INTRODUCTION
------------
This document provides a brief tutorial on the use of the mlt++ wrapper
and bindings.
Hello World
-----------
The mlt++ wrapper is a c++ wrapper for the mlt C library. As such, it
provides clean C++ access to the underlying library.
An example of use is as follows:
#include <mlt++/Mlt.h>
using namespace Mlt;
int main( void )
{
Factory::init( );
Producer p( "pango:", "Hello World" );
Consumer c( "sdl" );
c.connect( p );
c.run( );
return 0;
}
This is a fairly typical example of mlt++ usage - create a 'producer' (an
object which produces 'frames'), create a 'consumer' (an object which consumes
frames), connect them together, start the consumer and wait until done (here
we just wait for the user to close the window).
In this case, we construct a window as a consumer using the 'sdl' consumer
(SDL is a standard portable library which provides platform independent
access to accelerated video display and audio) and use the 'pango'
producer to generate frames with the words 'Hello World' (pango is a
library from the gtk toolkit).
The main point of this example is to show that mlt uses existing libraries
to provide its functionality - this keeps the framework itself very small.
Note that mlt is designed to be housed in GUI or server type applications -
typically, applications don't wait around for the consumer to be stopped in
the manner shown.
So far, we've introduced the Producer and Consumer mlt classes. We'll cover
each of these in more detail later in the tutorial, but for now, we'll
briefly cover the remaining classes.
Playlists
---------
Another simple class is the Playlist - this is direct extension of Producer
and it allows you to maintain a list of producer objects.
As a simple example of the Playlist in action, we'll convert the example
above into an application which plays multiple video or audio files.
#include <mlt++/Mlt.h>
using namespace Mlt;
int main( int argc, char **argv )
{
Factory::init( );
Playlist list;
for ( int i = 1; i < argc; i ++ )
{
Producer p( argv[i] );
if ( p.is_valid( ) )
list.append( p );
}
Consumer c( "sdl" );
c.connect( list );
c.run( );
return 0;
}
Now you can run the program as:
./player *.avi *.mp3 *.jpg etc
In this case, we construct a playlist by simply appending producers to it.
Notice that although the scope of the Producer is limited to the inner
for loop, we can safely add it to the playlist - this is due to the fact
that all mlt objects maintain reference counts and no object is really
destroyed until all the references are gone. In this case, when the list
object goes out of scope, all the producers we created will automatically
be destroyed.
Filters
-------
So far, we've shown how you can load and play media. We've given a brief
intro to the Playlist container, now it's time to start manipulating
things...
For the next example, I'll add a 'watermark' to the video - a watermark
is used by broadcasters to brand the channel and normally consists of a
logo of some sort. We'll just use some black text on a partially
transparent red background.
#include <mlt++/Mlt.h>
using namespace Mlt;
int main( int argc, char **argv )
{
Factory::init( );
Playlist list;
for ( int i = 1; i < argc; i ++ )
{
Producer p( argv[i] );
if ( p.is_valid( ) )
list.append( p );
}
Filter f( "watermark", "pango:" );
f.set( "producer.text", "MLT++" );
f.set( "producer.fgcolour", "0x000000ff" );
f.set( "producer.bgcolour", "0xff000080" );
list.attach( f );
Consumer c( "sdl" );
c.connect( list );
c.run( );
return 0;
}
Notice that the watermark filter reuses the 'pango' producer we showed in the
first example. In fact, you could use any producer here - if you wanted to
use a graphic or a video, you would just construct the filter with a full path
to that as the second argument.
We manipulate the filter using the set method - this method was also shown
in the first example.
Finally, we attach the filter to the playlist. This ensure that all frames
that are obtained from the playlist are watermarked.
Cuts
----
When you add a clip to a playlist, the a cut object is created - this is merely a
wrapper for the producer, spanning the specified in and out points.
Whenever you retrieve a clip from a playlist, you will always get a cut object.
This allows you to attach filters to a specific part of a producer and should
the position of the cut in the playlist change, then the filter will remain
correctly associated to it.
A producer and a cut are generally identical in behaviour, but should you need to
distinguish between them, you can use:
if ( producer.is_cut( ) )
and to retrieve the parent of a cut, you can use:
Producer parent = producer.parent_cut( );
Filters that are attached directly to a parent are executed before any filters
attached to the cut.
Tractor
-------
A tractor is an object that allows the manipulation of multiple video and audio
tracks.
Stepping away from the player example we've been tinkering with for a minute,
let's assume we want to do something like dub a video with some audio. This
a very trivial thing to do:
Tractor *dub( char *video_file, char *audio_file )
{
Tractor *tractor = new Tractor( );
Producer video( video_file );
Producer audio( audio_file );
tractor->set_track( video, 0 );
tractor->set_track( audio, 1 );
return tractor;
}
That's all that needs to be done - you can now connect the returned object to a
consumer, or add it to a playlist, or even apply it as a track to another tractor.
Transition
----------
Let's now assume we want to mix the audio between two tracks - to do this, we
need to introduce the concept of a transition. A transition in mlt is a service
which combines frames from two producers to produce a new frame.
Tractor *mix( char *video_file, char *audio_file )
{
Tractor *tractor = new Tractor( );
Transition mix( "mix" );
Producer video( video_file );
Producer audio( audio_file );
tractor.set_track( video, 0 );
tractor.set_track( audio, 1 );
tractor.field.plant_transition( mix, 0, 1 );
return tractor;
}
The tractor returned will now mix the audio from the original video and the
audio.
Mix
---
There is a convenience function which simplifies the process of applying
transitions betwee adjacent cuts on a playlist. This is often preferable
to use over the constuction of your own tractor and transition set up.
To apply a 25 frame luma transition between the first and second cut on
the playlist, you could use:
Transition luma;
playlist.mix( 0, 25, luma );
Events
------
Typically, applications need to be informed when changes occur in an mlt++ object.
This facilitates application services such as undo/redo management, or project
rendering in a timeline type widget and many other types of operations which an
application needs.
As an example, consider the following:
class Westley
{
private:
Consumer consumer;
Tractor &tractor;
public:
Westley( MltTractor &tractor ) :
tractor( tractor ),
consumer( "westley" )
{
consumer.connect( tractor );
tractor.listen( tractor, "producer-changed",
( mlt_listener )Westley::listener );
}
static void listener( Properties *tractor, Westley *object )
{
object->activate( );
}
void activate( )
{
consumer.start( );
}
};
Now, each time the tractor is changed, the westley representation is output to
stderr.
Servers and Westley Docs
------------------------
One of the key features of MLT is its server capabilities. This feature
allows you to pass westley documents seamlessly from one process to
another and even to different computers on your network.
The miracle playout server is one such example of an application which
uses this functionality - you can build your own servers into your own
processes with ease.
A server process would be running as follows:
#include <mlt++/Miracle>
using namespace Mlt;
int main( void )
{
Miracle miracle( "miracle", 5250 );
miracle.start( );
miracle.execute( "uadd sdl" );
miracle.execute( "play u0" );
miracle.wait_for_shutdown( );
return 0;
}
Typically, when you have an MLT object such as a producer or a playlist,
you can send a westley representation of this to a running server with:
Conumser valerie( "valerie", "localhost:5250" );
valerie.connect( producer );
valerie.start( );
The effect of the push will be to append the producer on to the first
unit (u0).
You can completely customise the miracle server - an example of this
is shown below.
That's All Folks...
-------------------
And that, believe it or not, is a fairly complete summary of the classes you'll
typically be interfacing with in mlt++. Obviously, there's a little more to it
than this - a couple of intrisinc classes have been glossed over (notably, the
Properties and Service base classes). The next section will cover all of the
above, but in much more detail...
DIGGING DEEPER
--------------
The previous section was designed to give you a whistle stop tour through the major
framework classes. This section will take you through the scenic route.
Introducing Base Classes
------------------------
Services in mlt are the collective noun for Producers, Filters, Transitions and
Consumer. A Service is also the base class from which all of these classes
extend. It provides the basic connectivity which has been shown throughout the
examples in the previous section.
Properties are the main way in which we communicate with the Services -
essentially, it provides get/set methods for named values. All services extend
Properties.
Properties
----------
Properties provide the general mechanism for communicating with Services -
through the Properties interface, we are able to manipulate and serialise
a services state.
For example, to dump all the properties to stdout, you can use something
like:
void dump( Properties &properties )
{
for ( int i = 0; i < properties.count( ); i ++ )
cout << Properties.get_name( i ) << " = " << Properties.get( i ) << endl;
}
Note that the properties object handles type conversion, so the following
is acceptable:
properties.set( "hello", "10.5" );
int hello_int = properties.get_int( "hello" );
double hello_double = properties.get_double( "hello" );
A couple of convenience methods are provide to examine or serialise property
objects.
For example:
properties.debug( );
will report all serialisable properties on stderr, in the form:
Object: [ ref=1, in=0, out=0, track=0, u=75, v=150, _unique_id=15,
mlt_type=filter, mlt_service=sepia ]
Services
--------
Typically, all the services are constructed via the specific classes
constructor. Often, you will receive Service objects rather than their
specific type. In order to access the extended classes interface,
you will need to create a reference.
For example, given an arbitrary Service object, you can determine its
type by using the type method - this will return a 'service_type' which
has values of producer_type, filter_type etc. Alternatively, you can
create a wrapping object and check on its validity.
bool do_we_have_a_producer( Service &service )
{
Producer producer( service );
return producer.is_valid( );
}
Events
------
Servers and Westley Docs
------------------------
For various reasons, you might want to serialise a producer to a string.
To do this, you just need to specify a property to write to:
Consumer westley( "westley", "buffer" );
westley.connect( producer );
westley.start( );
buffer = westley.get( "buffer" );
You can use any name you want, and you can change it using the "resource"
property. Any name with a '.' in it is considered to be a file. Hence, you
can use a westley consumer to store multiple instances of the same MLT
object - useful if you want to provide undo/redo capabilities in an
editing application.
Should you receive an xml document as a string, and you want to send it
on to a server, you can use:
Conumser valerie( "valerie", "localhost:5250" );
valerie.set( "westley", buffer );
valerie.start( );
If you need to obtain an MLT object from a string:
Producer producer( "westley-xml", buffer );
The following shows a working example of an extended server:
class ShotcutServer : public Miracle
{
public:
ShotcutServer( char *id, int port ) :
Miracle( id, port )
{
}
void set_receive_doc( bool doc )
{
set( "push-parser-off", doc );
}
// Reject all commands other than push/receive
Response *execute( char *command )
{
valerie_response response = valerie_response_init( );
valerie_response_set_error( response, 400, "Not OK" );
return new Response( response );
}
// Push document handler
Response *received( char *command, char *doc )
{
valerie_response response = valerie_response_init( );
// Use doc in some way and assign Response
if ( doc != NULL )
valerie_response_set_error( response, 200, "OK" );
return new Response( response );
}
// Push service handler
Response *push( char *command, Service *service )
{
valerie_response response = valerie_response_init( );
// Use service in some way and assign Response
if ( service != NULL )
valerie_response_set_error( response, 200, "OK" );
return new Response( response );
}
};
NB: Should you be incorporating this into a GUI application, remember that the
execute, received and push methods are invoked from a thread - make sure that
you honour the locking requirements of your GUI toolkit before interacting with
the UI.
