the filters and
164 transitions - these are told which tracks to deal and which regions of the
167 Note that the application communicates with the producer - it can alter
168 playback speed, position, or even which producer is connected to which
171 The consumer receives the first non-blank frame (see below). It has no say
172 in the order in which gets them (the sdl consumer when used with inigo might
173 appear to be an exception - it isn't - it simply has a route back to the
174 application to allow the application to interpret key presses).
176 Whether this is better or worse than a traditional graph approach is a moot
177 point. The author of this document likes it anyway :-).
182 To create a multitrack westley, we can use two playlists and introduce a
183 tractor. For the purposes of demonstration, I'll add a filter here too:
186 <producer id="producer0">
187 <property name="resource">clip1.dv</property>
189 <producer id="producer1">
190 <property name="resource">clip2.dv</property>
192 <playlist id="playlist0">
193 <entry producer="producer0" in="0" out="2999"/>
194 <blank length="1000"/>
195 <entry producer="producer0" in="3000" out="6999"/>
196 <playlist id="playlist1">
197 <blank length="3000"/>
198 <entry producer="producer1" in="0" out="999"/>
200 <tractor id="tractor0">
202 <track producer="playlist0"/>
203 <track producer="playlist1"/>
206 <property name="track">0</property>
207 <property name="mlt_service">greyscale</property>
212 Here we see that blank frames are inserted into the first playlist and a
213 blank is provided at the beginning of the second - this can be visualised in
214 the traditional timeline widget as follows:
216 +-------+ +-------------+
218 +-------+---+-------------+
222 Adding the filter on the top track, gives us:
224 +-------+ +-------------+
226 +-------+---+-------------+
228 --------+---+-------------+
232 Note that it's only applied to the visible parts of the top track.
234 The requirement to apply a filter to the output, as opposed to a specific
235 track leads us to the final item in the Rules section above. As an example,
236 let's assume we wish to watermark all output, then we could use the
240 <producer id="producer0">
241 <property name="resource">clip1.dv</property>
243 <producer id="producer1">
244 <property name="resource">clip2.dv</property>
246 <playlist id="playlist0">
247 <entry producer="producer0" in="0" out="2999"/>
248 <blank length="1000"/>
249 <entry producer="producer0" in="3000" out="6999"/>
250 <playlist id="playlist1">
251 <blank length="3000"/>
252 <entry producer="producer1" in="0" out="999"/>
254 <tractor id="tractor0">
256 <track producer="playlist0"/>
257 <track producer="playlist1"/>
260 <property name="track">0</property>
261 <property name="mlt_service">greyscale</property>
264 <tractor id="tractor1">
266 <track producer="tractor0"/>
269 <property name="mlt_service">watermark</property>
270 <property name="resource">watermark1.png</property>
275 Here we employ another tractor and we define a single track (being the
276 tractor we previously defined) and apply a watermarking filter there.
278 This is simply provided as an example - the watermarking functionality could
279 be better handled at the playout stage itself (ie: as a filter automatically
280 placed between all producers and the consumer).
282 Tracks act like "layers" in an image processing program like the GIMP. The
283 bottom-most track takes highest priority and higher layers are overlays
284 and do not appear unless there are gaps in the lower layers or unless
285 a transition is applied that merges the tracks on the specifed region.
286 Practically speaking, for A/B video editing it does not mean too much,
287 and it will work as expected; however, as a general rule apply any CGI
288 (graphic overlays with pixbuf or titles with pango) on tracks higher than
289 your video tracks. Also, this means that any audio-only tracks that are
290 lower than your video tracks will play rather than the audio from the video
291 clip. Remember, nothing is affected like mixing or compositing until one
292 applies a transition or appropriate filter.
295 <producer id="producer0">
296 <property name="resource">clip1.dv</property>
298 <playlist id="playlist0">
299 <entry producer="producer0"/>
301 <producer id="producer1">
302 <property name="resource">clip2.mpeg</property>
304 <playlist id="playlist1">
306 <entry producer="producer1"/>
308 <tractor id="tractor0" in="0" out="315">
309 <multitrack id="multitrack0">
310 <track producer="playlist0"/>
311 <track producer="playlist1"/>
313 <transition id="transition0" in="50" out="74">
314 <property name="a_track">0</property>
315 <property name="b_track">1</property>
316 <property name="mlt_service">luma</property>
318 <transition id="transition1" in="50" out="74">
319 <property name="a_track">0</property>
320 <property name="b_track">1</property>
321 <property name="mlt_service">mix</property>
322 <property name="start">0.0</property>
323 <property name="end">1.0</property>
328 A "luma" transition is a video wipe processor that takes a greyscale bitmap
329 for the wipe definition. When one does not specify a bitmap, luma performs
330 a dissolve. The "mix" transition does an audio mix, but it interpolates
331 between the gain scaling factors between the start and end properties -
332 in this example, from 0.0 (none of track B) to 1.0 (all of track B).
333 Because the bottom track starts out with a gap specified using the <blank>
334 element, the upper track appears during the blank segment. See the demos and
335 services.txt to get an idea of the capabilities of the included transitions.
339 The information presented above is considered the MLT Westley "normal"
340 form. This is the output generated by the westley consumer, for example,
341 when used with inigo. It is the output generated when you use the
342 "Westley to File" consumer in the demo script, which beginners will find
343 most useful for learning to use westley XML. This section describes
344 alternative forms the westley producer accepts.
346 First of all, the normal form is more of a linear format with producers
347 and playlists defined prior to their usage in a multitrack. Westley
348 also accepts a hierarchical format with producers as children of tracks
349 or playlist entries and with playlists as children of tracks:
358 <property name="resource">clip1.dv</property>
367 Obviously, this example is meant to demonstrate hierarchy and not effective
368 use of playlist or multitrack!
370 Secondly, as part of error handling, westley is forgiving if you fail to
371 supply <tractor>, <track>, and <entry> where one can be understood. This
372 affords an abbreviated syntax that is less verbose and perhaps less
373 intimidating for a human to read and understand. One can simplify the
380 <property name="resource">clip1.dv</property>
386 Yes, filters and transitions can be added to the above example after the
387 closing multitrack tag (</multitrack>) because it is still enclosed within
388 the westley body tags.
390 If you specify in and out on a producer and it has been enclosed within
391 an <entry> or <playlist>, then the edit points apply to the playlist
392 entry and not to the producer itself. This facilitates re-use of media:
395 <producer id="clip1" in="25" out="78">
396 <property name="resource">clip1.dv</property>
398 <entry producer="clip1" in="119" out="347"/>
401 In the above example, the producer attribute of the entry element is
402 a reference to the preceding producer. All references must follow the
403 definition. The edit points supplied on the producer above will not affect
404 the entry that references it below because westley knows the clip is a
405 playlist entry and optimises this situation. The advantage is that one
406 does not need to determine every clip to be included ahead of time
407 and specify them outside the context of the mutlitrack timeline.
409 This form of authoring will be easier for many to visualise as a non-linear
410 editor's timeline. Here is a more complex example:
415 <producer id="foo" in="100" out="149">
416 <property name="resource">clip2.mpeg</property>
419 <entry producer="foo" in="10" out="59"/>
423 <producer id="bar" in="100" out="199">
424 <property name="resource">clip3.mpeg</property>
426 <entry out="99" producer="bar"/>
429 <filter mlt_service="greyscale" track="0"/>
430 <transition mlt_service="luma" in="25" out="49" a_track="0" b_track="1"/>
431 <transition mlt_service="luma" in="75" out="99" a_track="0" b_track="1">
432 <property name="reverse" value="1"/>
436 Did you notice something different in the last example? Properties can be
437 expressed using XML attributes on the element as well. However, only
438 non-service-specific properties are supported in this way. For example,
439 "mlt_service" is available to any producer, filter, or transition. However,
440 "resource" is actually service-specific. Notice the syntax of the last
441 property, on the last transition. Westley accepts property values using
442 the "value" attribute as well as using element text.
444 We have seen a few different ways of expressing property values. There are
445 a couple more for properties that can accept XML data. For example, the
446 GDK pixbuf producer with librsvg can handle embedded SVG, and the Pango
447 producer can handle embedded Pango markup. You can enclose the embedded XML
448 using a CDATA section:
450 <property name="resource"><![CDATA[ <svg>...</svg> ]]></property>
452 Please ensure the opening CDATA tag immediately follows the opening
453 property tag and that the section closing tag immediately precedes the
454 closing property tag.
456 However, westley can also accept inline embedded XML:
458 <property name="resource">
463 Currently, there is no namespace handling so a conflict will occur only on
464 any embedded XML that contains an element named "property" because
465 westley collects embedded XML until it reaches a closing property tag.
468 Entities and Parameterisation:
470 The westley producer parser supports XML entities. An example:
472 <?xml version="1.0"?>
474 <!ENTITY msg "Hello world!">
477 <producer id="producer0">
478 <property name="mlt_service">pango</property>
479 <property name="text">&msg;</property>
483 If you are embedding another XML document into a property value not using
484 a CNODE section, then any DOCTYPE section must be relocated before any of
485 the xml elements to be well-formed. See demo/dvg.westley for an example.
487 Entities can be used to parameterise westley! Using the above example, the
488 entity declared serves as the default value for &msg;. The entity content
489 can be overridden from the resource property supplied to the westley
490 producer. The syntax is the familiar, url-encoded query string used with
491 HTTP, e.g.: file?name=value&name=value...
493 There are a couple of rules of usage. The Miracle LOAD command and inigo
494 command line tool require you to preface the URL with "westley:" because
495 the query string destroys the filename extension matching peformed by
496 Fezzik. Also, inigo looks for '=' to tokenise property settings. Therefore,
497 one uses ':' between name and value instead of '='. Finally, since inigo
498 is run from the shell, one must enclose the URL within single quotes to
499 prevent shell filename expansion, or similar.
501 Needless to say, the ability to parameterise westley XML compositions is
502 an extremely powerful tool. An example for you to play with is available in
503 demo/entity.westley. Try overriding the name from inigo:
504 inigo 'westley:entity.westley?name:Charlie'
506 Technically, the entity declaration is not needed in the head of the XML
507 document if you always supply the parameter. However, you run the risk
508 of unpredictable behviour without one. Therefore, it is safest and a best
509 practice to always supply an entity declaration. It is improves the
510 readability as one does not need to search for the entity references to
511 see what parameters are available.
516 If one finds the above hierarchical, abbreviated format intuitive,
517 start with a simple template and fill and extend as needed:
522 ...add a playlist for each track...
524 ...add filters and transitions...
527 By using a playlist for each track, it is easier to iteratively add new
528 clips and blank regions as you develop the project. You will not have to
529 use <track> or later add <playlist> when necessary.
531 A more advanced template that allows sequencing multitracks is:
537 ...add a playlist for each track...
539 ...add filters and transitions...
546 ...add a playlist for each track...
548 ...add filters and transitions...
552 If you end up making a collection of templates for various situations, then
553 consider using XML Entities to make the template more effective by moving
554 anything that should parameterised into an entity.
556 If you want to have a silent, black background for audio and video fades,
557 then make the top track simply <producer mlt_service="colour"/>. Then,
558 use composite and volume effects. See the "Fade from/to black/silence"
559 demo for an example (demo/mlt_fade_black).
561 If you apply the reverse=1 property to a transition like "luma," then
562 be careful because it also inherently swaps the roles of A and B tracks.
563 Therefore, you need to might need to swap the a_track and b_track values
564 if it did not turn out the way you expected. See the "Clock in and out"
565 for an example (demo/mlt_clock_in_and_out).