Patent GB-1319686-A (GB)
Title: Graphic arts process simulation system
IPC: G02B26/10
Filed: 1970-10-08
Abstract: 1319686 Printing process simulation system HAZELTINE CORP 8 Oct 1970 [6 Nov 1969] 47883/70 Heading H4F Relates to a colour printing process simulation system which derives an electronically displayed preview colour image that simulates a composite colour reproduction produced from a set of colour separations by depositing colour pigments on a base material by means of an adjustable half-tone colour reproduction printing process, and which is easily adjustable for simulating the variables in such process. The system comprises an electro-optical scanning arrangement in which a set of image signals are developed from the colour separations; a first analogue processor in which each image signal is processed in accordance with an electrical analogue of the reproduction printing process to develop area signals each representing the fractional portion of each corresponding elemental area of the reproduction that will be occupied by colour pigment; and a second analogue processor which processes the area signals in accordance with mathematical relationships which define the apparent colour of each corresponding printed elemental area for developing output signals which may be displayed (e.g. on a colour CRT or by using colour photographic techniques) to give a colour image simulating the composite colour printed reproduction which may be obtained from the set of separations by means of the adjustable half-tone colour reproduction printing process. Electro-optical arrangement comprises a table 48, Fig. 8, having four identical apertures containing glass plates 42a, 42b, 42c and 42d on which are placed four colour separation transparencies 49a, 49b, 49c and 49d representative of cyan, magenta, yellow and black prepared from an original colour image, a corner of each transparency occupying the central corner of its associated aperture as shown. The raster from a flying spot CRT is directed via a mirror (not shown) and four lenses 41a, 41b, 41c and 41d simultaneously through the four transparencies and the resulting modulated light beams are focused by Fresnel lenses on to photomultipliers 44a, 44b, 44c and 44d which develop a set of image signals Tc, Tm, Ty and Tk. Transparencies of different sizes may be scanned by ensuring that the optical axis of each lens 41 intersects a corner of the raster and the central corner of its associated transparency on table 48. The location of the flying spot scanner assembly and the set of lenses 41a-41d are independently adjustable in a vertical direction in order to accommodate separation transparency sets of different size, the orientation of the Fresnel lens for each channel being adjustable so that the plane thereof remains orthogonal to an imaginary line drawn through the centre of its corresponding imaging lens and the centre of the Fresnel lens (Fig. 9, not shown). First analogue processor comprises for each image signal Tc, Tm, Ty and Tk a half-tone simulator 16c, Fig. 3, a mode selector 17c and a printing simulator 18c coupled in cascade, which develop output signals representative of the fractional portion of each corresponding elemental area of the composite colour print (i.e. the fractional dot area). If the set of transparencies being proofed contain continuous-tone images each image signal is converted by half-tone simulator 16c to a corresponding signal representative of dot area on the corresponding half-tone transparency. Simulator 16c includes a transmissivity-to-density converter 19 (e.g. a log amplifier) and a screening characteristic simulator 20 having an adjustable nonlinear signal translation characteristic which can be altered to simulate any one of a number of different screening processes. The resulting output is coupled to the continuous tone CT input of mode selector 17c. When half tone transparencies are proofed switch 21 is placed in its half-tone HT position, the half tone simulator 16c being by-passed. Mode selector 17c inverts 22 the signal when it is representative of a positive separation transparency. Thus standard type signals representative of half-tone negative separation transparencies are fed into simulators 18c which include units that are the direct electrical analogues of corresponding portions of the printing process being simulated. Each simulator 18c includes a stroke adjustment simulator 24 (change in ink dot size), in ink spreading simulator 25 (simulates dot spreading effect when ink is deposited on paper and includes adjustable nonlinear amplifiers), and also preferably an inverter 26. Outputs (C) and (1-C) represent the fractional dot area of each elemental area of the composite colour print that will be occupied and not occupied respectively by cyan ink in the printing process being simulated. Similar signals are derived for the magenta, yellow and black channels. Second analogue processor comprises log amplifiers, adders, exponential amplifiers and tristimulus multipliers to which are fed the four signal pairs from the first analogue processor and which solves the Neugebauer equations to derive (in the embodiment described) the X, Y and Z colour representative signals (Fig. 4, not shown). The Neugebauer equations may be solved in terms of R, G and B although it is easier to convert the X, Y and Z signal to R, G and B signals for display on a colour CRT. For different inkpaper combinations different tristimulus multipliers (networks of resistors) may be plugged into the processor.