gtk2/docs/reference/gdk-pixbuf/tmpl/scaling.sgml

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Scaling

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Scaling pixbufs and scaling and compositing pixbufs

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  <para>
    The &gdk-pixbuf; contains functions to scale pixbufs, to scale
    pixbufs and composite against an existing image, and to scale
    pixbufs and composite against a solid color or checkerboard.
    Compositing a checkerboard is a common way to show an image with
    an alpha channel in image-viewing and editing software.
  </para>

  <para>
    Since the full-featured functions (gdk_pixbuf_scale(),
    gdk_pixbuf_composite(), and gdk_pixbuf_composite_color()) are
    rather complex to use and have many arguments, two simple
    convenience functions are provided, gdk_pixbuf_scale_simple() and
    gdk_pixbuf_composite_color_simple() which create a new pixbuf of a
    given size, scale an original image to fit, and then return the
    new pixmap.
  </para>

  <para>
    The following example demonstrates handling an expose event by
    rendering the appropriate area of a source image (which is scaled
    to fit the widget) onto the widget's window.  The source image is
    rendered against a checkerboard, which provides a visual
    representation of the alpha channel if the image has one. If the
    image doesn't have an alpha channel, calling
    gdk_pixbuf_composite_color() function has exactly the same effect
    as calling gdk_pixbuf_scale().
  </para>

  <programlisting>
gboolean
expose_cb (GtkWidget *widget, GdkEventExpose *event, gpointer data)
{
  GdkPixbuf *dest;

  gdk_window_set_back_pixmap (widget->window, NULL, FALSE);
  
  dest = gdk_pixbuf_new (GDK_COLORSPACE_RGB, FALSE, 8, event->area.width, event->area.height);

  gdk_pixbuf_composite_color (pixbuf, dest,
                              0, 0, event->area.width, event->area.height,
                              -event->area.x, -event->area.y,
                              (double) widget->allocation.width / gdk_pixbuf_get_width (pixbuf),
                              (double) widget->allocation.height / gdk_pixbuf_get_height (pixbuf),
                              GDK_INTERP_BILINEAR, 255,
                              event->area.x, event->area.y, 16, 0xaaaaaa, 0x555555);

  gdk_pixbuf_render_to_drawable (dest, widget->window, widget->style->fg_gc[GTK_STATE_NORMAL],
                                 0, 0, event->area.x, event->area.y,
                                 event->area.width, event->area.height,
                                 GDK_RGB_DITHER_NORMAL, event->area.x, event->area.y);
  
  gdk_pixbuf_unref (dest);
  
  return TRUE;
}
  </programlisting>

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  <para>
    GdkRGB
  </para>