Hi Nicholas,
Nicholas Dudfield wrote:
Lenard, Rene and co,
>> There is no unit test for blitting a SRCALPHA source with non-zero
alpha to a SRCALPHA destination with non-zero alpha
I'm writing some tests for surface.blit atm. What is the expected
behaviour of such an operation?
def test_blit__SRCALPHA_to_SRCALPHA_non_zero(self):
# " There is no unit test for blitting a SRCALPHA source with
non-zero
# alpha to a SRCALPHA destination with non-zero alpha " LL
w,h = size = 32,32
s = pygame.Surface(size, pygame.SRCALPHA, 32)
s2 = s.copy()
s.fill((32,32,32,111))
s2.fill((32,32,32,31))
s.blit(s2, (0,0))
# TODO:
# what is the correct behaviour ?? should it blend? what
algorithm?
self.assertEquals(s.get_at((0,0)), (32,32,32,31))
Cheers. I'm not sure here.
I get (32, 32, 32, 129).
Here is a floating point equation for alpha blend of an rgb color
element in a pixel. It is adapted from the one in the Python Image
Library (PIL):
(dst * (255.0 - alpha) + src * alpha) / 255.0
So when src = dst = c:
(c * (255.0 - alpha) + c * alpha) / 255.0 =
(c * 255.0 - c * alpha + c * alpha) =
c
If source and destination colors are the same the resulting color is
also the same, irregardless of alpha. Pygame calculates the new alpha
value with some averaging formula I don't recognize:
dA = sA + dA - ((sA * dA) / 255)
So sA = 31 and dA = 111 gives 129.
Also, wrt to the blitting opaque source behaviour, are we considering
this a bug? A failed test? I'm that way inclined and have written a
test for that, will wait for approval before committing.
def test_blit__SRCALPHA_opaque_source(self):
src = pygame.Surface( (256,256), SRCALPHA ,32)
dst = src.copy()
for i, j in test_utils.rect_area_pts(src.get_rect()):
dst.set_at( (i,j), (i,0,0,j) )
src.set_at( (i,j), (0,i,0,255) )
dst.blit(src, (0,0))
for pt in test_utils.rect_area_pts(src.get_rect()):
self.assertEquals ( dst.get_at(pt)[1], src.get_at(pt)[1] )
According to the above floating point blend equation and Pygame's alpha
equation this unit test should hold true. I have made it so in SVN. But
will the change stick this time? Maybe there are some other instances
where the new blend equation gives a slightly different value to the
replaced equation. But will this truly cause breakage?
--
Lenard Lindstrom
<[EMAIL PROTECTED]>
