Patent GB-1234975-A (GB)
Title: Improvements in or relating to the composition of rastered half tone pictures by means of electronic phototype setters operating by rastering
IPC: G06F3/13
Filed: 1968-08-12
Abstract: 1,234,975. Facsimile apparatus. DR.-ING. RUDOLF HELL. 12 Aug., 1968 [26 Aug., 1967], No. 38516/68. Heading H4F. In a method for composing rastered half-tone pictures the elemental raster areas are allocated code numbers depending on the quantized tonal value of a photo-electrically scanned continuous tone picture original, the code numbers being derived by dividing the range of tonal values from white to black into a finite number of successive grey stages, which are increasingly shaded according to a predetermined non-linear function, these stages being numbered and the stage numbers being coded. Depending on the code number allocated to the detected tonal value, the surface of an elemental raster area is covered with black by a predetermined amount to form a raster dot, the elemental raster areas, i.e. raster dots, being stored like character pictures in electronic form in the magazine store of the composing apparatus, e.g. a phototype setter. The raster information is read out from the store of the phototype setter for recording the raster dot on the picture screen of a cathode-ray tube which is focused on to a photographic film, the distances between the elemental raster areas being the same as the distances between the scanned dots of the continuous tone picture original. Each elemental raster area may be divided into a large number of surface elements, each individual element of which may be black or white, i.e. occupied or unoccupied by the electron beam in the cathode-ray recording tube. This permits almost any shaping, and even a non-uniform shaping, of the raster dots, Fig. 2 (not shown). The " raster dot " contained in one elemental raster area may comprise a plurality of unconnected dots. The raster dots may be arranged to overlap, or just touch one another, in the blackest part of the half-tone photographed image . Two non-linear functions to which the successive grey stages are numbered are described with reference to Fig. 4 (not shown), the degree of black covering being plotted on the ordinate axis whilst the abscissa is provided at equal intervals with the numbers of the elemental raster areas (i.e. stage numbers) allocated to the different grey stages. The functions correspond to the power of perception of the human eye (physiological curve), one curve relating to raster dots without overlapping, and the other curve to raster dots with overlapping. Instead of providing a store from which the picture content of the different raster dots are read out, the composing programme may control a raster dot generator which produces the voltages or currents necessary for beam deflection and brightness control of the recording cathode-ray tube, e.g. circular raster dots may be produced by out-of-focus control of the electron beam, or square or rhombic raster dots may be produced by recording Lissajous figures. This latter raster dot generator also preferably controls the intensity of the electron beam with respect to the velocity of the spot deflection and the focusing with respect to the raster dot size. Another raster dot generator is described in which rhombic raster dots are produced by modulating an A.C. voltage of high-frequency with a voltage at the raster dot frequency, subtracting from this modulated A.C. voltage a D.C. voltage which is inversely proportional to the covering values of the raster dots to be recorded, forming an inverse pulse from the residual voltage, and adding the inverse pulse to the residual voltage.