Hi Dan,

the last days I had some time and I worked on the jack integration in kdenlive. Now I'm some steps farther.
But I have two issues:

#1: MLT hangs on closing when filter_jackrack was active and videoglwidget.cpp is compiled in kdenlive (opengl activated)

It was tricky to find this problem but now its clear. When in Kdenlive OPENGL is activated (see src/CMakeLists.txt)

  list(APPEND kdenlive_SRCS videoglwidget.cpp)

then mlt (kdenlive) hangs forever on closing when the filter_jackrack was loaded. If you shutdown the jackd before closing the 
program it closes without hanging. If I compile kdenlive without videoglwidget.cpp all is working just perfect. It seems that there are
some side effects between jackd/filter_jackrack and opengl.

Any ideas what the problem is?

#2: funny sinus sound on transport stop when sdl_preview consumer is used instead of sdl 

Steps to reproduce: 

1) bash# MLT_CONSUMER=sdl_preview melt -jack my_sound.wav 
2) start jack transport
3) stop jack transport

Now you can hear a pretty cool sinus tone or something like that with different frequencies and volumes on every stop.

The reason for this is the code in consumer_sdl_preview.c:

static void *consumer_thread( void *arg )
        int eos_threshold = 20 + mlt_properties_get_int( MLT_CONSUMER_PROPERTIES( this->play ), "buffer" );
// Loop until told not to
while( this->running )
// Get a frame from the attached producer
frame = mlt_consumer_get_frame( consumer );

// Ensure that we have a frame
if ( this->running && frame != NULL )
// If we aren't playing normally, then use the still
if ( speed != 1 )
mlt_producer producer = MLT_PRODUCER( mlt_service_get_producer( MLT_CONSUMER_SERVICE( consumer ) ) );
mlt_position duration = producer? mlt_producer_get_playtime( producer ) : -1;
int pause = 0;

if ( this->active == this->play )
// Do not interrupt the play consumer near the end
if ( duration - this->last_position > eos_threshold )  <--- this criteria is true on jack transport stop, why? 
// Get a new frame at the sought position
mlt_frame_close( frame );
if ( producer )
mlt_producer_seek( producer, this->last_position );
frame = mlt_consumer_get_frame( consumer );
pause = 1;
pause = this->active == this->play;
if ( pause ) <--- pause is true and the this->play (sdl) consumer is stopped 
// Start the still consumer
mlt_consumer_stop( this->play );
this->last_speed = speed;
this->active = this->still;
this->ignore_change = 0;
mlt_consumer_start( this->still );

The result is that in filter_jackrack.c:

the function jackrack_get_audio() is not called and the jack "output_buffers" is not filled with data. Then in jack_process

static int jack_process (jack_nframes_t frames, void * data)
for ( i = 0; i < channels; i++ )
size_t jack_size = ( frames * sizeof(float) );
size_t ring_size;
// Send audio through out port
jack_output_buffers[i] = jack_port_get_buffer( jack_output_ports[i], frames );
if ( ! jack_output_buffers[i] )
mlt_log_error( MLT_FILTER_SERVICE(filter), "no buffer for output port %d\n", i );
err = 1;
ring_size = jack_ringbuffer_read_space( output_buffers[i] );  <--- ring_size = 0, nothing is copied to jackd
jack_ringbuffer_read( output_buffers[i], ( char * )jack_output_buffers[i], ring_size < jack_size ? ring_size : jack_size );

the ring_size is 0 and no data is transfered to the jack outport (jack_output_buffers). In this case jackd produces a very painful sound (the mlt jack outports are connected to system).
You get the same result if you set a breakpoint in the jack_process or the consumer_thread. 

The sdl consumer instead doesn't stop the generation of data and thats why you don't get this behaviour for sdl (default consumer). But the SDL Consumer doesn't work very well with video data. 

Kdenlive uses the sdl_preview consumer for the monitors. Thats why I need a solution for the sdl_preview consumer.

Is there a solution for this (my) problem?