For example, if we want to take an image, scale it to 600 x 600, make it a bit transparent, and composite it over a white and gray checkerboard pattern, we could do this: 

 1: <?php
 2:   $pixbuf = GdkPixbuf::new_from_file('shape.png');
 3: 
 4:   $cmap = gdk::colormap_get_system();
 5:   $c1 = $cmap->alloc('white');
 6:   $c2 = $cmap->alloc('gray50');
 7: 
 8:   $pixbuf = $pixbuf->composite_color_simple(600, 600, GDK_INTERP_BILINEAR,
 9:                                             230, 8, $c1->pixel, $c2->pixel);
10: ?>