Visualizza wikitesto di Catturare e riprodurre immagini video mediante una WebCam con le funzioni esterne delle API di GStreamer

La risorsa '''GStreamer''' consente anche di catturare e riprodurre immagini video mediante una ''WebCam''.

Sarà necessario avere installata nel sistema e richiamare nell'applicazione Gambas la libreria condivisa: "''libgstreamer-1.0.so.0.2605.0'' ".


=Effettuare un video senza audio=
Mostriamo di seguito due esempi di cattura e riproduzione di immagini video ''senza audio'' con una webCam in un'applicazione ''a riga di comando''.

==Impostazione ''passo passo'' della ''Pipeline'' con varie funzioni==
In questo esempio si costruirà mediante varie funzioni una ''Pipeline'' per connettere i vari elementi di ''GStreamer'':
 Private webcam As Pointer
 
 
 Library "libgstreamer-1.0:0.2605.0"
 
 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_pipeline_new (const gchar *name)''
 ' ''Create a new pipeline with the given name.''</font>
 Private Extern gst_pipeline_new(name As String) As Pointer
 
 <FONT Color=gray>' ''GstElement * gst_element_factory_make(const gchar *factoryname, Const gchar * name)''
 ' ''Create a new element of the type defined by the given element factory.''</font>
 Private Extern gst_element_factory_make(factoryname As String, name As String) As Pointer
  
 <FONT Color=gray>' ''void g_object_set(gpointer object, const gchar *first_property_name, ...)''
 ' ''Sets properties on an object.''</font>
 Private Extern g_object_set(gobject As Pointer, key As String, value As String, element_1 As Pointer)
 
 <FONT Color=gray>' ''void gst_bin_add_many (GstBin *bin, GstElement *element_1, ...)''
 ' ''Adds a NULL-terminated list of elements to a bin.''</font>
 Private Extern gst_bin_add_many(bin As Pointer, element_1 As Pointer, element_2 As Pointer, element_3 As Pointer, element_4 As Pointer)
 
 <FONT Color=gray>' ''gboolean gst_element_link_many (GstElement *element_1,  GstElement *element_2, ...)''
 ' ''Chain together a series of elements.''</font>
 Private Extern gst_element_link_many(element_1 As Pointer, element_2 As Pointer, element_3 As Pointer, element_4 As Pointer) As Boolean
 
 <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_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 pipeline, con, sink As Pointer
  Dim tempus As Long
      
  gst_init(0, 0)
   
  pipeline = gst_pipeline_new("webcam")
   
  webcam = gst_element_factory_make("v4l2src", "webcam")
   
  g_object_set(webcam, "device", "/dev/video0", 0)
  
  con = gst_element_factory_make("videoconvert", "convert")
 
  sink = gst_element_factory_make("xvimagesink", "image")
   
  gst_bin_add_many(pipeline, webcam, con, sink, 0)
   
  gst_element_link_many(webcam, con, sink, 0)
   
 <FONT Color=gray>' ''Avviamo la riproduzione video:''</font>
  gst_element_set_state(pipeline, GST_STATE_PLAYING)
 
  Do
    gst_element_query_position(webcam, GST_FORMAT_TIME, VarPtr(tempus))
    Write "\rTempo: " & Time(0, 0, 0, tempus / 1000000)
    Wait 0.01
  Loop
 
 End
 
 
 Public Sub Application_Read()
 
 Dim s As String
 
  Input #File.In, s
   
  Select Case s
    Case "p"
 <FONT Color=gray>' ''Pone in pausa la riproduzione del file mediale''</font>
      gst_element_set_state(webcam, GST_STATE_PAUSED)
    Case "r"
 <FONT Color=gray>' ''Riprende la riproduzione del file mediale''</font>
      gst_element_set_state(webcam, GST_STATE_PLAYING)
    Case "s"
 <FONT Color=gray>' ''Arresta la riproduzione del file mediale''</font>
      gst_element_set_state(webcam, GST_STATE_NULL)
      gst_object_unref(webcam)
      Print "\nEsecuzione terminata."
      Quit
  End Select
 
 End


==Impostazione della ''Pipeline'' con la funzione "gst_parse_launch()"==
In quest'altro esempio si costruirà una ''Pipeline'', per connettere i vari ''elementi'' di ''GStreamer'', con una linea di comando mediante la funzione esterna ''gst_parse_launch()''.

In questo esempio rendremo la cattura delle immagini più veloce e pertanto l'esecuzione del video risulterà essere più fluida:
 Private webcam As Pointer
 
 
 Library "libgstreamer-1.0:0.2605.0"
 
 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_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 tempus As Long
 
  gst_init(0, 0)
 
  webcam = gst_parse_launch("v4l2src ! tee name=t_vid ! queue ! videoconvert ! xvimagesink sync=false t_vid. ! queue ! videorate ! video/x-raw-yuv,framerate=30/1", 0)
 
 <FONT Color=gray>' ''Avviamo la riproduzione video:''</font>
  gst_element_set_state(webcam, GST_STATE_PLAYING)
 
  Do
    gst_element_query_position(webcam, GST_FORMAT_TIME, VarPtr(tempus))
    Write #File.Out, "\rTempo: " & Time(0, 0, 0, tempus / 1000000)
    Wait 0.01
  Loop
 
 End
 
 
 Public Sub Application_Read()
 
  Dim s As String
 
  Input #File.In, s
   
  Select Case s
    Case "p"
 <FONT Color=gray>' ''Pone in pausa la riproduzione del file mediale''</font>
      gst_element_set_state(webcam, GST_STATE_PAUSED)
    Case "r"
 <FONT Color=gray>' ''Riprende la riproduzione del file mediale''</font>
      gst_element_set_state(webcam, GST_STATE_PLAYING)
    Case "s"
 <FONT Color=gray>' ''Arresta la riproduzione del file mediale''</font>
      gst_element_set_state(webcam, GST_STATE_NULL)
      gst_object_unref(webcam)
      Print "\nEsecuzione terminata."
      Quit
  End Select
 
 End


====Altre possibili soluzioni di ripresa video con la funzione "gst_parse_launch()"====
Dimensionando la finestra video ad esempio su 640x480:
 webcam = gst_parse_launch("v4l2src ! video/x-raw,width=640,height=480,framerate=30/1 ! videoconvert ! xvimagesink", 0)

Aspetto 16/9:
 webcam = gst_parse_launch("v4l2src ! video/x-raw,width=640,height=480,framerate=30/1 ! aspectratiocrop aspect-ratio=16/9 ! videoconvert ! xvimagesink", 0)



=Riferimenti=
* http://gstreamer.freedesktop.org/
* http://gstreamer.com/