Dopo uno studio ed una ricerca matti e disperatissimi, sono riuscito a fare un passo in avanti nella lettura dei codici a barra con una WebCam mediante le risorse di
E' un piccolo passo, poiché per ora sarà ncessario richiamare alcune funzioni esterne della libreria di Gstreamer. Al riguardo va precisato che bisogna avere installata nel proprio sistema la libreria di
Private webcam As Pointer
Private bs As Pointer
Private cnt As Integer
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_MESSAGE_ERROR As Integer = 2
Private Const GST_MESSAGE_ELEMENT As Integer = 32768
' gst_init (int *argc, char **argv[])
' Initializes the GStreamer library, setting up internal path lists, registering built-in elements, and loading standard plugins.
Private Extern gst_init(argc As Pointer, argv As Pointer)
' GMainLoop * g_main_loop_new (GMainContext *context, gboolean is_running)
' Creates a new GMainLoop structure.
Private Extern g_main_loop_new(context As Pointer, is_running As Boolean) As Pointer
' GstElement * gst_parse_launch (const gchar *pipeline_description, GError **error)
' Create a new pipeline based on command line syntax.
Private Extern gst_parse_launch(description As String, gerror As Pointer) As Pointer
' GstBus * gst_pipeline_get_bus (GstPipeline *pipeline)
' Gets the GstBus of pipeline.
Private Extern gst_pipeline_get_bus(pipeline As Pointer) As Pointer
' guint gst_bus_add_watch (GstBus *bus, GstBusFunc func, gpointer user_data)
' Adds a bus watch to the default main context with the default priority.
Private Extern gst_bus_add_watch(bus As Pointer, func As Pointer, user_data As Pointer) As Integer
' GstStateChangeReturn gst_element_set_state(GstElement *element, GstState state)
' Sets the state of the element.
Private Extern gst_element_set_state(element As Pointer, state As Integer) As Integer
' void g_main_loop_run (GMainLoop *loop)
' Runs a main loop until g_main_loop_quit() is called on the loop.
Private Extern g_main_loop_run(gloop As Pointer)
' const gchar * gst_structure_get_name (const GstStructure *structure)
' Get the name of structure as a string.
Private Extern gst_structure_get_name(gstructure As Pointer) As String
' const GstStructure * gst_message_get_structure (GstMessage *message)
' Access the structure of the message.
Private Extern gst_message_get_structure(message As Pointer) As Pointer
' gint g_value_get_int (const GValue *value)
' Get the contents of a G_TYPE_INT GValue.
Private Extern g_value_get_int(value As Pointer) As Integer
' const GValue * gst_structure_get_value (const GstStructure *structure, const gchar *fieldname)
' Get the value of the field with name fieldname .
Private Extern gst_structure_get_value(gstructure As Pointer, fieldname As String) As Pointer
' const gchar * g_value_get_string (const GValue *value)
' Get the contents of a G_TYPE_STRING GValue.
Private Extern g_value_get_string(value As Pointer) As String
' void gst_object_unref(gpointer object)
' Decrements the reference count on object.
Private Extern gst_object_unref(gobject As Pointer)
Public Sub Main()
Dim lp As Pointer
gst_init(0, 0)
lp = g_main_loop_new(0, False)
' Imposta gli elementi necessari per la ripresa video e per la estrazione del codice a barre:
webcam = gst_parse_launch("v4l2src ! videoconvert ! zbar ! videoconvert ! xvimagesink", 0)
' Poiché l'elemento "zbar" inserisce il testo afferente al codice a barre nel puntatore "GstBus", è necessario estrapolarlo dalla pipeline:
bs = gst_pipeline_get_bus(webcam)
' Resta in osservazione se è pronto un messaggio nel bus, nel qual caso invoca la funzione "Gestione_messaggio_bus()":
gst_bus_add_watch(bs, Gestione_messaggio_bus, lp)
' Avvia la ripresa video:
gst_element_set_state(webcam, GST_STATE_PLAYING)
' Avvia un loop infinito:
g_main_loop_run(lp)
End
Public Function Gestione_messaggio_bus(bus As Pointer, msg As Pointer, data As Pointer) As Boolean
Select Case Int@(msg + 64) ' GstMessage->type
Case GST_MESSAGE_ELEMENT
Inc cnt
Print "\nMessaggio n. "; cnt, ": "; gst_structure_get_name(gst_message_get_structure(msg)),
' Informa sulla qualità della ripresa video, affinché così l'utente sia possa valutare il grado di affidabilità
' dei dati inviati dal plug-in "zbar" attinenti al codice a barre ripreso con la webcam:
Print "qualità: "; g_value_get_int(gst_structure_get_value(gst_message_get_structure(msg), "quality"))
Print " Tipo: "; g_value_get_string(gst_structure_get_value(gst_message_get_structure(msg), "type"))
Print " Codice: "; g_value_get_string(gst_structure_get_value(gst_message_get_structure(msg), "symbol"))
Case GST_MESSAGE_ERROR
Print "Errore durante la ripresa video !"
Chiude()
End Select
Return True
End
Private Procedure Chiude()
' Arresta la ripresa video e chiude la libreria GStreamer:
gst_element_set_state(webcam, GST_STATE_NULL)
gst_object_unref(bs)
gst_object_unref(webcam)
Print "\nEsecuzione terminata."
Quit
End
