GB2240237A - Colour reproduction using memory tables for correcting the colour signals - Google Patents

Abstract

The system has a memory circuit 14 storing tables with correction values for correction of incoming colour separation signals. A table addressing circuit 15 is used to direct the incoming signals to three relatively small inexpensive memory circuits namely static RAMS 14a, 14b and 14c, for generation of corrected signals. The table addressing circuit, which may be a hard-wired circuit or a dynamically controlled switching circuit, addresses all bits of an associated colour separation signal to a memory circuit together with some of the most significant bits of the other two signals. Thus, memory capacity is optimised with the result that the circuits are relatively simple and inexpensive to maintain and less complex processing circuitry is required. Further, there is no need for feedback circuits for dynamically updating the tables as these are set in response to the type of output device or scanner being used. <IMAGE>

Description

"An image generating system" The present invention relates to an image generating system and in particular to that of the type which represents values of an original picture by digital values which are generated by scanning of the original picture.

In such image generating systems, some steps involved may lead to degradation of the signals and accordingly the final image may not correspond satisfactorily to the original picture.

For example, in the field of comic book reproduction, a black keyline is coloured with cyan, magenta and yellow values.

Colouring may be carried out on a computer graphics system by scanning the black key line as a raster bitmap and assigning exact colour values to that bitmap. Such a bitmap generally includes three colour image RGB or CMY data. Once coloured, it is outputted as a three colour separation and merged with the original keyline. If these films are used in the four colour printing process the colour quality of the image may vary significantly from the intended visual values assigned at the colouring stage. This variation may be due to both optical and mechanical data gain on press. For example, mechanical data gain is affected by blanket pressures, ink/water balance and other variables. Such variations in colour or image values generally do not vary linearly with the colour values.

Variations or impurities may also occur in the scanning of an original picture, especially if a charge-coupled device array is used. This also leads to degradation of the quality of the final image.

In an effort to overcome these problems, relatively sophisticated and expensive scanners and presses have been used. Alternatively, correction systems such as that described in United Kingdom Patent Specification No. GB 2,070,381 B (DAINIPPON) may be used for correction of the image signals to compensate for degradation of the signals and variations in printing. This correction system includes a table of corrected signals which is accessed by colour separation signals. The corrected signals are outputted at the output devices and are monitored so that the tables may be updated by a colour controller. In general, a colour separation signal is 8 bits wide and thus, the signals addressing the table of this prior art would be 24 bits wide and would require 224 storage locations per colour. Such a memory is extremely expensive.Further, the circuits required for updating such a memory would of necessity be complex because of the very large number of locations and the complexity of the inter-relationships between them.

The present invention is directed towards providing an image generating system incorporating a correction system which is simpler and which is more simple to operate and maintain.

According to the invention, there is provided an image generating system comprising: means for reception of digital parallel colour separation signals representing an original picture, scanned by the scanner; a memory circuit having a table of corrected values associated with each colour separation signal; a table addressing circuit comprising means for directing to each table of the memory circuit, the associated colour separation signal and a bit of the other colour separation signals as an index for the table; and means for directing addressed corrected colour separation values from the tables as correction colour separation signals to the output device.

In one embodiment, in the range of one to four most significant bits are directed from each other colour separation signal to each table.

In another embodiment, the tables include corrected values determined according to charge-coupled device scanner characteristics.

Preferably, the memory circuit comprises static random access memory circuits. Ideally, there is one static random access memory circuit for each table.

The invention will be more clearly understood from the following description of some preferred embodiments thereof, given by way of example only with reference to the accompanying drawings in which: Fig. 1 is a schematic representation of an image generating system of the invention; Fig. 2 is a circuit diagram illustrating a correction system incorporated in the image generating system; and Fig. 3 is a graphical representation of variations in paper dot values caused by a printing process.

Referring to the drawings, and initially to Fig. 1, there is illustrated an image generating system of the invention, indicated generally by the reference numeral 1. The system 1 comprises a scanner 2 of the charge-coupled device array type having three 8-bit output buses for transmission of red, green and blue colour separation signals representing a scanned original picture. The buses 3 are connected to a colour correction system 4 having output buses 5, again 8 bits wide, for transmission of corrected colour separation signals to an output device, not shown. The output device may be a graphics display terminal or a printer or any other suitable device.

The correction system 4 comprises a peripheral interface circuit 10 connected to disk drives 11, a user visual display terminal 12 and a keyboard 13. The peripheral interface circuit 10 is connected to a memory circuit, namely, a set of three static RAMS of 16 KByte capacity. These are described in more detail below with reference to Fig. 2. A memory circuit 14 is connected to a table addressing circuit 15.

Referring now to Fig. 2, portion of the correction system 4 is illustrated in more detail. Parts similar to those described with reference to the previous drawings are identified by the same reference numerals. The memory circuit 14 comprises three static RAM (S-RAM) chips 14(a), 14(b) and 14(c) of 16 KByte capacity and having 14 input lines and 8 output lines.

Before describing operation of the image generating system of the invention, the characteristics of various image generating devices will now be described. In most printing devices, there is a difference in intensity between colours printed and the intensity represented by the colour separation signals.

For example, the following table illustrates a typical example: - ORIGINAL R 40% G 40% B 40% PRINT R 48% G 46% B 43% CORRECTION R -8% G -6% B -3% These correction factors are normally non-linear.

Referring now to Fig. 3, it will be seen for example that during a printing process paper dot values vary in a nonlinear fashion during the printing process. The correction system of the invention must allow for such non-linearity.

If, as in the above embodiment, the image generating system includes a scanner of the charge-coupled device array type, the problem is more complex. Impurities caused by such a device and its associated colour filters may be represented as follows: R = Rin + Ri + Gi + Bi G = Gin + Ri + Gi + Bi B = Bin + Ri + Gi + Bi The terms with the "i" suffix are impurities caused by the physical limitations of the charge-coupled device technology and vary from one device array or set of filters to another.

It will be seen that the impurities introduced in, for example, the red data sample are dependent not only on the value of red at that position on the original picture but also on the values of green and blue at that position on the picture. Thus, a pure red colour scanned on a charge-coupled device array may have a blue or green component in addition to an inaccuracy in the red component, which inaccuracy does not change in a linear manner with the value of the red colour being sampled.

The image generating system 1 of the invention is set up by initially determining correction factors for the type of output device being used. This may be an automated process which would be carried out at intervals for, say, a certain charge-coupled device array, as the response characteristics of the device would vary with age. The correction factors are used to determine values for a correction table for each of the red, green and blue colour components. The values storedin each of these tables is a correction value for that component in the corrected colour separation signals as a function of the original colour separation signals. Each table is stored in the respective S-RAM circuit 14(a), 14(b) or 14(c). Each table has 214 locations because it has 14 address lines operating in this embodiment. The corrected signal values stored at each location is 8 bits wide.It will be seen in Fig. 2 that the table addressing circuit 15 comprises a hard wired circuit wherein each S-RAM has all 8 lines of its associated bus forming an input in addition to three lines carrying the most significant bits of the other two colour separation signals. For example; the S-RAM 14(a) has all 8 lines carrying the red colour separation signal in addition to the lines carrying the three most significant bits from the green and blue colour separation signals.

In operation, the scanner 2 scans an original picture and generates colour separation signals which are outputted on the buses 3. The S-RAM 14(a), 14(b) and 14(c) continuously receive the associated signal together with the three most significant bits from the other two colour separation signals and this amounts to a 14 bit address which locates the relevant location in the table. The relevant location stores an 8-bit corrected colour separation value and this is delivered to the output buses 5 as a corrected colour separation signal.

It will thus be appreciated that relatively little memory capacity is required as there are only three chips, each requiring only 2l4 memory locations. This is a significant improvement over the prior art where memory circuits having 224 memory locations per colour were required, which circuits are extremely expensive. A further advantage is that the memory circuit 14 is simpler and if one of the circuits fail, it is relatively simple and inexpensive to replace it. It will also be appreciated that there is no need to provide feedback circuits and complex addressing circuits for addressing of a very large memory circuit. All that is required is to set the memory circuit 14 with the correct tables for the particular output device and this needs to be updated only on a relatively infrequent basis as the output device ages or is replaced.

The table addressing circuit need not be necessarily be a hard wired circuit. It is envisaged that it would be a dynamically controlled switching circuit which may switch any desired selection of the other colour separation signals to a particular S-RAM circuit. The important point is that there is no need to have all 24 lines going into the one circuit with resulting complexity and large memory circuits being required.

It will also be appreciated that because the memory circuit 14 is relatively small, speed of access is generally maintained at a high rate.

The invention is not limited to the embodiments hereinbefore described but may be varied in construction and detail.

Claims (6)

1. An image generating system comprising a scanner for scanning an original picture and an output device for output of an image, the system also including a correction system comprising: means for reception of digital parallel colour separation signals representing an original picture, scanned by the scanner; a memory circuit having a table of corrected values associated with each colour separation signal; a table addressing circuit comprising means for directing to each table of the memory circuit, the associated colour separation signal and a bit of the other colour separation signals as an index for the table; and means for directing addressed corrected colour separation values from the tables as correction colour separation signals to the output device.

2. An image generating system as claimed in claim 1 wherein in the range of one to four most significant bits are directed from each other colour separation signal to each table.

3. An image generating system as claimed in claim 1 or claim 2 wherein the tables include corrected values determined according to charge-coupled device scanner characteristics.

4. An image generating system as claimed in any preceding claim wherein the memory circuit comprises static random access memory circuits.

5. An image generating system as claimed in claim 4, wherein there is one static random access memory circuit for each table.

6. An image generating system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.