Imposed weight matrix error diffusion halftoning of image data

A method of halftoning a digital image defined by a rectangular array of scan lines of pixels, each pixel having a gray value, for forming an output image, the method comprising the steps of:

• for each possible gray value in the image, providing a corresponding imposed matrix, each imposed matrix having a respective average gray value approximately equal to the corresponding pixel gray value;
• for each image pixel,
• reading the image data to determine the pixel gray value;
• selecting an imposed matrix that corresponds to the pixel gray value;
• logically tiling the image with the selected imposed matrix to determine a location in the matrix that corresponds to the image pixel location;
• and
• adjusting the image pixel gray value to a corresponding new gray value defined by the corresponding location in the selected imposed matrix;
• and then
• error diffusing the pixel using the new gray value.

A method of halftoning a digital image formed of a rectangular array of scan lines of pixels, each pixel having a gray value, for forming an output image, the method comprising the steps of:

• for each scan line of pixels, selecting a corresponding processing direction such that the selected processing direction varies from each scan line to the next according to a blue noise function, the blue noise function having a predetermined minimum frequency of at least one direction change per three scan lines;
• for each possible gray value in the image, providing a corresponding imposed matrix, each matrix having a respective average gray value approximately equal to its respective image gray value;
• and
• for each image pixel,
• reading the image data to determine the pixel gray value;
• selecting the imposed matrix that corresponds to the pixel gray value;
• logically tiling the image with the selected imposed matrix to determine a location in the matrix that corresponds to the image pixel location;
• adjusting each image pixel gray value so as to form a corresponding new gray value defined by the corresponding location in the selected imposed matrix;
• and then error diffusing the pixel.