Bit mask generation system

An image processing method for converting multi-level image data into half-tone image data comprising:

• receiving items of multi-level image data, said items of multi-level image data associating positions in an image with a respective one of a range of shades;
• storing data representative of a set of binary bit mask arrays, each of said arrays being associated with a respective shade of said range of shades, the entries in said arrays defining a pattern of dots and gaps for representing said associated shade;
• and
• converting items of multi-level image data into half-tone image data by:
• selecting the bit mask array associated with the shade for an item of multi-level image data being converted;
• identifying one or more entries in said selected bit mask utilizing the position identified by said item of multi-level data;
• and
• outputting said one or more identified entries in said bit mask array as half-tone data for said position,
• wherein the entries in the binary bit mask arrays defined by stored data are arranged such that each entry forms part of a cluster of two or more adjacent entries of the same type within said array or is adjacent to an entry of the same type which forms part of a cluster of entries of the same type in said array, where the entries at the edges of a binary bit mask array are considered adjacent to corresponding entries at the opposite edge of the binary bit mask array.

A method of generating bit mask arrays comprising:

• storing data representative of a binary array of entries of a first and a second type wherein the entries in said array are arranged such that each entry forms part of a cluster of two or more adjacent entries of the same type within said array or is adjacent to an entry of the same type which forms part of a cluster of entries of the same type in said array, the entries at the edges of a binary bit mask array being adjacent to corresponding entries at the opposite edge of the binary bit mask array;
• determining a set of entries of a first type to be considered for modification;
• sequentially selecting and processing entries in a determined set to determine whether modifying a selected entry to become an entry of said second type would cause said stored array to define an array of entries where at least some entries no longer form part of a cluster of two or more entries of the same type or are entries which are no longer adjacent to a cluster of two or more entries of the same type;
• updating said stored array by modifying a selected entry if such modification is not such to cause said stored array to define an array of entries where at least some entries no longer form part of a cluster of two or more entries of the same type or are entries which are no longer adjacent to a cluster of two or more entries of the same type;
• determining an alternative modification of a plurality of adjacent entries of said first type including said selected entry which is not such to cause said stored array to define a array of entries where at least some entries no longer form part of a cluster of two or more entries of the same type or are entries which are no longer adjacent to a cluster of two or more entries of the same type if modification of said single entry is such to cause said stored array to define an array of entries where at least some entries no longer form part of a cluster of two or more entries of the same type or are entries which are no longer adjacent to a cluster of two or more entries of the same type;
• and
• if it is determined that modifying any individual entries of said selected set of entries of said first type would cause said stored array to define an array of entries where at least some entries no longer form part of a cluster of two or more entries of the same type or are entries which are no longer adjacent to a cluster of two or more entries of the same type, updating said stored array utilizing a determined alternative modification associated with one of said selected set of entries of said first type.

A method of generating a set of bit mask array for use in converting multilevel image data into half-tone image data for printing using a printing system where the vertical and horizontal extent of dots in a rendered half tone image are not equal, the method comprising:

• storing data defining a bit mask array;
• computing for each entry of a first type in said bit mask array the value of a distance function indicative of the distance of the position in the array of the entry of that type from entries in said array of a second type;
• selecting as an entry for modification an entry of said first type associated with a value indicative of the greatest distance of the position of said entry from entries of said second type in the array;
• and
• modifying said array by changing said selected entry for modification to become an entry of said second type, wherein said distance function comprises a distance function determined from the difference in x co-ordinates and y co-ordinates in two points in an array respectively in using the following equations:
• δx=[|x1−x2|−1]*λx+1
• δy=[|y1−y2|−1]*λy+1
• where x1, y1 and x2, y2 are coordinates for the two points in the and array and λx and λy are scaling factors for scaling the distances in terms of co-ordinates to actual distances in output images in terms of the smallest dimensions of pixel in output images.