1. Field of the Invention
The present invention relates generally to a computer processing of colour information, and more particularly to a method of and a device for transforming colour information for use in different colour processing and/or reproduction systems or devices such as computers, CRTs, colour printers, colour copiers, and so on.
2. Description of the Prior Art
Color information generally comprises a combination of at least three colour data, including a set of colour components such as cyan (C), magenta (M) and yellow (Y) or cyan (C), magenta (M), yellow (Y) and black (K) in the subtractive colour mixing, or red (R), green (G) and blue (B) in the additive colour mixing, three psychological attributes of colour such as hue (H), chroma (C) and value (V), and so on. Further, the concept of uniform colour space is known in the field of colour science, assuming a three dimensional space defined by three coordinates having scales of L*, a* and b*, respectively.
Artificial reproduction of colours is generally obtained by either giving a certain colour uniformly over the entire area when viewed as of each minute unit area of an object so that the colour on such each minute unit area provides directly the colour at that portion of the object, or by dividing each minute unit area of an object into a plurality of, generally 2 to 8, sub-unit areas separated from one another or overlapped in part with one another and giving different colours on the respective sub-unit areas so that those different colours provided by a set of sub-unit areas produce a required colour at that portion of the object as a result of mixing of colour in the retina. The photogravure printing by CMY is an example of the former art of uniformly colouring each minute unit area, according to the subtractive colour mixing, and a spot light by RGB is an example of said former art, according to the additive colour mixing. The process printing by CMY is an example of the latter art of differently colouring sub-unit areas of each minute unit area, according to the subtractive colour mixing, and a colour display by RGB is an example of said latter art, according to the additive colour mixing.
According to the developments of printing techniques and digital computers in these years there have been developed various systems for editing and operating digital pictures and/or characters obtained by a scanner, on the computer display, so as to reproduce colour images by various colour output devices such as colour printers and colour copiers based upon digital colour data processed in the computer. According to the developments of these arts it has become necessary to transform colour data for use in various different colour reproduction systems or devices with a high precision of prediction of reproduced colours on the display thereof in a computer, also with availability of modifications colour data based upon the computer display thereof.
However, the electronic, or numerical, colour data based upon various different colour reproduction systems or devices are generally not based upon comparable scales and/or not distributed in comparable ranges, while on the other hand the correspondence between the colours displayed in a monitor and the colours reproduced by colour reproduction devices such as colour printers, colour copiers, etc. varies widely in accordance with the varieties of the colour reproduction performances depending upon physical, chemical and/or technical factors particular to respective colour processing or reproduction systems or devices. Therefore, it has been attempted to compensate for such differences in the scale and/or range of distribution of colour data and/or in the correspondence in colour reproduction in various colour processing or reproduction systems or devices, so that the reproduction of colour is more accurately predicted.
Conventionally, the compensation for the difference in colour reproduction according to different colour reproduction systems or devices has been generally made by referring to look-up tables prepared for such compensation, with a help of interpolation when required data points are not available in such tables. It has also been tried to conceive the problem of solving the compensation for a difference in colour or distribution of colour data in a colour space as a problem with regard to geometric transformation, like the geometric rotation or shifting of images, and to apply a geometric linear transformation to such compensation. Further, it has also been tried to use the art of matrix transformation or solving linear equations to compensate for a difference in the correspondence in the computer colour data and/or the colour reproduction performances of various colour reproduction systems or devices.
However, the table look up method requires a large number of data, and nevertheless is not satisfactory in its precision, because a linear interpolation is applied to nonlinear colour spaces, resulting in a considerable discrepancy between a colour predicted and a colour actually reproduced. Mathematical approaches such as the geometric linear transformation, matrix transformation and solving linear equations are also not satisfactory, because they are also dependent upon a linear interpolation applied to nonlinear colour spaces.