Block based halftoning at 45 degree “screen” angle

Zakhor et al. (1993) introduced a block based halftoning algorithm which was based on dividing the image into small blocks and minimizing the distortion between the original continuous tone image and the filtered version of its halftone. While the resulting images using this technique look promising, there remains an underlying “cheesecloth screen” artifact. The present authors show that by rotating the small optimization blocks by 45 degrees, i.e. turning them into rhombic blocks, one can eliminate this horizontal/vertical artifact, thus improving the perceived quality of the resulting halftoned images. They also propose ways of speeding up the optimization algorithm by one or two orders of magnitude; of course, this is achieved at the expense of slightly lower quality halftoned images