Patent GB-986151-A (GB)
Title: Apparatus for correction of half-tone color images
IPC: H04N1/60
Filed: 1962-04-05
Abstract: 986,151. Copying in colour by scanning. FAIRCHILD CAMERA & INSTRUMENT CORPORATION. April 5, 1962 [April 13, 1961], No. 13151/62. Heading H4F. Black printers.-A corrected black-printer signal K for use in the production of half-tone colour images from a scanned original colour copy is derived in accordance with the equation K=H. Maximum of (R, G, B) - α R - # G - γ#, where R, G and B are the red, green and blue additive primary colour components of the original and H, α, # and γ are coefficients predetermined by the characteristics of the printing inks and the reflectances of the medium used for reproduction. A signal representing the first term of the equation is derived at terminal 13 by biased diodes 14, 15, 16, which select the maximum of negative-going R, G and B primary colour signals applied at terminals 10, 11, 12, a signal representing the last three terms of the equation is developed across resistor 22 by appropriate choice of resistors 19, 20, 21 in accordance with coefficients α, # and γ, and the difference of the two signals giving the desired signal K is obtained by subtracting circuit 25, 26. The signal K appears through amplifier 32 and may be utilized in a conventional engraving machine to cut a black printer plate. An adjustable portion of the output signal is fed back via resistor 34 to circuit 25, 26. The signal is also applied to circuits for developing corrected colour separation signals (see below). Colour printers.-Corrected colour printer signals for producing yellow, magenta and cyan printing plates and incorporating under-colour removal in accordance with amplitude of the corrected black printer signal K (see above) are derived each in accordance with an equation of the form Ap 1 =bp 1  R+cp 1  G+dp 1  B+ep 1  K, where R, G and B are the additive primary colour components, bp 1 , Cp 1  and dp 1  are the proportions (different for each printing colour) of the primary colours necessary to produce a fully saturated printing colour p1 and ep1 is a factor which is determined from the solution of a series of simultaneous equations set out in the Specification. Units 40, 41 and 42 show in detail the circuits for developing the yellow (Y) printer signal. Similar circuits 70, 71, 72 and 80, 81, 82 are provided for the magenta (M) and cyan (C) printer signals. The red, green and blue signals appear at terminals 43, 44, 45 and the K signal appears at terminal 46. The desired proportions of the signals are selected by choice of resistors 47, 48, 49 and 50, the signals in accordance with positive coefficients being combined across resistor 51 and applied via cathode-follower 57 whilst the signals with negative coefficients are combined across resistor 52 and applied via cathode-follower 58. As illustrated, for the Y signal, K, R and G are positive whilst B is negative. The two groups of signals are then subtracted in circuit 53, 54, the final signal being applied through an amplifier 42 to output terminals 69. The amplitude of the signal is controllable by adjustment of potentiometers 61a, 65a. Means are provided for adding to the signals in one colour circuit proportions of the signals developed in the other colour circuits for the purpose of developing signals representative of a changed set of inks. Thus the signals from potentiometer 61a and 65a in the Y circuit are applied to the M and C circuits, and similarly signals from the M and C circuits are each applied to the other two circuits. For example, the signals across potentiometers 93a and 98a in the M circuit are applied via resistors 101 and 102 respectively to the subtracting circuit 53, 54 of the Y circuit. Reference is made to the construction of the assembly of resistors 47, 48, 49 and 50 in the Y circuit and the corresponding resistors in the M and C circuits in the form of interchangeable plugboards so that the apparatus may be readily adapted for use with printing processes having different inks or with scanners involving different primary colours. The derivation of corrected colour printer signals as described above forms the subject of Specification 986,152. The Specification refers to printing colours having specified characteristics and deduces circuit component values for use with such colours. Specifications 692,349 and 692,400 are referred to in connection with engraving machines. Specification 907,978 is referred to in connection with corrections to be applied to colour printer signals so as to compress colour saturation and luminance.