Method and arrangement for producing rastered color separations and printing forms

A system for producing rastered printing forms with rasters having arbitrary raster angles and raster widths with a recording element moving relatively over a recording medium, the recording medium being subdivided into a plurality of surface elements and wherein raster dots are composed of the surface elements are recorded by the recording element, comprising:

• a memory matrix for addressable deposit of raster thresholds which represent a periodic basic structure of a raster mesh of the raster;
• a coordinate transformation unit connected to the memory matrix for calculating addresses for the memory matrix from coordinate values of the surface elements on the recording medium swept by the recording element taking a respective raster angle into consideration;
• a comparator whose inputs are connected with a raster threshold readout from the memory matrix and with image signal values acquired by point-by-point and line-by-line optoelectronic scanning of an original and whose output is connected to the recording element, said comparator comparing raster thresholds and image signal values so that a decision is respectively made on a basis of a comparison as to whether a surface element is recorded as a part of a raster dot or not;
• said coordinate transformation unit comprising
• an arithmetic unit for subdividing the respective raster angle into a first sub-angle whose tangent is a rational number and into a second sub-angle whose tangent is an irrational number;
• a first read-write memory connected to the arithmetic unit for identification of first coordinate parts of a second orthogonal coordinate system aligned in a direction of the raster from coordinate values of the surface elements in a first orthogonal coordinate system oriented in a recording direction and describing the respective raster angles with the second coordinate system, the identification of the first coordinate parts taking the first sub-angle into consideration;
• a random generator connected to the arithmetic unit for calculating second coordinate parts of the second coordinate system from coordinate values of the first coordinate system taking said second sub-angle into consideration, and for randomly modifying the calculated second coordinate parts of the second coordinate system;
• an adder connected to a respective output of said first read-write memory and of said random generator for addition of said first coordinate parts of said second coordinate system and the randomly modified second coordinate parts of said second coordinate system to form total amounts;
• an accumulator connected to an output of the adder for formation of second coordinate system coordinate values from said total amounts by accumulation and for calculating addresses for said memory matrix;
• and
• a register connected to an output of the accumulator for deposit of said addresses for the memory matrix.