Adding Noise To A Picture

It’s easy enough to generate a bunch of random pixel values, to make an
image of pure noise. Adding noice to an existing image is another
matter. Ideally what you want is exclusive-or operation between the
bits of the noise pixels and the image pixels.

After experimenting with various Cairo operators, the best effect I
could find was with OPERATOR_DIFFERENCE. I use paint_with_alpha to
apply the noise pattern as a mask, and the alpha becomes the noise
strength: crank this up to 1, and much (thought not all) of the
original image detail disappears.

The script is here
<https://github.com/ldo/qahirah_examples/blob/master/noisy_picture>,
and you can see the output here
<http://default-cube.deviantart.com/art/Noisy-Picture-520465768>.

Try modifying the script if you have some other ideas.
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