[cairo] how to use alpha blending by cairo?

[lao wb]

Hi
   I am using GTK+ to do with my application. I have a bitmap without alpha
channel as my background, another bitmap with alpha channel to show above
through to see the background. At first, I tried to use gdk to do this,
still not succeed yet, always  show only the last one gdk_draw_pixbuf
invokes in the expose handler.
   here is my gdk sample code:

#include <gtk/gtk.h>
#define STD_WIDTH    680
#define STD_HEIGHT  480

static GdkPixbuf * s_basePix;
static GdkPixbuf * s_homeMenuPix;

gboolean on_expose(GtkWidget * widget,
                   GdkEventExpose * ps,
                   gpointer data)
{
    GdkPixbuf * basePix;
    GdkPixbuf * homeMenuPix;

    homeMenuPix = s_homeMenuPix;
    basePix = s_basePix;

    gdk_draw_pixbuf(widget->window, widget->style->bg_gc[GTK_STATE_NORMAL],
                    s_basePix, 0, 0, 0, 0,
                    STD_WIDTH,
                    STD_HEIGHT,
                    GDK_RGB_DITHER_NONE,
                    0, 0);

    gdk_draw_pixbuf(widget->window, widget->style->bg_gc[GTK_STATE_NORMAL],
                    s_homeMenuPix, 0, 0, 0, 0,
                    STD_WIDTH,
                    STD_HEIGHT,
                    GDK_RGB_DITHER_NONE,
                    0, 0);

    return FALSE;
}


int main(int argc, char * argv[])
{
    GtkWidget * mainWnd        = NULL;
    GdkDrawable *    pixMap    = NULL;
    GtkWidget * img            = NULL;
    GtkWidget * box            = NULL;
    GtkWidget * button        = NULL;
    GdkBitmap      * msk        = NULL;
    GdkBitmap * homeMenu    = NULL;
    GdkColormap      * colorMap = NULL;
    GdkScreen    * screen    = NULL;

    gtk_init(&argc, &argv);

    mainWnd = gtk_window_new(GTK_WINDOW_TOPLEVEL);

    gtk_window_set_title(GTK_WINDOW(mainWnd), "test draw...\n");

    gtk_signal_connect(GTK_OBJECT(mainWnd), "destroy", gtk_main_quit, NULL);

    img = gtk_image_new();
    gtk_container_add(GTK_CONTAINER(mainWnd), img);
    gtk_signal_connect(GTK_OBJECT(img),
                       "expose-event",
                       GTK_SIGNAL_FUNC(on_expose),
                       NULL);

    gtk_container_set_border_width(GTK_CONTAINER(mainWnd), 5);

    gtk_widget_show_all(mainWnd);

    screen = gdk_screen_get_default();
    colorMap = gdk_screen_get_rgba_colormap(screen);
    if(!colorMap)
        colorMap = gdk_screen_get_rgb_colormap(screen);

    homeMenu = gdk_pixmap_create_from_xpm(mainWnd->window,
                                          &msk,

&mainWnd->style->bg[GTK_STATE_NORMAL],
                                          "f:/homemenu.bmp");
    s_homeMenuPix =
    gdk_pixbuf_get_from_drawable(s_homeMenuPix,
                                 homeMenu,
                                 colorMap,
                                 0, 0, 0, 0,
                                 STD_WIDTH,
                                 STD_HEIGHT);

    pixMap = gdk_pixmap_create_from_xpm(mainWnd->window,
                                        &msk,

&mainWnd->style->bg[GTK_STATE_NORMAL],
                                        "f:/Sunset.bmp");
    s_basePix =
    gdk_pixbuf_get_from_drawable(s_basePix,
                                 pixMap,
                                 colorMap,
                                 0, 0, 0, 0,
                                 STD_WIDTH,
                                 STD_HEIGHT);

    gtk_main();

    return 0;
}

I have posted this to gtk maillist, but noone reply.
Is there any cairo api can help me to composite pixbuf as my expect.
Any advice would be appreciated!


Laowb
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