How can you draw semi-transparent primitives such as filled polygons to a Drawable in GTK?
Its 2010, and I google isn't finding how to put an alpha value into a colour for me. What am I missing?
Use gtk.gdk.Drawable.cairo_create() and operate on the returned gtk.gdk.CairoContext. See documentation. Cairo is a generic vector 2D library that is used in GTK+ since many versions ago; GDK drawing primitives are deprecated in its favor (though not the whole GDK).
Here is actual source code to do it. I just wanted to gray out the entire pixbuf, but to do a shape you'd just have to use the appropriate path methods:
def getBlendedPixbuf(pixbuf, (r,g,b,a)):
"""Turn a pixbuf into a blended version of the pixbuf by drawing a
transparent alpha blend on it."""
pixbuf = pixbuf.copy()
#convert pixbuf to Drawable:
visual = gtk.gdk.visual_get_best()
screen = visual.get_screen()
cmap = screen.get_default_colormap()
w,h = pixbuf.get_width(), pixbuf.get_height()
drawable = gtk.gdk.Pixmap(
None, w, h, visual.depth)
gc = drawable.new_gc()
drawable.draw_pixbuf(
gc, pixbuf, 0,0,0,0,-1,-1)
#do all the drawing on it
cc = drawable.cairo_create()
cc.set_source_rgba(r,g,b,a)
cc.paint()
#put it back into a pixbhf
pixbuf.get_from_drawable(
drawable,cmap,0,0,0,0,w,h)
return pixbuf
来源：https://stackoverflow.com/questions/3048148/alpha-blending-in-gtk