HyperDither is an OS X app that uses an elegant color reduction (dithering) routine to turn color or grayscale images into 1-bit black and white. The visual effect produced by this technique creates rich, velvety tones, and is higher-quality than the “Diffusion Dither” method used by Photoshop.
In the early 1980s, while developing the graphics technology for the first Macintosh, Bill Atkinson (HyperCard, QuickDraw, MacPaint) discovered a very elegant filter to convert greyscale image data for display on the Mac's 1-bit black-and-white screen.
Later, this graphics routine was implemented as an option in Apple’s HyperScan software, connecting early Macs to flatbed scanners, and nowhere else. HyperScan, and the excellent Atkinson dithering routine, were subsequently unavailable for many years—but not forgotten! We emailed him to inquire about the details of the algorithm and he was kind enough to respond with a brief write-up of the routine that is now available in HyperDither.