Conventional error diffusion halftoning uses a causal error filter. We propose the iterative error diffusion algorithm by extending the error diffusion to accommodate noncausal error filters. We realize the importance of the phase response of the error filter in the error diffusion halftoning method, and demonstrate it using examples. Iterative error diffusion is able to realize a zero phase error filter. We also trace a drawback of error diffusion to the shape of the error filter, and provide a remedy. The results obtained using a zero phase error filter in the iterative error diffusion algorithm are, in our opinion, superior to the error diffusion halftones.

The halftone quality of an error diffusion halftone depends on the error filter response. Apart form the magnitude response being a low pass, it is desirable to have a zero phase response. We propose the iterative error diffusion halftoning algorithm that allows use of non-causal error filter (such as zero phase error filter). We present simulation results showing that the proposed halftoning scheme is substantially better, specially in retaining the sharpness of the edges and in truthfullness.