If you happen to be writing your simulated vintage mac site on a modern mac, vImage (part of the Accelerate framework) supports Atkinson dithering from OS X 10.9 onward via vImageConvert_Planar8to[Planar|Indexed][124].
Not overly useful for one image, but if you have lots of them, it's nice to have a programmatic solution.
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If you happen to be writing your simulated vintage mac site on a modern mac, vImage (part of the Accelerate framework) supports Atkinson dithering from OS X 10.9 onward via vImageConvert_Planar8to[Planar|Indexed][124].
Not overly useful for one image, but if you have lots of them, it's nice to have a programmatic solution.
For instance, Atkinson dithering on video. Like Vine, but 8bit.
For that, you'll want the new vImage CV pixel buffer functions in iOS 8 and Yosemite =)