1. A method of converting an image in an input bitmap to an image in an output bitmap, each bitmap being a set of pixels having values, comprising: (A) defining a mapping between pixels in the input bitmap and pixels in the output bitmap; (B) providing for storage of an error term; (C) for each current pixel in the output bitmap, determining an output value as follows: (1) determining a test value from a combination of the error term and the values of the input pixels mapped to the current output pixel, (2) determining the output value from the test value, and (3) updating the error term based on the test value and the output value.