Visualizza wikitesto di Estrarre una traccia di un CD audio convertendola in un file WAV con la funzione 'gst parse launch()' di GStreamer

La funzione esterna ''gst_parse_launch()'' della libreria ''GStreamer'' consente di costruire e gestire in modo semplice una pipeline ''GStreamer''. La riga della pipeline gestita dalla funzione esterna ''gst_parse_launch()'' è un insieme di elementi separati da punti esclamativi (!). Le proprietà possono essere aggiunte agli elementi, sotto forma di: ''proprietà = valore''.

Sarà necessario avere installata nel sistema e richiamare nell'applicazione Gambas la libreria dinamica condivisa: "''libgstreamer-1.0''"


Mostriamo un esempio pratico, nel quale verrà estratta e convertita in un file WAV la quarta traccia di un CD audio:
 Private audio As Pointer
 
 
 Library "libgstreamer-1.0"
 
 Private Enum GST_STATE_VOID_PENDING = 0, GST_STATE_NULL, GST_STATE_READY, GST_STATE_PAUSED, GST_STATE_PLAYING
 Private Const GST_FORMAT_TIME As Integer = 3
 
 <FONT Color=gray>' ''gst_init (int *argc, char **argv[])''
 ' ''Initializes the GStreamer library, setting up internal path lists, registering built-in elements, and loading standard plugins.''</font>
 Private Extern gst_init(argc As Pointer, argv As Pointer)
 
 <FONT Color=gray>' ''GstElement * gst_parse_launch (const gchar *pipeline_description, GError **error)''
 ' ''Create a new pipeline based on command line syntax.''</font>
 Private Extern gst_parse_launch(description As String, GError As Pointer) As Pointer
 
 <FONT Color=gray>' ''GstStateChangeReturn gst_element_set_state(GstElement *element, GstState state)''
 ' ''Sets the state of the element.''</font>
 Private Extern gst_element_set_state(gstelement As Pointer, state As Integer) As Integer
 
 <FONT Color=gray>' ''gboolean gst_element_query_duration(GstElement *element, GstFormat format, gint64 *duration)''
 ' ''Queries an element for the total stream duration in nanoseconds.''</font>
 Private Extern gst_element_query_duration(gselement As Pointer, formatI As Integer, duration As Pointer) As Boolean
  
 <FONT Color=gray>' ''gboolean gst_element_query_position (GstElement *element, GstFormat format, gint64 *cur)''
 ' ''Queries an element for the stream position in nanoseconds.''</font>
 Private Extern gst_element_query_position(gstelement As Pointer, gstformat As Pointer, cur As Pointer) As Boolean
 
 <FONT Color=gray>' ''void gst_object_unref(gpointer object)''
 ' ''Decrements the reference count on object.''</font>
 Private Extern gst_object_unref(gobject As Pointer)
 
 
 '''Public''' Sub Main()
  
  Dim pos, dur As Long
  Dim po, du As Date
      
   gst_init(0, 0)
   
   audio = gst_parse_launch("cdparanoiasrc <FONT Color=#B22222>track=4</font> ! queue ! audioconvert ! wavenc ! filesink location=<FONT Color=gray>''/percorso/del/file.wav''</font>", 0)
        
 <FONT Color=gray>' ''Avviamo l'estrazione della traccia dal CD audio:''</font>
   gst_element_set_state(audio, GST_STATE_PLAYING)
   
   While True
     gst_element_query_position(audio, GST_FORMAT_TIME, VarPtr(pos))
     gst_element_query_duration(audio, GST_FORMAT_TIME, VarPtr(dur))
     po = Date(0, 0, 0, 0, 0, 0, pos / 1000000)
     du = Date(0, 0, 0, 0, 0, 0, dur / 1000000)
     If (pos > 0) And (CStr(po) >= CStr(du)) Then Break
     Write #File.Out, "\rDurata: " & du & "      " & "Tempo trascorso: " & po
   Wend
   
   Termina()
       
 '''End'''
 
 
 '''Public''' Sub Application_Read()
 
  Dim s As String
  
   Input #File.In, s
   
 <FONT Color=gray>' ''Se viene inviato dalla console il carattere "t", termina l'applicazione:''</font>
   If "t" Then Termina()
   
 '''End'''
 
 
 '''Private''' Procedure Termina()
    
   gst_object_unref(audio)
   Print "\nEstrazione terminata."
   Quit
   
 '''End'''




=Riferimenti=
* http://docs.gstreamer.com/display/GstSDK/gst-launch
* http://gstreamer.freedesktop.org/
* http://gstreamer.com/
* https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-wavenc.html