We present a clustered minority pixel error diffusion halftoning algorithm for which the quantizer threshold is modified based on the past output and a dot activation map. Dot size, dot shape, and dot distribution are more controllable, compared with other clustered dot halftone algorithms such as R. Levien's algorithm (see IS&T 8th Int. Congress on Advances in Non-Impact Printing Technologies, p.280-2, 1992). This method also effectively reduces structured worm-like artifacts in midtones that occur in Levien's algorithm. The dot distribution is further improved by using different error diffusion weights for different input gray levels.