In the graphic arts industry, before a color image is printed, very often a proof of the image is made. The proof is used for inspection and approval by the printer or his customer before printing the color image on the production printing press. Nowadays, such a proof is usually made by means of digital color proofing, wherein digital data that represent the color image are directly sent to a high resolution, high-quality printer that prints the color proof on a receiving substrate, such as paper. High-quality ink-jet printers may be used for this purpose. In this document, the printer on which the proof will be made is called the proofing device, or simply the proofer, while the printer on which the image is finally to be printed is called the printing device.
The color image that is to be printed can be represented as a digital image, by a number of pixels, i.e. small discrete elements that together constitute the digital image. Each pixel has a set of color values in a device independent color space, such as CIELAB.
If the color image is to be printed on a particular printing device, the characteristics of that printing device have to be taken into account. Each printing device has a device dependent colorant space; for a printing device that uses cyan (C), magenta (M), yellow (Y) and black (K) marking particles (i.e. particles such as ink or toner), the device dependent colorant space is a CMYK space. A “colorant” is an independent variable with which a printing device can be addressed; in case of a CMYK printer, the colorants are C, M, Y and K. It is customary to express the range of physically achievable values for the colorants of a device in %. Therefore, the so-called “colorant values”, denoted as c, usually range from c=0% to c=100%.
Different printing devices have different characteristics, e.g. the two cyan inks used by two different printers will usually have different colors (the color difference may be small but such small differences may be very significant in high-quality printing). The printer model of a particular printing device reflects how that particular device reproduces color. A “printer model” is a mathematical relation that expresses the printer's output color values as a function of the input colorant values for a given printer. Thus, when inputting a set of colorant values to the printer model, the output of the printer model is the set of color values that will result when the printer is addressed with that set of colorant values. The printer model can be inverted; when using the inverted printer model, the colorant values result that are required to obtain given color values with the particular printer.
For a CMYK printing device, the “primary colors” are cyan, magenta, yellow and black. The “secondary colors” for a CMYK device are red (R), green (G) and blue (B); they are obtained by combining two primary colors (R=M+Y, G=C+Y, B=C+M). For an RGB device, red, green and blue are the primary colors while cyan, magenta and yellow are the secondary colors.
More information on colorants, colorant space, color space, gamuts, printer model and other relevant terms can be found in EP-A-1 083 739 herein incorporated by reference in its entirety for background information.
To make a digital color proof on a proofer of a color image that is to be printed on a specific printing device, it is customary to proceed as follows.
In a first step, a transformation from the specific printing device to the proofer is determined, so that the transformation has as input data colorant values in the device dependent colorant space of the specific printing device, and as output data colorant values in the device dependent colorant space of the proofer. This transformation is composed of the printer model of the specific printing device followed by the inverted printer model of the proofer. Then, in a second step, the transformation that was determined in the first step is applied to the color image, pixel per pixel (remark: the input data for the transformation can e.g. be obtained by transforming the color values of the image in CIELAB space to the CMYK space of the specific printing device). By applying the transformation, the input data in the device dependent colorant space of the specific printing device are thus transformed initially to a device independent color space, by means of the printer model of the specific printing device, and subsequently from this device independent color space to the device dependent colorant space of the proofer, by means of the inverted printer model of the proofer. The obtained output data, in the device dependent colorant space of the proofer, can now be used to address the proofer in order to make the proof of the color image.
By starting from input data in the device dependent colorant space of the printing device, and transforming them to the proofer, a faithful proof of the image is obtained, since the transformation suitably takes account of the characteristics of the printing device and of the proofer.
However, the resulting proof is often not satisfactory. A typical example are color strips of primary and secondary colors that are printed in the border of the receiving substrate as control patches. E.g. in the yellow patch, some spots of other colors may appear that are disturbing to the customer.
There is thus a need for improving the transformation method described above.