Halftoning, which is a technology of printing continuoustone images imitatively on bi-level printing devices such as ink jet printers and electrophotographic printers, has been studied extensively, and many techniques such as error diffusion have been proposed. The premise of them is that the size and pitch of each dot is equal. In the case of ink-jet printers however, the dot position can be controlled with a smaller unit than dot size by electrostatic deflection, etc, though the dot size is fixed because it is determined by the size of a drop of ink. Yanaka et al. have already proposed an algorithm in which the position of each black dot is completely continuous, and each black dot is regarded to be a charged particle. In this paper another solution has been proposed, in which the position of each black dot is digital, but can be controlled with a smaller unit than the dot size. The algorithm is an extension of conventional error diffusion. In order to get better halftone images, scalable error filter has been proposed, in which error filter of arbitrary size can be generated simply by giving a parameter.