Method for generating and storing digitalized density threshold values for the rastering of a half-tone picture original

Claim 1

A method for producing a half-tone picture from an input picture using rotated screens, comprising the steps of: making up the rotated screens with a plurality of orthogonally oriented comparison fields forming a supercell with each comparison field being formed of a plurality of sub-cells, and wherein each sub-cell comprises a plurality of elements with each element having a storage word assigned thereto and corresponding to a pixel of the half-tone image to be blackened or not to be blackened depending on a size of a half-tone raster spot to be grown by use of each sub-cell for a corresponding gray tone level of the input picture to be reproduced as said half-tone spot in the half-tone picture, and wherein the sub-cells have different sizes caused by a number of storage words of the sub-cells differing from a nominal value; providing a two-dimensional spot function for describing a shape of the spot to be grown by use of each sub-cell, said spot function defining a threshold value to be assigned to each of the elements in each sub-cell; creating a sorted list of position addresses of the storage words of the elements in each sub-cell depending on the spot function; assigning density threshold values defined by the spot function to the storage words according to positions of the respective storage words in the sorted list; as a threshold value correction, if an actual number of storage words is greater than said nominal value for a respective sub-cell, density threshold values are raised for storage words at a beginning of the sorted list and density threshold values are lowered for storage words at an end of the sorted list, and if an actual number of storage words is less than said nominal value for a respective sub-cell, density threshold values are lowered for storage words positioned at the beginning of the sorted list and density threshold values are raised for storage words at the end of the sorted list; and comparing tonal values of the input picture to be reproduced as said half-tone picture to the threshold values of the sub-cells to create a bit map of on/off signals for a binary recording device which decides during reproduction of the half-tone picture by the binary recording device whether a pixel of the half-tone picture is blackened or not.

Claim 2

A method for producing a half-tone picture from an input picture using rotated screens, comprising the steps of: making up the rotated screens with a plurality of orthogonally oriented comparison fields forming a supercell with each comparison field being formed of a plurality of sub-cells, and wherein each sub-cell comprises a plurality of elements with each element having a storage word assigned thereto and corresponding to a pixel of the half-tone image to be blackened or not to be blackened depending on a size of a half-tone raster spot to be grown by use of each sub-cell for a corresponding gray tone level of the input picture to be reproduced as said half-tone spot in the half-tone picture, and wherein the sub-cells have different sizes caused by a number of storage words of the sub-cells differing from a nominal value; providing a two-dimensional spot function for describing a shape of the spot to be grown by use of each sub-cell, said spot function defining a threshold value to be assigned to each of the elements in each sub-cell; creating a sorted list of position addresses of the storage words of the elements in each sub-cell depending on the spot function; assigning density threshold values defined by the spot function to the storage words according to positions of the respective storage words in the sorted list; as a threshold value correction, if an actual number of storage words is greater than said nominal value for a respective sub-cell, density threshold values are raised for storage words at a beginning of the sorted list and density threshold values are lowered for storage words at an end of the sorted list, and if an actual number of storage words is less than said nominal value for a respective sub-cell, density threshold values are lowered for storage words positioned at the beginning of the sorted list and density threshold values are raised for storage words at the end of the sorted list; and comparing tonal values of the input picture to be reproduced as said half-tone picture to the threshold values of the sub-cells to create a bit map of on/off signals for a binary recording device which decides during reproduction of the half-tone picture by the binary recording device whether a pixel of the half-tone picture is blackened or not.