Patent ID: 7170640

Claim:
The method for transforming an input image comprising pixels having a first state out of a first set of M (M>2) possible states into a halftone image comprising quantized pixels having a second state out of a second set of N possible states, said second set being a real sub-set of said first set, the method comprising the steps of: obtaining an input pixel from said input image, said input pixel having a first error corresponding to an error between a first modified pixel value and a first quantized pixel value; modifying said input pixel to obtain a next modified pixel value by adding at least a portion of the first error; selecting a third set of P possible states, said third set being a real sub-set of said second set; quantizing said next modified pixel value to obtain a next quantized pixel value by selecting one state out of said third set; and calculating a next error as a difference between said next modified pixel value and said next quantized pixel value, wherein the step of selecting a third set depends on a state of said input pixel.