4 This document provides a brief tutorial on the use of the mlt++ wrapper
11 The mlt++ wrapper is a c++ wrapper for the mlt C library. As such, it
12 provides clean C++ access to the underlying library.
14 An example of use is as follows:
17 #include <mlt++/Mlt.h>
23 Producer p( "pango:" );
24 p.set( "text", "Hello World" );
26 Event *e = Consumer.setup_wait_for( "consumer-stopped" );
34 This is a fairly typical example of use of mlt++ - create a 'producer' (an
35 object which produces 'frames'), create a 'consumer' (an object which consumes
36 frames), connect them together, start the consumer and wait until done (here
37 we just wait for the user to close the window).
39 In this case, we construct a window as a consumer using the 'sdl' consumer
40 (SDL is a standard portable library which provides platform independent
41 access to accelerated video display and audio) and use the 'pango'
42 producer to generate frames with the words 'Hello World' (pango is a
43 library from the gtk toolkit).
45 The main point of this example is to show that mlt uses existing libraries
46 to provide its functionality - this keeps the framework itself very small.
48 Note that mlt is designed to be housed in GUI or server type applications -
49 typically, applications don't wait around for the consumer to be stopped in
52 So far, we've introduced the Producer and Consumer mlt classes. We'll cover
53 each of these in more detail later in the tutorial, but for now, we'll
54 briefly cover the remaining classes.
60 Another simple class is the Playlist - this is direct extension of Producer
61 and it allows you to maintain a list of producer objects.
63 As a simple example of the Playlist in action, we'll convert the example
64 above into an application which plays multiple video or audio files.
67 #include <mlt++/Mlt.h>
70 int main( int argc, char **argv )
74 for ( int i = 1; i < argc; i ++ )
76 Producer p( argv[i] );
82 Event *e = Consumer.setup_wait_for( "consumer-stopped" );
89 Now you can run the program as:
91 ./player *.avi *.mp3 *.jpg etc
93 In this case, we construct a playlist by simply appending producers to it.
94 Notice that although the scope of the Producer is limited to the inner
95 for loop, we can safely add it to the playlist - this is due to the fact
96 that all mlt objects maintain reference counts and no object is really
97 destroyed until all the references are gone. In this case, when the list
98 object goes out of scope, all the producers we created will automatically
105 So far, we've shown how you can load and play media. We've given a brief
106 intro to the Playlist container, now it's time to start manipulating
109 For the next example, I'll add a 'watermark' to the video - a watermark
110 is used by broadcasters to brand the channel and normally consists of a
111 logo of some sort. We'll just use some black text on a partially
112 transparent red background.
115 #include <mlt++/Mlt.h>
118 int main( int argc, char **argv )
122 for ( int i = 1; i < argc; i ++ )
124 Producer p( argv[i] );
128 Filter f( "watermark", "pango:" );
129 f.set( "producer.text", "MLT++" );
130 f.set( "producer.fgcolour", "0x000000ff" );
131 f.set( "producer.bgcolour", "0xff000080" );
135 Event *e = Consumer.setup_wait_for( "consumer-stopped" );
142 Notice that the watermark filter reuses the 'pango' producer we showed in the
143 first example. In fact, you could use any producer here - if you wanted to
144 use a graphic or a video, you would just construct the filter with a full path
145 to that as the second argument.
147 We manipulate the filter using the set method - this method was also shown
148 in the first example.
150 Finally, we attach the filter to the playlist. This ensure that all frames
151 that are obtained from the playlist are watermarked.
157 A tractor is an object that allows the manipulation of multiple video and audio
160 Stepping away from the player example we've been tinkering with for a minute,
161 let's assume we want to do something like dubbing a video with some audio. This
162 a very trivial thing to do:
164 Tractor *dub( char *video_file, char *audio_file )
166 Tractor *tractor = new Tractor( );
167 Producer video( video_file );
168 Producer audio( audio_file );
169 tractor->set_track( video, 0 );
170 tractor->set_track( audio, 1 );
174 That's all that needs to be done - you can now connect the returned object to a
175 consumer, or add it to a playlist, or even apply it as a track to another tractor.
181 Let's now assume we want to mix the audio between two tracks - to do this, we
182 need to introduce the concept of a transition. A transition in mlt is a service
183 which combines frames from two producers to produce a new frame.
185 Tractor *mix( char *video_file, char *audio_file )
187 Tractor *tractor = new Tractor( );
188 Transition mix( "mix" );
189 Producer video( video_file );
190 Producer audio( audio_file );
191 tractor.set_track( video, 0 );
192 tractor.set_track( audio, 1 );
193 tractor.field.plant_transition( mix, 0, 1 );
197 The tractor returned will now mix the audio from the original video and the audio.
203 And that, believe it or not, is a fairly complete summary of the classes you'll
204 typically be interfacing with in mlt++. Obviously, there's a little more to it
205 than this - a couple of intrisinc classes have been glossed over (notably, the
206 Properties and Service base classes). The next section will cover all of the
207 above, but in much more detail...
213 The previous section was designed to give you a whistle stop tour through the major
214 framework classes. This section will take you through the scenic route.
217 Introducing Base Classes
218 ------------------------
220 Services in mlt are the collective noun for Producers, Filters, Transitions and
221 Consumer. A Service is also the base class from which all of these classes
222 extend. It provides the basic connectivity which has been shown throughout the
223 examples in the previous section.
225 Properties are the main way in which we communicate with the Services -
226 essentially, it provides get/set methods for named values. All services extend
233 Properties provide the general mechanism for communicating with Services -
234 through the Properties interface, we are able to manipulate and serialise
237 For example, to dump all the properties to stdout, you can use something
240 void dump( Properties &properties )
242 for ( int i = 0; i < properties.count( ); i ++ )
243 cout << Properties.get_name( i ) << " = " << Properties.get( i ) << endl;
246 Note that the properties object handles type conversion, so the following
249 properties.set( "hello", "10.5" );
250 int hello_int = properties.get_int( "hello" );
251 double hello_double = properties.get_double( "hello" );
253 A couple of convenience methods are provide to examine or serialise property
260 will report all serialisable properties on stderr, in the form:
262 Object: [ ref=1, in=0, out=0, track=0, u=75, v=150, _unique_id=15,
263 mlt_type=filter, mlt_service=sepia ]
269 Typically, all the services are constructed via the specific classes
270 constructor. Often, you will receive Service objects rather than their
271 specific type. In order to access the extended classes interface,
272 you will need to create a reference.
274 For example, given an arbitrary Service object, you can determine its
275 type by using the type method - this will return a 'service_type' which
276 has values of producer_type, filter_type etc. Alternatively, you can
277 create a wrapping object and check on its validity.
279 bool do_we_have_a_producer( Service &service )
281 Producer producer( service );
282 return producer.is_valid( );