1 Framework Documentation
3 Copyright (C) 2004 Ushodaya Enterprises Limited
4 Author: Charles Yates <charles.yates@pandora.be>
5 Last Revision: 2004-03-20
13 MLT is a multimedia framework designed for television broadcasting. As such,
14 it provides a pluggable architecture for the inclusion of new audio/video
15 sources, filters, transitions and playback devices.
17 The framework provides the structure and utility functionality on which
18 all of the MLT applications and services are defined.
20 On its own, the framework provides little more than 'abstract classes' and
21 utilities for managing resources, such as memory, properties, dynamic object
22 loading and service instantiation.
24 This document is split roughly into 3 sections. The first section provides a
25 basic overview of MLT, the second section shows how it's used and the final
26 section shows shows structure and design, with an emphasis on how the system
32 This document is provided as a 'road map' for the framework and should be
33 considered mandatory reading for anyone wishing to develop code at the MLT
38 1. framework maintainers;
40 3. application developers;
41 4. anyone interested in MLT.
43 The emphasis of the document is in explaining the public interfaces, as
44 opposed to the implementation details.
46 It is not required reading for the MLT client/server integration - please
47 refer to valerie.txt and dvcp.txt for more details on this area.
50 SECTION 1 - BASIC OVERVIEW
51 --------------------------
53 Basic Design Information:
57 The framework has no dependencies other than the standard C99 and POSIX
60 It follows a basic Object Oriented design paradigm, and as such, much of the
61 design is loosely based on the Producer/Consumer design pattern.
63 It employs Reverse Polish Notation for the application of audio and video FX.
65 The framework is designed to be colour space neutral - the currently
66 implemented modules, however, are very much 8bit YUV422 oriented. In theory,
67 the modules could be entirely replaced.
69 A vague understanding of these terms is assumed throughout the remainder of
75 The general structure of an MLT 'network' is simply the connection of a
76 'producer' to a 'consumer':
79 |Producer|-->|Consumer|
82 A typical consumer requests MLT Frame objects from the producer, does
83 something with them and when finished with a frame, closes it.
85 /\ A common confusion with the producer/consumer terminology used here is
86 /!!\ that a consumer may 'produce' something. For example, the libdv consumer
87 \!!/ produces DV and the libdv producer seems to consume DV. However, the
88 \/ naming conventions refer only to producers and consumers of MLT Frames.
90 To put it another way - a producer produces MLT Frame objects and a consumer
91 consumes MLT Frame objects.
93 An MLT Frame essentially provides an uncompressed image and its associated
96 Filters may also be placed between the producer and the consumer:
98 +--------+ +------+ +--------+
99 |Producer|-->|Filter|-->|Consumer|
100 +--------+ +------+ +--------+
102 A service is the collective name for producers, filters, transitions and
105 The communications between a connected consumer and producer or service are
106 carried out in 3 phases:
112 MLT employs 'lazy evaluation' - the image and audio need not be extracted
113 from the source until the get image and audio methods are invoked.
115 In essence, the consumer pulls from what it's connected to - this means that
116 threading is typically in the domain of the consumer implementation and some
117 basic functionality is provided on the consumer class to ensure realtime
126 Before we go in to the specifics of the framework architecture, a working
127 example of usage is provided.
129 The following simply provides a media player:
133 #include <framework/mlt.h>
135 int main( int argc, char *argv[] )
137 // Initialise the factory
138 if ( mlt_factory_init( NULL ) == 0 )
140 // Create the default consumer
141 mlt_consumer hello = mlt_factory_consumer( NULL, NULL );
143 // Create via the default producer
144 mlt_producer world = mlt_factory_producer( NULL, argv[ 1 ] );
146 // Connect the producer to the consumer
147 mlt_consumer_connect( hello, mlt_producer_service( world ) );
149 // Start the consumer
150 mlt_consumer_start( hello );
152 // Wait for the consumer to terminate
153 while( !mlt_consumer_is_stopped( hello ) )
156 // Close the consumer
157 mlt_consumer_close( hello );
159 // Close the producer
160 mlt_producer_close( world );
163 mlt_factory_close( );
167 // Report an error during initialisation
168 fprintf( stderr, "Unable to locate factory modules\n" );
175 This is a simple example - it doesn't provide any seeking capabilities or
176 runtime configuration options.
178 The first step of any MLT application is the factory initialisation - this
179 ensures that the environment is configured and MLT can function. The factory
180 is covered in more detail below.
182 All services are instantiated via the factories, as shown by the
183 mlt_factory_consumer and mlt_factory_producer calls above. There are similar
184 factories for filters and transitions. There are details on all the standard
185 services in services.txt.
187 The defaults requested here are a special case - the NULL usage requests
188 that we use the default producers and consumers.
190 The default producer is "fezzik". This producer matches file names to
191 locate a service to use and attaches 'normalising filters' (such as scalers,
192 deinterlacers, resamplers and field normalisers) to the loaded content -
193 these filters ensure that the consumer gets what it asks for.
195 The default consumer is "sdl". The combination of fezzik and sdl will
196 provide a media player.
198 In this example, we connect the producer and then start the consumer. We
199 then wait until the consumer is stopped (in this case, by the action of the
200 user closing the SDL window) and finally close the consumer, producer and
201 factory before exiting the application.
203 Note that the consumer is threaded - waiting for an event of some sort is
204 always required after starting and before stopping or closing the consumer.
206 Also note, you can override the defaults as follows:
208 $ MLT_CONSUMER=westley ./hello file.avi
210 This will create a westley xml document on stdout.
212 $ MLT_CONSUMER=westley MLT_PRODUCER=avformat ./hello file.avi
214 This will play the video using the avformat producer directly, thus it will
215 bypass the normalising functions.
217 $ MLT_CONSUMER=libdv ./hello file.avi > /dev/dv1394
219 This might, if you're lucky, do on the fly, realtime conversions of file.avi
220 to DV and broadcast it to your DV device.
225 As shown in the 'Hello World' example, factories create service objects.
227 The framework itself provides no services - they are provided in the form of
228 a plugin structure. A plugin is organised in the form of a 'module' and a
229 module can provide many services of different types.
231 Once the factory is initialised, all the configured services are available
234 The complete set of methods associated to the factory are as follows:
236 int mlt_factory_init( char *prefix );
237 const char *mlt_factory_prefix( );
238 char *mlt_environment( char *name );
239 mlt_producer mlt_factory_producer( char *name, void *input );
240 mlt_filter mlt_factory_filter( char *name, void *input );
241 mlt_transition mlt_factory_transition( char *name, void *input );
242 mlt_consumer mlt_factory_consumer( char *name, void *input );
243 void mlt_factory_close( );
245 The mlt_factory_prefix returns the path to the location of the installed
246 modules directory. This can be specified in the mlt_factory_init call
247 itself, or it can be specified via the MLT_REPOSITORY environment variable,
248 or in the absence of either of those, it will default to the install
249 prefix/shared/mlt/modules.
251 The mlt_environment provides read only access to a collection of name=value
252 pairs as shown in the following table:
254 +------------------+------------------------------------+------------------+
255 |Name |Description |Values |
256 +------------------+------------------------------------+------------------+
257 |MLT_NORMALISATION |The normalisation of the system |PAL or NTSC |
258 +------------------+------------------------------------+------------------+
259 |MLT_PRODUCER |The default producer |"fezzik" or other |
260 +------------------+------------------------------------+------------------+
261 |MLT_CONSUMER |The default consumer |"sdl" or other |
262 +------------------+------------------------------------+------------------+
264 These values are initialised from the environment variables of the same
267 As shown above, a producer can be created using the 'default normalising'
268 producer, and they can also be requested by name. Filters and transitions
269 are always requested by name - there is no concept of a 'default' for these.
274 As shown in the services.txt document, all services have their own set of
275 properties than can be manipulated to affect their behaviour.
277 In order to set properties on a service, we need to retrieve the properties
278 object associated to it. For producers, this is done by invoking:
280 mlt_properties properties = mlt_producer_properties( producer );
282 All services have a similar method associated to them.
284 Once retrieved, setting and getting properties can be done directly on this
287 mlt_properties_set( properties, "name", "value" );
289 A more complete description of the properties object is found below.
294 So far, we've shown a simple producer/consumer configuration - the next
295 phase is to organise producers in playlists.
297 Let's assume that we're adapting the Hello World example, and wish to queue
298 a number of files for playout, ie:
302 Instead of invoking mlt_factory_producer directly, we'll create a new
303 function called create_playlist. This function is responsible for creating
304 the playlist, creating each producer, appending to the playlist and ensuring
305 that all the producers are cleaned up when the playlist is destroyed. The
306 last point is important - a close on the playlist won't explicitly close these
307 producers. In this example, we use unique "data" properties with destructors
310 mlt_producer create_playlist( int argc, char **argv )
312 // We're creating a playlist here
313 mlt_playlist playlist = mlt_playlist_init( );
315 // We need the playlist properties to ensure clean up
316 mlt_properties properties = mlt_playlist_properties( playlist );
318 // Loop through each of the arguments
320 for ( i = 1; i < argc; i ++ )
322 // Define the unique key
325 // Create the producer
326 mlt_producer producer = mlt_factory_producer( NULL, argv[ i ] );
328 // Add it to the playlist
329 mlt_playlist_append( playlist, producer );
331 // Create a unique key for this producer
332 sprintf( key, "producer%d", i );
334 // Now we need to ensure the producers are destroyed
335 mlt_properties_set_data( properties, key, producer, 0, ( mlt_destructor )mlt_producer_close, NULL );
338 // Return the playlist as a producer
339 return mlt_playlist_producer( playlist );
342 Now all we need do is to replace these lines in the main function:
344 // Create a normalised producer
345 mlt_producer world = mlt_factory_producer( NULL, argv[ 1 ] );
350 mlt_producer world = create_playlist( argc, argv );
352 and we have a means to play multiple clips.
357 Inserting filters between the producer and consumer is just a case of
358 instantiating the filters, connecting the first to the producer, the next
359 to the previous filter and the last filter to the consumer.
363 // Create a producer from something
364 mlt_producer producer = mlt_factory_producer( ... );
366 // Create a consumer from something
367 mlt_consumer consumer = mlt_factory_consumer( ... );
369 // Create a greyscale filter
370 mlt_filter filter = mlt_factory_filter( "greyscale", NULL );
372 // Connect the filter to the producer
373 mlt_filter_connect( filter, mlt_producer_service( producer ), 0 );
375 // Connect the consumer to filter
376 mlt_consumer_connect( consumer, mlt_filter_service( filter ) );
378 As with producers and consumers, filters can be manipulated via their
379 properties object - the mlt_filter_properties method can be invoked and
380 properties can be set as needed.
382 The additional argument in the filter connection is an important one as it
383 dictates the 'track' on which the filter operates. For basic producers and
384 playlists, there's only one track (0), and as you will see in the next
385 section, even multiple tracks have a single track output.
388 Multiple Tracks and Transitions:
390 MLT's approach to multiple tracks is governed by two requirements:
392 1) The need for a consumer and producer to communicate with one another via
394 2) The desire to be able to serialise and manipulate a 'network' (or filter
395 graph if you prefer).
397 We can visualise a multitrack in the way that an NLE presents it:
399 +-----------------+ +-----------------------+
401 +---------------+-+--------------------------+-+---------------------+
403 +------------------------------+
405 The overlapping areas of track 0 and 1 would (presumably) have some kind of
406 transition - without a transition, the frames from b1 and b2 would be shown
407 during the areas of overlap (ie: by default, the higher numbered track takes
408 precedence over the lower numbered track).
410 MLT has a multitrack object, but it is not a producer in the sense that it
411 can be connected directly to a consumer and everything will work correctly.
412 A consumer would treat it precisely as it would a normal producer, and, in
413 the case of the multitrack above, you would never see anything from track 1
414 other than the transitions between the clips - the gap between a1 and a2
415 would show test frames.
417 This happens because a consumer pulls one frame from the producer it's
418 connected to while a multitrack will provide one frame per track.
419 Something, somewhere, must ensure that all frames are pulled from the
420 multitrack and elect the correct frame to pass on.
422 Hence, MLT provides a wrapper for the multitrack, which is called a
423 'tractor', and its the tractors task to ensure that all tracks are pulled
424 evenly, the correct frame is output and that we have 'producer like'
427 Thus, a multitrack is conceptually 'pulled' by a tractor as shown here:
431 | +------+ | +-------+
432 | |track0|-|--->|tractor|
436 | |track1|-|--->|---o---|--->
441 | +------+ | +-------+
444 With a combination of the two, we can now connect multitracks to consumers.
445 The last non-test card will be retrieved and passed on.
447 The tracks can be producers, playlists, or even other tractors.
449 Now we wish to insert filters and transitions between the multitrack and the
450 tractor. We can do this directly by inserting filters directly between the
451 tractor and the multitrack, but this involves a lot of connecting and
452 reconnecting left and right producers and consumers, and it seemed only fair
453 that we should be able to automate that process.
455 So in keeping with our agricultural theme, the concept of the 'field' was
456 born. We 'plant' filters and transitions in the field and the tractor pulls
457 the multitrack (think of a combine harvester :-)) over the field and
458 produces a 'bail' (sorry - kidding - frame :-)).
460 Conceptually, we can see it like this:
464 | +------+ | +-------------+ +-------+
465 | |track0|-|--->|field |--->|tractor|
466 | +------+ | | | |\ |
467 | | | filters | | \ |
468 | +------+ | | and | | \ |
469 | |track1|-|--->| transitions |--->|---o---|--->
470 | +------+ | | | | / |
472 | +------+ | | | |/ |
473 | |track2|-|--->| |--->| |
474 | +------+ | +-------------+ +-------+
477 In reality, we create a field first, and from that we obtain a multitrack
478 and a tractor. We can then populate the multitrack, field and finally,
479 connect the tractor to the consumer.
481 The reasoning behind this is possibly flawed - it might have made more
482 sense to produce the tractor and have it encapsulate the field and the
483 multitrack as that is how it looks to a connected consumer:
485 +-----------------------------------------------+
486 |tractor +--------------------------+ |
487 | +----------+ | +-+ +-+ +-+ +-+ | |
488 | |multitrack| | |f| |f| |t| |t| | |
489 | | +------+ | | |i| |i| |r| |r| | |
490 | | |track0|-|--->| |l|- ->|l|- ->|a|--->|a|\| |
491 | | +------+ | | |t| |t| |n| |n| | |
492 | | | | |e| |e| |s| |s| |\ |
493 | | +------+ | | |r| |r| |i| |i| | \|
494 | | |track1|-|- ->| |0|--->|1|--->|t|--->|t|-|--o--->
495 | | +------+ | | | | | | |i| |i| | /|
496 | | | | | | | | |o| |o| |/ |
497 | | +------+ | | | | | | |n| |n| | |
498 | | |track2|-|- ->| | |- ->| |--->|0|- ->|1|/| |
499 | | +------+ | | | | | | | | | | | |
500 | +----------+ | +-+ +-+ +-+ +-+ | |
501 | +--------------------------+ |
502 +-----------------------------------------------+
504 An example will hopefully clarify this.
506 Let's assume that we want to provide a 'watermark' to our hello world
507 example. We have already extended the example to play multiple clips,
508 and now we will place a text based watermark, reading 'Hello World' in
509 the top left hand corner:
511 mlt_producer create_tracks( int argc, char **argv )
514 mlt_field field = mlt_field_init( );
516 // Obtain the multitrack
517 mlt_multitrack multitrack = mlt_field_multitrack( field );
519 // Obtain the tractor
520 mlt_tractor tractor = mlt_field_tractor( field );
522 // Obtain a composite transition
523 mlt_transition transition = mlt_factory_transition( "composite", "10%,10%:15%x15%" );
526 mlt_producer track0 = create_playlist( argc, argv );
528 // Create the watermark track - note we NEED fezzik for scaling here
529 mlt_producer track1 = mlt_factory_producer( "fezzik", "pango" );
531 // Get the length of track0
532 mlt_position length = mlt_producer_get_playtime( track0 );
534 // Set the properties of track1
535 mlt_properties properties = mlt_producer_properties( track1 );
536 mlt_properties_set( properties, "text", "Hello\nWorld" );
537 mlt_properties_set_position( properties, "in", 0 );
538 mlt_properties_set_position( properties, "out", length - 1 );
539 mlt_properties_set_position( properties, "length", length );
540 mlt_properties_set_int( properties, "a_track", 0 );
541 mlt_properties_set_int( properties, "b_track", 1 );
543 // Now set the properties on the transition
544 properties = mlt_transition_properties( transition );
545 mlt_properties_set_position( properties, "in", 0 );
546 mlt_properties_set_position( properties, "out", length - 1 );
548 // Add our tracks to the multitrack
549 mlt_multitrack_connect( multitrack, track0, 0 );
550 mlt_multitrack_connect( multitrack, track1, 1 );
552 // Now plant the transition
553 mlt_field_plant_transition( field, transition, 0, 1 );
555 // Now set the properties on the tractor
556 properties = mlt_tractor_properties( tractor );
557 mlt_properties_set_data( properties, "multitrack", multitrack, 0, ( mlt_destructor )mlt_multitrack_close, NULL );
558 mlt_properties_set_data( properties, "field", field, 0, ( mlt_destructor )mlt_field_close, NULL );
559 mlt_properties_set_data( properties, "track0", track0, 0, ( mlt_destructor )mlt_producer_close, NULL );
560 mlt_properties_set_data( properties, "track1", track1, 0, ( mlt_destructor )mlt_producer_close, NULL );
561 mlt_properties_set_data( properties, "transition", transition, 0, ( mlt_destructor )mlt_transition_close, NULL );
563 // Return the tractor
564 return mlt_tractor_producer( tractor );
567 Now all we need do is to replace these lines in the main function:
570 mlt_producer world = create_playlist( argc, argv );
574 // Create a watermarked playlist
575 mlt_producer world = create_tracks( argc, argv );
577 and we have a means to play multiple clips with a horribly obtrusive
578 watermark - just what the world needed, right? ;-)
581 SECTION 3 - STRUCTURE AND DESIGN
582 --------------------------------
586 The mlt framework consists of an OO class hierarchy which consists of the
587 following public classes and abstractions:
602 Each class defined above can be read as extending the classes above and to
605 The following sections describe the properties, stacking/queuing and memory
606 pooling functionality provided by the framework - these are key components
607 and a basic understanding of these is required for the remainder of the
613 The properties class is the base class for the frame and service classes.
615 It is designed to provide an efficient lookup table for various types of
616 information, such as strings, integers, floating points values and pointers
617 to data and data structures.
619 All properties are indexed by a unique string.
621 The most basic use of properties is as follows:
623 // 1. Create a new, empty properties set;
624 mlt_properties properties = mlt_properties_new( );
626 // 2. Assign the value "world" to the property "hello";
627 mlt_properties_set( properties, "hello", "world" );
629 // 3. Retrieve and print the value of "hello";
630 printf( "%s\n", mlt_properties_get( properties, "hello" ) );
632 // 4. Reassign "hello" to "world!";
633 mlt_properties_set( properties, "hello", "world!" );
635 // 5. Retrieve and print the value of "hello";
636 printf( "%s\n", mlt_properties_get( properties, "hello" ) );
638 // 6. Assign the value "0" to "int";
639 mlt_properties_set( properties, "int", "0" );
641 // 7. Retrieve and print the integer value of "int";
642 printf( "%d\n", mlt_properties_get_int( properties, "int" ) );
644 // 8. Assign the integer value 50 to "int2";
645 mlt_properties_set_int( properties, "int2", 50 );
647 // 9. Retrieve and print the double value of "int2";
648 printf( "%s\n", mlt_properties_get( properties, "int2" ) );
650 Steps 2 through 5 demonstrate that the "name" is unique - set operations on
651 an existing "name" change the value. They also free up memory associated to
652 the previous value. Note that it also possible to change type in this way
655 Steps 6 and 7 demonstrate that the properties object handles deserialisation
656 from strings. The string value of "0" is set, the integer value of 0 is
659 Steps 8 and 9 demonstrate that the properties object handles serialisation
662 To show all the name/value pairs in a properties, it is possible to iterate
665 for ( i = 0; i < mlt_properties_count( properties ); i ++ )
666 printf( "%s = %s\n", mlt_properties_get_name( properties, i ),
667 mlt_properties_get_value( properties, i ) );
669 Note that properties are retrieved in the order in which they are set.
671 Properties are also used to hold pointers to memory. This is done via the
674 uint8_t *image = malloc( size );
675 mlt_properties_set_data( properties, "image", image, size, NULL, NULL );
677 In this example, we specify that the pointer can be retrieved from
678 properties by a subsequent request to get_data:
680 image = mlt_properties_get_data( properties, "image", &size );
684 image = mlt_properties_get_data( properties, "image", NULL );
686 if we don't wish to retrieve the size.
690 1) The allocated memory remains after the properties object is closed unless
691 you specify a destructor. In the case above, this can be done with:
693 mlt_properties_set_data( properties, "image", image, size, free, NULL );
695 When the properties are closed, or the value of "image" is changed, the
696 destructor is invoked.
698 2) The string value returned by mlt_properties_get is NULL. Typically, you
699 wouldn't wish to serialise an image as a string, but other structures
700 might need such functionality - you can specify a serialiser as the last
701 argument if required (declaration is char *serialise( void * )).
703 Properties also provides some more advanced usage capabilities.
705 It has the ability to inherit all serialisable values from another properties
708 mlt_properties_inherit( this, that );
710 It has the ability to mirror properties set on this on another set of
713 mlt_properties_mirror( this, that );
715 After this call, all serialisable values set on this are passed on to that.
720 Stacks and queues are essential components in the MLT framework. Being of a
721 lazy disposition, we elected to implement a 'Double Ended Queue' (deque) -
722 this encapsulates the functionality of both.
724 The API of the deque is defined as follows:
726 mlt_deque mlt_deque_init( );
727 int mlt_deque_count( mlt_deque this );
728 int mlt_deque_push_back( mlt_deque this, void *item );
729 void *mlt_deque_pop_back( mlt_deque this );
730 int mlt_deque_push_front( mlt_deque this, void *item );
731 void *mlt_deque_pop_front( mlt_deque this );
732 void *mlt_deque_peek_back( mlt_deque this );
733 void *mlt_deque_peek_front( mlt_deque this );
734 void mlt_deque_close( mlt_deque this );
736 The stacking operations are used in a number of places:
738 * Reverse Polish Notation (RPN) image and audio operations
741 The queuing operations are used in:
743 * the consumer base class;
744 * consumer implementations may require further queues.
749 The MLT framework provides memory pooling capabilities through the mlt_pool
750 API. Once initilialised, these can be seen as a straightforward drop in
751 replacement for malloc/realloc/free functionality.
753 The background behind this API is that malloc/free operations are
754 notoriously inefficient, especially when dealing with large blocks of memory
755 (such as an image). On linux, malloc is optimised for memory allocations
756 less than 128k - memory blocks allocated of these sizes or less are retained
757 in the process heap for subsequent reuse, thus bypassing the kernel calls
758 for repeated allocation/frees for small blocks of memory. However, blocks of
759 memory larger than that require kernel calls and this has a detrimental
760 impact on performance.
762 The mlt_pool design is simply to hold a list of stacks - there is one stack
763 per 2^n bytes (where n is between 8 and 31). When an alloc is called, the
764 requested size is rounded to the next 2^n, the stack is retrieved for that
765 size, and an item is popped or created if the stack is empty.
767 Each item has a 'header', situated immediately before the returned address -
768 this holds the 'stack' to which the item belongs.
770 When an item is released, we retrieve the header, obtain the stack and push
773 Thus, from the programmers point of view, the API is the same as the
774 traditional malloc/realloc/free calls:
776 void *mlt_pool_alloc( int size );
777 void *mlt_pool_realloc( void *ptr, int size );
778 void mlt_pool_release( void *release );
783 A frame object is essentially defined as:
793 The life cycle of a frame can be represented as follows:
795 +-----+----------------------+-----------------------+---------------------+
796 |Stage|Producer |Filter |Consumer |
797 +-----+----------------------+-----------------------+---------------------+
798 | 0.0 | | |Request frame |
799 +-----+----------------------+-----------------------+---------------------+
800 | 0.1 | |Receives request | |
801 | | |Request frame | |
802 +-----+----------------------+-----------------------+---------------------+
803 | 0.2 |Receives request | | |
804 | |Generates frame for | | |
805 | |current position | | |
806 | |Increments position | | |
807 +-----+----------------------+-----------------------+---------------------+
808 | 0.3 | |Receives frame | |
809 | | |Updates frame | |
810 +-----+----------------------+-----------------------+---------------------+
811 | 0.4 | | |Receives frame |
812 +-----+----------------------+-----------------------+---------------------+
814 Note that neither the filter nor the consumer have any conception of
815 'position' until they receive a frame. Speed and position are properties of
816 the producer, and they are assigned to the frame object when the producer
819 Step 0.3 is a critical one here - if the filter determines that the frame is
820 of interest to it, then it should manipulate the image and/or audio stacks
821 and properties as required.
823 Assuming that the filter deals with both image and audio, then it should
824 push data and methods on to the stacks which will deal with the processing.
825 This can be done with the mlt_frame_push_image and audio methods. In order for
826 the filter to register interest in the frame, the stacks should hold:
829 [ producer_get_image ] [ data1 ] [ data2 ] [ filter_get_image ]
832 [ producer_get_audio ] [ data ] [ filter_get_audio ]
834 The filter_get methods are invoked automatically when the consumer invokes a
835 get_image on the frame.
837 +-----+----------------------+-----------------------+---------------------+
838 |Stage|Producer |Filter |Consumer |
839 +-----+----------------------+-----------------------+---------------------+
840 | 1.0 | | |frame_get_image |
841 +-----+----------------------+-----------------------+---------------------+
842 | 1.1 | |filter_get_image: | |
843 | | | pop data2 and data1 | |
844 | | | frame_get_image | |
845 +-----+----------------------+-----------------------+---------------------+
846 | 1.2 |producer_get_image | | |
847 | | Generates image | | |
848 +-----+----------------------+-----------------------+---------------------+
849 | 1.3 | |Receives image | |
850 | | |Updates image | |
851 +-----+----------------------+-----------------------+---------------------+
852 | 1.4 | | |Receives image |
853 +-----+----------------------+-----------------------+---------------------+
855 Obviously, if the filter isn't interested in the image, then it should leave
856 the stack alone, and then the consumer will retrieve its image directly from
859 Similarly, audio is handled as follows:
861 +-----+----------------------+-----------------------+---------------------+
862 |Stage|Producer |Filter |Consumer |
863 +-----+----------------------+-----------------------+---------------------+
864 | 2.0 | | |frame_get_audio |
865 +-----+----------------------+-----------------------+---------------------+
866 | 2.1 | |filter_get_audio: | |
868 | | | frame_get_audio | |
869 +-----+----------------------+-----------------------+---------------------+
870 | 2.2 |producer_get_audio | | |
871 | | Generates audio | | |
872 +-----+----------------------+-----------------------+---------------------+
873 | 2.3 | |Receives audio | |
874 | | |Updates audio | |
875 +-----+----------------------+-----------------------+---------------------+
876 | 2.4 | | |Receives audio |
877 +-----+----------------------+-----------------------+---------------------+
879 And finally, when the consumer is done with the frame, it should close it.
881 Note that a consumer may not evaluate both image and audio for any given
882 frame, especially in a realtime environment. See 'Realtime Considerations'
885 By default, a frame has the following properties:
887 +------------------+------------------------------------+------------------+
888 |Name |Description |Values |
889 +------------------+------------------------------------+------------------+
890 |_position |The producers frame position |0 to n |
891 +------------------+------------------------------------+------------------+
892 |_speed |The producers speed |double |
893 +------------------+------------------------------------+------------------+
894 |image |The generated image |NULL or pointer |
895 +------------------+------------------------------------+------------------+
896 |alpha |The generated alpha mask |NULL or pointer |
897 +------------------+------------------------------------+------------------+
898 |width |The width of the image | |
899 +------------------+------------------------------------+------------------+
900 |height |The height of the image | |
901 +------------------+------------------------------------+------------------+
902 |normalised_width |The normalised width of the image |720 |
903 +------------------+------------------------------------+------------------+
904 |normalised_height |The normalised height of the image |576 or 480 |
905 +------------------+------------------------------------+------------------+
906 |progressive |Indicates progressive/interlaced |0 or 1 |
907 +------------------+------------------------------------+------------------+
908 |top_field_first |Indicates top field first |0 or 1 |
909 +------------------+------------------------------------+------------------+
910 |audio |The generated audio |NULL or pointer |
911 +------------------+------------------------------------+------------------+
912 |frequency |The frequency of the audio | |
913 +------------------+------------------------------------+------------------+
914 |channels |The channels of the audio | |
915 +------------------+------------------------------------+------------------+
916 |samples |The samples of the audio | |
917 +------------------+------------------------------------+------------------+
918 |aspect_ratio |The sample aspect ratio of the image|double |
919 +------------------+------------------------------------+------------------+
920 |test_image |Used to indicate no image available |0 or 1 |
921 +------------------+------------------------------------+------------------+
922 |test_audio |Used to indicate no audio available |0 or 1 |
923 +------------------+------------------------------------+------------------+
925 The consumer can attach the following properties which affect the default
926 behaviour of a frame:
928 +------------------+------------------------------------+------------------+
929 |test_card_producer|Synthesise test images from here |NULL or pointer |
930 +------------------+------------------------------------+------------------+
931 |consumer_aspect_ |Apply this aspect ratio to the test |double |
932 |ratio |card producer | |
933 +------------------+------------------------------------+------------------+
934 |rescale.interp |Use this scale method for test image|"string" |
935 +------------------+------------------------------------+------------------+
937 While most of these are mainly self explanatory, the normalised_width and
938 normalised_height values require a little explanation. These are required
939 to ensure that effects are consistently handled as PAL or NTSC, regardless
940 of the consumers or producers width/height image request.
942 The test_image and audio flags are used to determine when images and audio
943 should be synthesised.
945 Additional properties may be provided by the producer implementation, and
946 filters, transitions and consumers may add additional properties to
947 communicate specific requests. These are documented in modules.txt.
949 The complete API for the mlt frame is as follows:
951 mlt_frame mlt_frame_init( );
952 mlt_properties mlt_frame_properties( mlt_frame this );
953 int mlt_frame_is_test_card( mlt_frame this );
954 int mlt_frame_is_test_audio( mlt_frame this );
955 double mlt_frame_get_aspect_ratio( mlt_frame this );
956 int mlt_frame_set_aspect_ratio( mlt_frame this, double value );
957 mlt_position mlt_frame_get_position( mlt_frame this );
958 int mlt_frame_set_position( mlt_frame this, mlt_position value );
959 int mlt_frame_get_image( mlt_frame this, uint8_t **buffer, mlt_image_format *format, int *width, int *height, int writable );
960 uint8_t *mlt_frame_get_alpha_mask( mlt_frame this );
961 int mlt_frame_get_audio( mlt_frame this, int16_t **buffer, mlt_audio_format *format, int *frequency, int *channels, int *samples );
962 int mlt_frame_push_get_image( mlt_frame this, mlt_get_image get_image );
963 mlt_get_image mlt_frame_pop_get_image( mlt_frame this );
964 int mlt_frame_push_frame( mlt_frame this, mlt_frame that );
965 mlt_frame mlt_frame_pop_frame( mlt_frame this );
966 int mlt_frame_push_service( mlt_frame this, void *that );
967 void *mlt_frame_pop_service( mlt_frame this );
968 int mlt_frame_push_audio( mlt_frame this, void *that );
969 void *mlt_frame_pop_audio( mlt_frame this );
970 void mlt_frame_close( mlt_frame this );
975 The service base class extends properties and allows 0 to m inputs and 0 to
976 n outputs and is represented as follows:
986 Descendents of service impose restrictions on how inputs and outputs can be
987 connected and will provide a basic set of properties. Typically, the service
988 instance is encapsulated by the descendent in order for it to ensure that
989 its connection rules are followed.
991 A service does not define any properties when constructed. It should be
992 noted that producers, filters and transitions my be serialised (say, via the
993 westley consumer), and care should be taken to distinguish between
994 serialisable and transient properties. The convention used is to prefix
995 transient properties with an underscore.
997 The public interface is defined by the following functions:
999 int mlt_service_init( mlt_service this, void *child );
1000 mlt_properties mlt_service_properties( mlt_service this );
1001 int mlt_service_connect_producer( mlt_service this, mlt_service producer, int index );
1002 int mlt_service_get_frame( mlt_service this, mlt_frame_ptr frame, int index );
1003 void mlt_service_close( mlt_service this );
1005 Typically, only direct descendents of services need invoke these methods and
1006 developers are encouraged to use those extensions when defining new services.
1011 A producer has 0 inputs and 1 output:
1021 A producer provides an abstraction for file readers, pipes, streams or any
1022 other image or audio input.
1024 When instantiated, a producer has the following properties:
1026 +------------------+------------------------------------+------------------+
1027 |Name |Description |Values |
1028 +------------------+------------------------------------+------------------+
1029 |mlt_type |The producers type |mlt_producer |
1030 +------------------+------------------------------------+------------------+
1031 |_position |The producers frame position |0 to n |
1032 +------------------+------------------------------------+------------------+
1033 |_speed |The producers speed |double |
1034 +------------------+------------------------------------+------------------+
1035 |fps |The output frames per second |25 or 29.97 |
1036 +------------------+------------------------------------+------------------+
1037 |in |The in point in frames |0 to length - 1 |
1038 +------------------+------------------------------------+------------------+
1039 |out |The out point in frames |in to length - 1 |
1040 +------------------+------------------------------------+------------------+
1041 |length |The length of the input in frames |0 to n |
1042 +------------------+------------------------------------+------------------+
1043 |aspect_ratio |aspect_ratio of the source |0 to n |
1044 +------------------+------------------------------------+------------------+
1045 |eof |end of clip behaviour |"pause" or "loop" |
1046 +------------------+------------------------------------+------------------+
1047 |resource |Constructor argument (ie: file name)|"<resource>" |
1048 +------------------+------------------------------------+------------------+
1050 Additional properties may be provided by the producer implementation.
1052 The public interface is defined by the following functions:
1054 mlt_producer mlt_producer_new( );
1055 int mlt_producer_init( mlt_producer this, void *child );
1056 mlt_service mlt_producer_service( mlt_producer this );
1057 mlt_properties mlt_producer_properties( mlt_producer this );
1058 int mlt_producer_seek( mlt_producer this, mlt_position position );
1059 mlt_position mlt_producer_position( mlt_producer this );
1060 mlt_position mlt_producer_frame( mlt_producer this );
1061 int mlt_producer_set_speed( mlt_producer this, double speed );
1062 double mlt_producer_get_speed( mlt_producer this );
1063 double mlt_producer_get_fps( mlt_producer this );
1064 int mlt_producer_set_in_and_out( mlt_producer this, mlt_position in, mlt_position out );
1065 mlt_position mlt_producer_get_in( mlt_producer this );
1066 mlt_position mlt_producer_get_out( mlt_producer this );
1067 mlt_position mlt_producer_get_playtime( mlt_producer this );
1068 mlt_position mlt_producer_get_length( mlt_producer this );
1069 void mlt_producer_prepare_next( mlt_producer this );
1070 void mlt_producer_close( mlt_producer this );
1075 The public interface is defined by the following functions:
1077 int mlt_filter_init( mlt_filter this, void *child );
1078 mlt_filter mlt_filter_new( );
1079 mlt_service mlt_filter_service( mlt_filter this );
1080 mlt_properties mlt_filter_properties( mlt_filter this );
1081 mlt_frame mlt_filter_process( mlt_filter this, mlt_frame that );
1082 int mlt_filter_connect( mlt_filter this, mlt_service producer, int index );
1083 void mlt_filter_set_in_and_out( mlt_filter this, mlt_position in, mlt_position out );
1084 int mlt_filter_get_track( mlt_filter this );
1085 mlt_position mlt_filter_get_in( mlt_filter this );
1086 mlt_position mlt_filter_get_out( mlt_filter this );
1087 void mlt_filter_close( mlt_filter );
1092 The public interface is defined by the following functions:
1094 int mlt_transition_init( mlt_transition this, void *child );
1095 mlt_transition mlt_transition_new( );
1096 mlt_service mlt_transition_service( mlt_transition this );
1097 mlt_properties mlt_transition_properties( mlt_transition this );
1098 int mlt_transition_connect( mlt_transition this, mlt_service producer, int a_track, int b_track );
1099 void mlt_transition_set_in_and_out( mlt_transition this, mlt_position in, mlt_position out );
1100 int mlt_transition_get_a_track( mlt_transition this );
1101 int mlt_transition_get_b_track( mlt_transition this );
1102 mlt_position mlt_transition_get_in( mlt_transition this );
1103 mlt_position mlt_transition_get_out( mlt_transition this );
1104 mlt_frame mlt_transition_process( mlt_transition this, mlt_frame a_frame, mlt_frame b_frame );
1105 void mlt_transition_close( mlt_transition this );
1110 The public interface is defined by the following functions:
1112 int mlt_consumer_init( mlt_consumer this, void *child );
1113 mlt_service mlt_consumer_service( mlt_consumer this );
1114 mlt_properties mlt_consumer_properties( mlt_consumer this );
1115 int mlt_consumer_connect( mlt_consumer this, mlt_service producer );
1116 int mlt_consumer_start( mlt_consumer this );
1117 mlt_frame mlt_consumer_get_frame( mlt_consumer this );
1118 mlt_frame mlt_consumer_rt_frame( mlt_consumer this );
1119 int mlt_consumer_stop( mlt_consumer this );
1120 int mlt_consumer_is_stopped( mlt_consumer this );
1121 void mlt_consumer_close( mlt_consumer );
1124 Specialised Producers:
1126 There are two major types of specialised producers - playlists and tractors.
1128 The following sections describe these.
1133 mlt_playlist mlt_playlist_init( );
1134 mlt_producer mlt_playlist_producer( mlt_playlist this );
1135 mlt_service mlt_playlist_service( mlt_playlist this );
1136 mlt_properties mlt_playlist_properties( mlt_playlist this );
1137 int mlt_playlist_count( mlt_playlist this );
1138 int mlt_playlist_clear( mlt_playlist this );
1139 int mlt_playlist_append( mlt_playlist this, mlt_producer producer );
1140 int mlt_playlist_append_io( mlt_playlist this, mlt_producer producer, mlt_position in, mlt_position out );
1141 int mlt_playlist_blank( mlt_playlist this, mlt_position length );
1142 mlt_position mlt_playlist_clip( mlt_playlist this, mlt_whence whence, int index );
1143 int mlt_playlist_current_clip( mlt_playlist this );
1144 mlt_producer mlt_playlist_current( mlt_playlist this );
1145 int mlt_playlist_get_clip_info( mlt_playlist this, mlt_playlist_clip_info *info, int index );
1146 int mlt_playlist_insert( mlt_playlist this, mlt_producer producer, int where, mlt_position in, mlt_position out );
1147 int mlt_playlist_remove( mlt_playlist this, int where );
1148 int mlt_playlist_move( mlt_playlist this, int from, int to );
1149 int mlt_playlist_resize_clip( mlt_playlist this, int clip, mlt_position in, mlt_position out );
1150 void mlt_playlist_close( mlt_playlist this );