Patent GB-1489433-A (GB)
Title: Method for dot matrix recording
IPC: B41J2/205
Filed: 1974-11-29
Abstract: 1489433 Halftone reproductions MEAD CORP 29 Nov 1974 [30 Nov 1973] 51904/74 Heading H4F A method of producing a halftone image by recording 2 x 2 dot matrices I, II, III, IV, Fig. 2, representative of elemental areas thereof, derived from a reference store of dot matrices, A, Fig. 1A, which form components of larger matrix groups 10, each group being representative of a different grey level, wherein the entire of one or more groups is recorded in image areas, of the same grey level, large enough for accommodation thereof and the smaller component dot matrices, selected from groups representing the relevant grey level, are recorded in those areas where changes of grey level occur. The reference store comprises four sets A, B, C, D each of 21 groups giving four different versions of each of 21 grey levels ranging from 0% (white) to 100% (black). The image is produced by an ink spot recorder of known type, Fig. 4, (not shown). The array of elements comprising the image forms uniform rows and columns on the record medium. Elements situated on: odd numbered rows and columns are chosen from the I position, Fig. 2, in the respective grey level group; odd numbered rows and even numbered columns from the II position; even numbered rows and odd numbered columns from the III position; and even numbered rows and columns from position IV. So that, for example, when an element of 30% grey level, located on an odd numbered row and column is to be printed, the 2Î2 dot matrix in the I position of one of the four 30% grey level groups, in this case the 30% group from set A, Fig. 1A, is chosen. When adjacent elements of the image are of the same grey level they are represented collectively by printing the entire group representative of that level in the chosen set otherwise they are represented individually by dot matrices derived from corresponding positions in differing matrix groups. To avoid the occurrence of patterning effects the four sets ABCD are established, the criteria for choosing from the sets being which of the elements above, below, right or left of the current element has highest grey level, averaging in the event of a plurality of elements above or below when printing entire groups at once, or by employing a random sample technique. In order to provide as high a number as 21 grey levels it is necessary to include among the groups, special groups 11 to 14. Fig. 1, comprising a plurality of groups 10 in side-by-side fashion. As described the method requires a binary data stream representing the presence or absence of dots in the spaces 15. This stream is produced on a magnetic tape, used to control the reproducer, by converting grey level data, generated by a scanning device of known type, Fig. 5 (not shown) and stored on another magnetic tape. A logic circuit for the control of a computer effecting the conversion is disclosed. Alternatively the recorder inking head may be controlled by the direct output of the conversion means which receives as input grey level data generated by the scanner direct or by reading said magnetic tape in either of which cases synchronism must be achieved between recorder head and data source.