When using cairo.ImageSurface.create_for_data in order to create an image in
cairo.FORMAT_ARGB32 using alpha-values != 255 the image contains wrong pixel data.

Please create those two generated images (python 2.5 code using pycairo 1.2.6)
in order to simplify the (follwing) problem-explanation:

# ---- code snippet start ----

imgW = 255
imgH = 255

pelData = array.array('c', '\0x0' * imgW * imgH * 4)

for y in range(imgH):
  for x in range(imgW):
    offset = 4 * ((y * imgW) + x)
    
    pelData[offset+0] = chr(0)   # B
    pelData[offset+1] = chr(0)   # G
    pelData[offset+2] = chr(x)   # R
    pelData[offset+3] = chr(255) # A, when is CONSTANT=255 image is okay

img = cairo.ImageSurface.create_for_data(pelData, cairo.FORMAT_ARGB32, imgW,
imgH, imgW * 4)

img.write_to_png("channel_test_alpha_const.png")



pelData = array.array('c', '\0x0' * imgW * imgH * 4)

for y in range(imgH):
  for x in range(imgW):
    offset = 4 * ((y * imgW) + x)
    
    pelData[offset+0] = chr(0) # B
    pelData[offset+1] = chr(0) # G
    pelData[offset+2] = chr(x) # R
    pelData[offset+3] = chr(y) # A, when varies image seems to be f*cked up

img = cairo.ImageSurface.create_for_data(pelData, cairo.FORMAT_ARGB32, imgW,
imgH, imgW * 4)

img.write_to_png("channel_test_alpha_var.png")

# ---- code snippet end ----



When executed, two images are created for illustrating the problem:

- "channel_test_alpha_const.png" contains a gradient from black (left image
border) to red (right image border), using 255 as constant alpha value for each
pixel

- ""channel_test_alpha_var.png"" SHOULD contain the same gradient as the one in
the previous image, BUT: the alpha-value for each pixel-row is identical to it's
y-index, means in top-most pixel row the gradient should be invisible due to
alpha=0 and in the bottom-most row the gradient should be full saturated due to
alpha=255

Due to some reason that's not true: the gradient-image using variable alpha
values instead of a constant one looks totally different. This seems to be
dependent to the alpha value: if alpha differs from 255 the image seems to have
a "misaligned" stride depending on the alpha value or something like that. There
are "gradient-like" edges in the image, always from top-left to some
"bottom-right" angle.

I hope the two test images illustrate the problem well, because it's difficult
to explain it here. And I need to mention that on the one hand I'm new to Python
(so it's possible I made some mistake creating the array), but on the other hand
the only difference in the code is the byte I set for the alpha-values in each
pixel's data, so I'm quite sure the problem must depend on the data I pass to
the create_for_data method.

Further (hopefully useful) information:

- Win XP
- Python 2.5
- pycairo 1.2.6