GB1565694A - Image reproducing methods and apparatus - Google Patents

Description

(54) IMAGE REPRODUCING METHODS AND APPARATUS (71) We, CROSFIELD ELECTRON ICS LIMITED, a British company of 766 Holloway Road, London N19 3JG, do hereby declare the invention, for which we pray that a patent may be granted to and the method by which it is to be performed to be particularly described in and by the following statement: In a colour scanner for the graphic arts some form of adjustable computer is required to convert signals derived from the scanner photomultipliers or photocells into signals which, when applied to the output means of the scanner, will give rise to colour separations or plates which will result in a printed image which is an acceptable reproduction of the original subject. To this end the computer must modify the signals in a manner which takes into account the characteristics of the input and output means of the scanner, the tone or gradation curve appropriate to the particular subject to be scanned, the absorbtions and printing characteristics of the ink, and the editorial modifications to the original subject which may on occasion be required.

The corrections required are however, complicated functions and are not all independent of one another; instead they interact and this makes it difficult to provide for an operator to make adjustments as required for a particular subject or for editorial reasons. Also, the more complicated the functions provided in the correction circuits, the less stable are these circuits.

In recent years, the modified analogue signals, have been converted into digital form in order to permit storage of these signals without losing the image quality in the final reproduction. The signals are extracted from store as required to expose or otherwise treat an output surface. Such storage permits, for example, enlarging by the method described in our Patent No. 1166091. Of course, when signals are converted to digital form it is necessary to provide a large number of digital or "quantising" levels, because if the steps between levels are too great the transitions between two adjacent quantising levels will be visible on a finished picture as undesirable contours. The number of levels cannot be increased indefinitely for reasons of storage capacity. The number required can be somewhat reduced by superimposing noise upon the signals.

Because of the difficulty of setting up and maintaining the analogue corrections referred to above and of providing adjustment that an average operator can manage, we suggested in our British Patent No. 1369702 that the correction should be carried out by means of a digitally operating look-up table; a store was provided with a matrix of colour and tone corrected output values responding to a matrix of colour-component input signal combinations, each representing a point in colour space; these values were loaded in advance of the image-reproducing operation and took into account the modifications required by an operator for the particular work in hand; image-representing signals derived from photomultipliers in the analysing head as it scanned the image to be reproduced were then converted to digital form and were applied to the "look-up table" constituted by the above-mentioned store to derive the required corrected values at the store output. For the preliminary loading of the store with the matrix of output values, parameter values were set up in accordance with the system characteristics and the characteristics of the original to be reproduced. These values were entered into a digital computer programmed to provide the required output-input relationship and, for example, to derive a black printer signal and remove "undercolour" from the colourcomponent signals.

The use of a digital "look-up" table of this kind gave the advantage of stability in the correction system and overcame the interaction problems. It also facilitated the setting up of the image reproducing apparatus to reproduce a particular image. However, some pictures or parts of pictures reproduced with this apparatus exhibited slight contouring. One solution to this would have been to increase the number of digitising levels, together with the store capacity.

We have found however, that this contouring can be avoided without increasing the number of digitising levels and stable correc- tion can still be achieved; image reproducing apparatus according to the present invention comprises means for generating signals representing colour-component densities of points of an image to be reproduced; preset analogue signal-modifying means for modifying the said signals in accordance with predetermined fixed characteristics such that at least one modified signal is a function of more than one unmodified colourcomponent signal; an analogue-digital converter for obtaining digital signals corresponding to the modified analogue signals; adjustable digital correcting means for further modifying the digital signals to obtain selected colour characteristics; a digital-toanalogue converter for deriving analogue signals corresponding to the further modified digital signals; and output means responsive.

to the latter analogue signals to treat output surfaces for the preparation of colour separations corresponding to each of the colour component images of the said original.

In a method of image reproduction according to the present invention, the major part of the correction is effected on the analogue signals prior to the analogue-to-digital conversion.

Thus, the analogue correcting means may provide a standard correction for each signal for typical operating conditions, taking into account the characteristics of input and output scanner devices, transparency colour correction and colour correction for ink absorbtions, and so on. Alternatively, the analogue colour correction may be designed on a theoretical basis so as to distribute the quantising contours equally with respect to just noticeable colour differences in all directions in all parts of colour space. The digital colour correction then takes account of the difference between this signal domain and the required output form.

In either case, the analogue correction is designed to carry out a relatively simple tranformation and to be highly stable. The correction which is required by an operator for an individual picture or to achieve a particular effect is effected by the look-up store which has been loaded under the control of the operator in a preliminary operation.

In this way, stability is achieved by simplifying the analogue correcting system, although this correcting is still responsible for the major part of the signal transformation between the input scanner and the output scanner, and at the same time contouring is avoided. The explanatation of the lack of contouring appears to be that in some parts of three dimensional colour space the eye is especially sensitive to small colour changes; if the number of quantising levels is reduced, contouring appears in such parts before it appears in other parts of the picture. A logarithmic circuit is customarily used following the photomultiplier of the analysing scanning head to modify the values of the signals of the photomultipliers. As a consequence, in the apparatus of Patent Specification No. 1369702, the quantising levels are distributed in an approximately uniform subjective manner with respect to picture brightness. When the present invention is used, however, because only a minor part of the correction is carried. out after the analogue-to-digital conversion, the quantising levels are substantially uniformly distributed with respect to colour separation density.

In order that the invention may be better understood, an explanatory diagram, together with two forms of apparatus embodying the invention, will now be described with reference to the accompanying drawings. In the drawings: Figure 1 is a chromaticity diagram illustrat- ing the problem solved by the invention; Figure 2 shows a first form of apparatus embodying the invention; and Figure 3 illustrates in the form of a block diagram a second form of apparatus embodying the invention.

The problem of colour discrimination is illustrated by Figure 1, which shows a C.I.E.

chromaticity diagram with ellipses round selected points, the radii of the ellipses representing standard deviation for a match in different directions. In this- diagram, the magnitudes of the radii of the ellipses have been enlarged relative to the scale of the chromaticity diagram in order to illustrate the problem more clearly. A full explanation of this diagram wilL be found in "Colour in Theory and Practice", edited by H. D. Murray, pages 159 and 160. The different sizes of these ellipses indicate the different sensitivities of the eye to colour differences in different parts of colour space.

One apparatus for overcoming this problem is illustrated in-Figure 2. In Figure 2, an original 10 is supported on a cylinder 12 which is driven in rotation about an axis 14.

A scanning head- 16 is given a linear motion parallel to the cylinder axis and in synchronism with the rotation of the cylinder. This motion is such that photomultipliers mounted in the scanning head scan a helical track of very fine pitch around the cylinder 12 and therefore scan the original 10 in a series of parallel lines. The scanning head 16 contains colour filters for three photomultipliers such that the output signals from the photomultipliers represent the red, blue and green components of scanned points of the original. A fourth photomultiplier scans the original 10 through an aperture larger than the apertures of the other photomultipliers and provides an "unsharp" signal in known manner. The three colour component "sharp" signals and the sharp signal are applied to logarithmic preamplifiers 18 in order to obtain signals varying substantially linearly with density, and thence to a circuit 20 in which the unsharp signal is combined with each sharp signal in known manner to provide contrast enhancement at edges.

The three colour-component signals from the circuit 20 are applied to an analogue correction circuit 22 where their amplitudes are modified to take into account the nonlinearities of the photographic, photochemical and printing parts of the overall imagereproduction system; the signal correction carried out in circuit 22 is simple and its amount is fixed; it represents a standard correction for typical operating conditions.

Next, the three corrected analogue signals are applied to an analogue-to-digital conversion circuit 24, where the signal values are samplied and converted into binary digital form. As an example, the digital signals may comprise seven binary digits, giving 128 levels. The binary signal values for each scanned point of the original are now applied to a digital enlarger 26, which may be of the kind disclosed in our British Patent No.

1166091. Briefly, the digital enlarger comprises a store and means for varying the rate of sampling signals extracted from the store in accordance with the degree of enlargement required, the rate of movement of the reproducing head in a direction parallel to the cylinder axis being also varied in accordance with the desired degree of enlargement.

The digital signals derived from the enlarger 26 are now applied to an adjustable digital correction circuit 28 comprising a look-up store of the kind proposed in British Patent Specification No. 1369702. This store has been pre-loaded with a matrix of tone and colour-corrected output values corresponding to a matrix of colour-component input signal combinations, each representing a point in colour space. In this example, for each point in colour space represented by an input signal combination, there are four output values representing the three colour component signals and a black printer signal; the three colour component signals values are reduced as a function of the black printer signal to remove "undercolour" in the colour component separations. The correction provided by the lock 28 is adjustable in that the values preloaded into the look-up store can be selected by an operator prior to the commencement of image-reproducing operation.

These corrections provide a "fine trim" to the simple fixed analogue correction provided by the lock 22.

The corrected signals from the digital correction unit 28 are now applied to the digital-to-analogue circuit 30. The resulting corrected analogue signals are selected one at a time by means of a switch 32 and are applied to a reproducing head 34 which scans a film 36 mounted on a cylinder 38 rotating about the axis 14 at the same rate as the cylinder 12. A light source in the reproducing head exposes the film 36 to permit a colour separation printer to be formed for the colour (black, magenta, yellow or cyan) selected by the switch 32.

Thereafter, the switch 32 is moved to select another output of the digital-toanalogue converter 30 and a fresh film is exposed, and so on.

The arrangement of the scanning and reproducing heads relative to the cylinders 12 and 34 is well known and may for example follow the lines indicated in British Patent specification No.1,166,091. The logarithmic preamplifiers 18 are standard circuits for following photomultipliers. An unsharp masking circuit suitable for the detail enhancement section of the circuit 20 is described in our British Patent No. 1,244,748.

Analogue correction circuits are also well known in themselves, one example being described in our British Patent No.

1,241,852. Analogue-to-digital conversion circuits and digital-to-analogue conversion circuits are available commercially. The above-mentioned British Patent No.

1,369,702, which describes the digital correction system, also includes a description of an interpolation circuit for use with the analogue-to-digital converter.

It will be appreciated that the example shown may be varied in many ways without departing from the scope of the invention.

Thus, the signals from the analogue-todigital converter may be stored on a longterm basis for use at a later time or in a different location. Furthermore, it is possible to expose all four separations at one time, for example by mounting two films in line around one track of the circumference of the drum 34 and two more films along a parallel track; the reproducing head then comprises two light sources and signals for the different separations are extracted from an intermediate store as required by the position of the separations on the drum 34 in relation to the corresponding exposing light sources in the reproducing head.

Also, the output surface which is being treated is not necessarily a photographic emulsion exposed to a light source in the reproducing head. The output surface may, for example, be a film with a very thin metal layer in which minute holes are formed by exposure to a pulsed laser source in the reproducing head; in a further possibility, the reproducing head includes a laser which directly evaporates material from the surface of the cylinder 38, which may have a plastics coating on a metal core or may have plasticsfilled recesses in the metal base.

Figure 3 shows diagramatically a further arrangement embodying the invention in which a more theoretical approach to the problem illustrated in Figure 1 is adopted. In Figure 3, signals from the scanning head 16 are applied to a transform unit 40 in which they are subjected to a transformation such that the differences in colour signal values of equal magnitude correspond to approximately uniform differences in apparent colour space, i.e. a transformation such that in terms of signal differences the ellipses of Figure 1 are all of approximately the same size.

Such transforms are well known in the theory of colour difference measurement and can be put into effect by the use of operational amplifiers and analogue multipliers in a manner apparent to one skilled in the art. The resulting signals are applied to analogue-to-digital converter 24. As in the case of Figure 2, the digital signals from the converter 24 are applied to a digital enlarging unit 26.

From the unit 26, the digital signals are applied to a digital correction unit 42 which may again be of the look-up table kind but which in this case provides output values representing the effect of digital inverse transformation (i.e. the inverse of the transformation carried out in the unit 40) plus colour and tone correction. Again, the transformation effected in the unit 40 is fixed and the correction component of the signal modification carried out by the unit 42 is adjustable for the particular work in hand. The resulting corrected digital signals are applied. as in Figure 2, to a digital-to-analogue conversion circuit 44 and thence to the reproducing head 34.

WHAT WE CLAIM IS: 1. Image-reproducing apparatus comprising: means for generating signals representing colour-component densities of points of an image to be reproduced; preset analogue signal-modifying means for modifying the said signals in accordance with predetermined fixed characteristics such that at least one modified signal is a function of more than one unmodified colourcomponent signal; an analogue-digital converter for obtaining digital signals corresponding to the modified analogue signals; adjustable digital correcting means for further modifying the digital signals to obtain selected colour characteristics: a digital-toanalogue converter for deriving analogue signals corresponding to the further modified digital signals; and output means responsive to the latter analogue signals to treat output surfaces for the preparation of colour separations corresponding to each of the colour component images of the said original.

2. Image reproducing apparatus in accordance with claim 1, in which the said analogue signal modifiying means provides a fixed amount of colour correction to impart predetermined fixed coloir characteristics to the analogue signals, and in which the adjustable digital correcting means provides further colour correction for trimming the fixed correction provided by the analogue correcting means.

3. Image-reproducing apparatus in accordance with claim 1, in which the preset analogue signal-modifying means effects a transformation of the signals from the scanning head to obtain, for predetermined differences between colour-component signal combinations, approximately uniform apparent differences in colour space, and in which the adjustable digital correcting means makes a digital inverse transformation in addition to adjustable colour correction.

4. A method of image reproduction in which analogue signals representing the colour component densities of all points of the image to be reproduced are subjected to fixed modification to obtain predetermined characteristics such that at least one modified signal is a function of more than one unmodified colour-component signal, in which the modified analogue signals for the said image points are converted into digital form, the digital signals are subjected to adjustable colour correction, the corrected digital signals are subsequently converted to analogue form, and an output surface is treated in accordance with the signals reconverted to analogue form to derive a colour separation corresponding to a colour component of the original image.

5. A method in accordance with claim 4 in which the analogue signals are initially subjected to fixed colour modification to obtain predetermined colour characteristics, the major part of the total colour correction being effected on the analogue signals prior to the analogue-to-digital conversion.

6. A method in accordance with claim 4, in which the analogue signals representing the colour component densities of the image to be reproduced are subjected to a transformation such that differences between colour-component signal combinations correspond to approximately uniform apparent differences in colour space, and in which the adjustable colour correction to which the digital signals are subjected is accompanied by a digital transformation which is the inverse of the transformation effected on the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. emulsion exposed to a light source in the reproducing head. The output surface may, for example, be a film with a very thin metal layer in which minute holes are formed by exposure to a pulsed laser source in the reproducing head; in a further possibility, the reproducing head includes a laser which directly evaporates material from the surface of the cylinder 38, which may have a plastics coating on a metal core or may have plasticsfilled recesses in the metal base. Figure 3 shows diagramatically a further arrangement embodying the invention in which a more theoretical approach to the problem illustrated in Figure 1 is adopted. In Figure 3, signals from the scanning head 16 are applied to a transform unit 40 in which they are subjected to a transformation such that the differences in colour signal values of equal magnitude correspond to approximately uniform differences in apparent colour space, i.e. a transformation such that in terms of signal differences the ellipses of Figure 1 are all of approximately the same size. Such transforms are well known in the theory of colour difference measurement and can be put into effect by the use of operational amplifiers and analogue multipliers in a manner apparent to one skilled in the art. The resulting signals are applied to analogue-to-digital converter 24. As in the case of Figure 2, the digital signals from the converter 24 are applied to a digital enlarging unit 26. From the unit 26, the digital signals are applied to a digital correction unit 42 which may again be of the look-up table kind but which in this case provides output values representing the effect of digital inverse transformation (i.e. the inverse of the transformation carried out in the unit 40) plus colour and tone correction. Again, the transformation effected in the unit 40 is fixed and the correction component of the signal modification carried out by the unit 42 is adjustable for the particular work in hand. The resulting corrected digital signals are applied. as in Figure 2, to a digital-to-analogue conversion circuit 44 and thence to the reproducing head 34. WHAT WE CLAIM IS:

1. Image-reproducing apparatus comprising: means for generating signals representing colour-component densities of points of an image to be reproduced; preset analogue signal-modifying means for modifying the said signals in accordance with predetermined fixed characteristics such that at least one modified signal is a function of more than one unmodified colourcomponent signal; an analogue-digital converter for obtaining digital signals corresponding to the modified analogue signals; adjustable digital correcting means for further modifying the digital signals to obtain selected colour characteristics: a digital-toanalogue converter for deriving analogue signals corresponding to the further modified digital signals; and output means responsive to the latter analogue signals to treat output surfaces for the preparation of colour separations corresponding to each of the colour component images of the said original.

2. Image reproducing apparatus in accordance with claim 1, in which the said analogue signal modifiying means provides a fixed amount of colour correction to impart predetermined fixed coloir characteristics to the analogue signals, and in which the adjustable digital correcting means provides further colour correction for trimming the fixed correction provided by the analogue correcting means.

3. Image-reproducing apparatus in accordance with claim 1, in which the preset analogue signal-modifying means effects a transformation of the signals from the scanning head to obtain, for predetermined differences between colour-component signal combinations, approximately uniform apparent differences in colour space, and in which the adjustable digital correcting means makes a digital inverse transformation in addition to adjustable colour correction.

4. A method of image reproduction in which analogue signals representing the colour component densities of all points of the image to be reproduced are subjected to fixed modification to obtain predetermined characteristics such that at least one modified signal is a function of more than one unmodified colour-component signal, in which the modified analogue signals for the said image points are converted into digital form, the digital signals are subjected to adjustable colour correction, the corrected digital signals are subsequently converted to analogue form, and an output surface is treated in accordance with the signals reconverted to analogue form to derive a colour separation corresponding to a colour component of the original image.

5. A method in accordance with claim 4 in which the analogue signals are initially subjected to fixed colour modification to obtain predetermined colour characteristics, the major part of the total colour correction being effected on the analogue signals prior to the analogue-to-digital conversion.

6. A method in accordance with claim 4, in which the analogue signals representing the colour component densities of the image to be reproduced are subjected to a transformation such that differences between colour-component signal combinations correspond to approximately uniform apparent differences in colour space, and in which the adjustable colour correction to which the digital signals are subjected is accompanied by a digital transformation which is the inverse of the transformation effected on the

analogue signals.

7. Image-reproducing apparatus, substantially as herein described with reference to Figure 2 or Figure 3 of the accompanying drawings.

8. A method of image reproduction substantially as herein described with reference to Figure 2 or Figure 3 of the accompanying drawings.