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 dinamica condivisa: "libgstreamer-1.0"

Effettuare e salvare 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"

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

' 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)

' GstElement* gst_pipeline_new (const gchar *name)
' Create a new pipeline with the given name.
Private Extern gst_pipeline_new(name As String) As Pointer

' GstElement * gst_element_factory_make(const gchar *factoryname, Const gchar * name)
' Create a new element of the type defined by the given element factory.
Private Extern gst_element_factory_make(factoryname As String, name As String) As Pointer
 
' void g_object_set(gpointer object, const gchar *first_property_name, ...)
' Sets properties on an object.
Private Extern g_object_set(gobject As Pointer, key As String, value As String)

' void gst_bin_add_many (GstBin *bin, GstElement *element_1, ...)
' Adds a NULL-terminated list of elements to a bin.
Private Extern gst_bin_add_many(bin As Pointer, element_1 As Pointer, element_2 As Pointer)

' gboolean gst_element_link_many (GstElement *element_1,  GstElement *element_2, ...)
' Chain together a series of elements.
Private Extern gst_element_link_many(element_1 As Pointer, element_2 As Pointer) As Boolean

' GstStateChangeReturn gst_element_set_state(GstElement *element, GstState state)
' Sets the state of the element.
Private Extern gst_element_set_state(gstelement As Pointer, state As Integer) As Integer
 
' gboolean gst_element_query_position (GstElement *element, GstFormat format, gint64 *cur)
' Queries an element for the stream position in nanoseconds.
Private Extern gst_element_query_position(gstelement As Pointer, gstformat As Pointer, cur As Pointer) As Boolean

' void gst_object_unref(gpointer object)
' Decrements the reference count on object.
Private Extern gst_object_unref(gobject As Pointer)


Public Sub Main()

 Dim pipeline, 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")
  
  sink = gst_element_factory_make("xvimagesink", "image")
  
  gst_bin_add_many(pipeline, webcam, sink)
  
  gst_element_link_many(webcam, sink)
  
' Avviamo la riproduzione video:
  gst_element_set_state(pipeline, GST_STATE_PLAYING)
  
  While True
    gst_element_query_position(webcam, GST_FORMAT_TIME, VarPtr(tempus))
    Write #File.Out, "\rTempo: " & Date(0, 0, 0, 0, 0, 0, tempus / 1000000)
    Wait 0.01
  Wend
      
End


Public Sub Application_Read()

Dim s As String

 Input #File.In, s
  
 Select Case s
   Case "p"
' Pone in pausa la riproduzione del file mediale
     gst_element_set_state(webcam, GST_STATE_PAUSED)
   Case "r"
' Riprende la riproduzione del file mediale
     gst_element_set_state(webcam, GST_STATE_PLAYING)
   Case "s"
' Arresta la riproduzione del file mediale
     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"

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

' 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)

' 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

' GstStateChangeReturn gst_element_set_state(GstElement *element, GstState state)
' Sets the state of the element.
Private Extern gst_element_set_state(gstelement As Pointer, state As Integer) As Integer
 
' gboolean gst_element_query_position (GstElement *element, GstFormat format, gint64 *cur)
' Queries an element for the stream position in nanoseconds.
Private Extern gst_element_query_position(gstelement As Pointer, gstformat As Pointer, cur As Pointer) As Boolean

' void gst_object_unref(gpointer object)
' Decrements the reference count on object.
Private Extern gst_object_unref(gobject As Pointer)


Public Sub Main()
 
 Dim tempus As Long
     
  gst_init(0, 0)
  
  webcam = gst_parse_launch("v4l2src /dev/video0 ! tee name=t_vid ! queue ! xvimagesink sync=false t_vid. ! queue ! videorate ! video/x-raw-yuv,framerate=30/1", 0)
     
' Avviamo la riproduzione video:
  gst_element_set_state(webcam, GST_STATE_PLAYING)
  
  While True
    gst_element_query_position(webcam, GST_FORMAT_TIME, VarPtr(tempus))
    Write #File.Out, "\rTempo: " & Date(0, 0, 0, 0, 0, 0, tempus / 1000000)
    Wait 0.01
  Wend
      
End


Public Sub Application_Read()

Dim s As String

 Input #File.In, s
  
 Select Case s
   Case "p"
' Pone in pausa la riproduzione del file mediale
     gst_element_set_state(webcam, GST_STATE_PAUSED)
   Case "r"
' Riprende la riproduzione del file mediale
     gst_element_set_state(webcam, GST_STATE_PLAYING)
   Case "s"
' Arresta la riproduzione del file mediale
     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-launch("v4l2src ! video/x-raw,width=640,height=480,framerate=30/1 ! ffmpegcolorspace ! 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 ! ffmpegcolorspace ! xvimagesink", 0)

Effettuare e salvare un video con audio

In quest'altro capitolo vedremo le modalità per salvare una ripresa video comprensiva di audio mediante una WebCam. Il file finale creato sarà di formato OGV.

Impostazione della Pipeline con la funzione gst_parse_launch()

In questo 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"

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

' 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)

' 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

' GstStateChangeReturn gst_element_set_state(GstElement *element, GstState state)
' Sets the state of the element.
Private Extern gst_element_set_state(gstelement As Pointer, state As Integer) As Integer
 
' gboolean gst_element_query_position (GstElement *element, GstFormat format, gint64 *cur)
' Queries an element for the stream position in nanoseconds.
Private Extern gst_element_query_position(gstelement As Pointer, gstformat As Pointer, cur As Pointer) As Boolean

' void gst_object_unref(gpointer object)
' Decrements the reference count on object.
Private Extern gst_object_unref(gobject As Pointer)


Public Sub Main()
 
 Dim tempus As Long
     
  gst_init(0, 0)
  
  webcam = gst_parse_launch("v4l2src ! videorate ! video/x-raw,framerate=5/1 ! queue ! theoraenc ! queue ! " &
                            "mux. alsasrc ! audio/x-raw,rate=44100,channels=2,depth=16 ! queue ! audioconvert ! queue ! " &
                            "vorbisenc ! queue ! mux. oggmux name=mux ! filesink location=/percorso/del/file.ogv", 0)
     
' Avviamo la riproduzione audio-video:
  gst_element_set_state(webcam, GST_STATE_PLAYING)
  
  While True
    gst_element_query_position(webcam, GST_FORMAT_TIME, VarPtr(tempus))
    Write #File.Out, "\rTempo: " & Date(0, 0, 0, 0, 0, 0, tempus / 1000000)
    Wait 0.01
  Wend
      
End


Public Sub Application_Read()

Dim s As String

 Input #File.In, s
  
 Select Case s
   Case "p"
' Pone in pausa la riproduzione del file mediale
     gst_element_set_state(webcam, GST_STATE_PAUSED)
   Case "r"
' Riprende la riproduzione del file mediale
     gst_element_set_state(webcam, GST_STATE_PLAYING)
   Case "s"
' Arresta la riproduzione del file mediale
     gst_element_set_state(webcam, GST_STATE_NULL)
     gst_object_unref(webcam)
     Print "\nEsecuzione terminata."
     Quit
 End Select

End

Riferimenti

• http://gstreamer.freedesktop.org/
• http://gstreamer.com/