As noted, the draw functions in PyGame don t actually do blending the way you might expect. From the PyGame documentation:
Most of the arguments accept a color argument that is an RGB triplet. These can also accept an RGBA quadruplet. The alpha value will be written directly into the Surface if it contains pixel alphas, but the draw function will not draw transparently.
What you ll probably want to do is essentially what Alex Martelli said--create an intermediate software surface with an alpha channel, draw your shape onto that surface, then blit with blending onto the final surface. If you ve used PyGame s font rendering, notice that it works roughly the same, except that Font.render creates the temporary surface for you.
I m a bit rusty with Python and PyGame these days, but here s a quick example that hopefully doesn t have too many mistakes:
# returns a surface with the circle drawn onto it
def render_transparent_circle(color, radius, width=0):
size = radius * 2
temp_surf = pygame.Surface((size, size), SRCALPHA)
temp_surf.fill(Color(0, 0, 0, 0))
pygame.draw.circle(temp_surf, color, (radius, radius), radius, width)
return temp_surf
# draw a half-opaque blue circle of radius 30 to the display at point (50, 100)
def test():
transp_blue = Color(0, 0, 255, 128)
screen = pygame.display.get_surface()
circle = render_transparent_circle(transp_blue, 30)
screen.blit(circle, (50, 100))
p.s. -- If you re going to display the same shape multiple times, you may want to consider caching the intermediate surface (but don t forget to profile to find out if it helps).
