Visualizza wikitesto di Convertire un'immagine a colori in un'immagine a scala di grigi mediante le funzioni esterne del API di Libgdk pixbuf

La libreria '''GDK-PixBuf''' consente il caricamento delle immagini e la manipolazione del buffer dei pixel.

Per poter fruire in Gambas delle risorse di questa libreria è necessario installare e richiamare la libreria condivisa: "''libgdk_pixbuf-2.0.so.0.4200.8'' ".

Con le risorse della libreria ''GDK-PixBuf'' è possibile anche convertire un'immagine a colori in un'immagine a scala di grigi.
<BR>Mostriamo un semplice esempio, nel quale un'immagine a colori di formato .jpg viene convertita in immagine .png a scala di grigi:
 Library "libgdk_pixbuf-2.0:0.4200.8"
 
 <FONT Color=gray>' ''GdkPixbuf * gdk_pixbuf_new_from_file (const char *filename, GError **error)''
 ' ''Creates a new pixbuf by loading an image from a file.''</font>
 Private Extern gdk_pixbuf_new_from_file(filename As String, GError As Pointer) As Pointer
 
 <FONT Color=gray>' ''GdkPixbuf * gdk_pixbuf_copy (const GdkPixbuf *pixbuf)''
 ' ''Creates a new GdkPixbuf with a copy of the information in the specified pixbuf.''</font>
 Private Extern gdk_pixbuf_copy(GdkPixbuf As Pointer) As Pointer
 
 <FONT Color=gray>' ''gboolean gdk_pixbuf_save (GdkPixbuf *pixbuf, const char *filename, const char *type, GError **error, ...)''
 ' ''Saves pixbuf to a file in format type. By default, "jpeg", "png", "ico" and "bmp" are possible file formats to save in.''</font>
 Private Extern gdk_pixbuf_save(GdkPixbuf As Pointer, filename As String, type As String, GErr As Pointer, altro As String) As Boolean
 
 <FONT Color=gray>' ''int gdk_pixbuf_get_rowstride (const GdkPixbuf *pixbuf)''
 ' ''Queries the rowstride of a pixbuf, which is the number of bytes between the start of a row and the start of the next row.''</font>
 Private Extern gdk_pixbuf_get_rowstride(GdkPixbuf As Pointer) As Integer
 
 <FONT Color=gray>' ''gboolean gdk_pixbuf_get_has_alpha (const GdkPixbuf *pixbuf)''
 ' ''Queries whether a pixbuf has an alpha channel (opacity information).''</font>
 Private Extern gdk_pixbuf_get_has_alpha(GdkPixbuf As Pointer) As Boolean
 
 <FONT Color=gray>' ''guchar * gdk_pixbuf_get_pixels (const GdkPixbuf *pixbuf)''
 ' ''Queries a pointer to the pixel data of a pixbuf.''</font>
 Private Extern gdk_pixbuf_get_pixels(GdkPixbuf As Pointer) As Pointer
 
 <FONT Color=gray>' ''int gdk_pixbuf_get_height (const GdkPixbuf *pixbuf)''
 ' ''Queries the height of a pixbuf.''</font>
 Private Extern gdk_pixbuf_get_height(GdkPixbuf As Pointer) As Integer
 
 <FONT Color=gray>' ''int gdk_pixbuf_get_width(GdkPixbuf As Pointer) As Integer''
 ' ''Queries the width of a pixbuf.''</font>
 Private Extern gdk_pixbuf_get_width(GdkPixbuf As Pointer) As Integer
 
 <FONT Color=gray>' ''void g_object_unref (gpointer object)''
 ' ''Decreases the reference count of object.''</font>
 Private Extern g_object_unref(gobject As Pointer)
 
 
 '''Public''' Sub Main()
 
  Dim pixbuf, err As Pointer
  Dim imm As String = "<FONT Color=gray>''/percorso/della/immagine/da/convertire''</font>"
 
  Print "File immagine: "; imm
 
  pixbuf = gdk_pixbuf_new_from_file(imm, VarPtr(err))
  If Val(String@(err)) <> 0 Then Error.Raise("Impossibile caricare l'immagine !")
 
  gray = grayscale_pixbuf(pixbuf)
 
  gdk_pixbuf_save(gray, "/tmp/icona.png", "png", VarPtr(err), Null)
  If Val(String@(err)) <> 0 Then Error.Raise("Impossibile salvare l'immagine in scala di grigi!")
 
  g_object_unref(pixbuf)
 
 '''End'''
 
 '''Private''' Function grayscale_pixbuf(pixbuf As Pointer) As Pointer
  
  Dim gray, pixels, endp, p As Pointer
  Dim rowstride, pixstride, row, n_rows, width As Integer
  Dim v As Float
  Dim p1, p2, p3 As Pointer
  Dim st As Stream
   
  gray = gdk_pixbuf_copy(pixbuf)
  If gray == 0 Then Error.Raise("Errore !")
  
  rowstride = gdk_pixbuf_get_rowstride(gray)
  If gdk_pixbuf_get_has_alpha(gray) Then
    pixstride = 4
  Else 
    pixstride = 3
  Endif
  pixels = gdk_pixbuf_get_pixels(gray)
  If pixels == 0 Then Error.Raise("Errore !")
  n_rows = gdk_pixbuf_get_height(gray)
  width = gdk_pixbuf_get_width(gray)
  
  Print "Larghezza: "; width; " pixel"
  Print "Altezza:   "; n_rows; " pixel"
 
  While (row < n_rows)
    p = pixels + (row * rowstride)
    endp = p + (pixstride * width)
    st = Memory p For Write
      While p <> endp
        p1 = p
        p2 = p + 1
        p3 = p + 2
        v = CFloat(Byte@(p1) * 0.30) + (Byte@(p2) * 0.59) + (Byte@(p3) * 0.11)
        Write #st, CByte(v) As Byte
        Write #st, CByte(v) As Byte
        Write #st, CByte(v) As Byte
        p += pixstride
      Wend
    Inc row
  Wend
  
  st.Close
  
  Return gray
 
 '''End'''



=Riferimenti=
* https://developer.gnome.org/gdk-pixbuf/stable/index.html
* http://openbooks.sourceforge.net/books/wga/graphics-gdk-pixbuf.html