Image processing apparatus and method, computer program and computer-readable storage medium

An image processing method for quantizing N(N≧2) color multivalue image data to data where each of color data representing said multivalue image data has a smaller number of bits, comprising:

• an addition step of adding errors generated by quantization of respective colors to respective color multivalue data;
• a first conversion step of performing conversion by using a first look-up table for converting the results of addition at said addition step into multivalue data of smaller number of bits;
• a second conversion step of performing conversion by using a second look-up table for inputting output values of the respective colors obtained at said first conversion step as N-dimensional addresses and outputting N quantized data;
• and
• a calculation step of calculating differences between the quantized data obtained at said second conversion step and the respective colors added at said addition step, as errors to be added to unquantized pixel positions.

A computer program which functions as an image processing apparatus which quantizes N(N≧2) color multivalue image data to data where each of color data representing said multivalue image data has a smaller number of bits, comprising:

• program code of addition process for adding errors generated by quantization of respective colors to respective color multivalue data;
• program code of first conversion process for performing conversion by using a first look-up table for converting the results of addition at said addition process into multivalue data of smaller number of bits;
• program code of second conversion process for performing conversion by using a second look-up table for inputting output values of the respective colors obtained at said first conversion process as N-dimensional addresses and outputting N quantized data;
• and
• program code of calculation process for calculating differences between the quantized data obtained at said second conversion process and the respective colors added at said addition process, as errors to be added to unquantized pixel positions.