Convertire un'immagine a colori in un'immagine a scala di grigi mediante le funzioni esterne del API di Libgdk pixbuf
Da Gambas-it.org - Wikipedia.
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.
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"
' GdkPixbuf * gdk_pixbuf_new_from_file (const char *filename, GError **error)
' Creates a new pixbuf by loading an image from a file.
Private Extern gdk_pixbuf_new_from_file(filename As String, GError As Pointer) As Pointer
' GdkPixbuf * gdk_pixbuf_copy (const GdkPixbuf *pixbuf)
' Creates a new GdkPixbuf with a copy of the information in the specified pixbuf.
Private Extern gdk_pixbuf_copy(GdkPixbuf As Pointer) As Pointer
' 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.
Private Extern gdk_pixbuf_save(GdkPixbuf As Pointer, filename As String, type As String, GErr As Pointer, altro As String) As Boolean
' 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.
Private Extern gdk_pixbuf_get_rowstride(GdkPixbuf As Pointer) As Integer
' gboolean gdk_pixbuf_get_has_alpha (const GdkPixbuf *pixbuf)
' Queries whether a pixbuf has an alpha channel (opacity information).
Private Extern gdk_pixbuf_get_has_alpha(GdkPixbuf As Pointer) As Boolean
' guchar * gdk_pixbuf_get_pixels (const GdkPixbuf *pixbuf)
' Queries a pointer to the pixel data of a pixbuf.
Private Extern gdk_pixbuf_get_pixels(GdkPixbuf As Pointer) As Pointer
' int gdk_pixbuf_get_height (const GdkPixbuf *pixbuf)
' Queries the height of a pixbuf.
Private Extern gdk_pixbuf_get_height(GdkPixbuf As Pointer) As Integer
' int gdk_pixbuf_get_width(GdkPixbuf As Pointer) As Integer
' Queries the width of a pixbuf.
Private Extern gdk_pixbuf_get_width(GdkPixbuf As Pointer) As Integer
' void g_object_unref (gpointer object)
' Decreases the reference count of object.
Private Extern g_object_unref(gobject As Pointer)
Public Sub Main()
Dim pixbuf, err As Pointer
Dim imm As String = "/percorso/della/immagine/da/convertire"
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
