Process to avoid image defects in multicolour printing caused by non coincident printing of the colour separations.

Technical field

The invention relates to the field of multicolor printing and relates to a method and apparatus for preventing image defects due to faulty overprinting of the color separations of the individual inks (magenta (M), cyan (C), yellow (Y) and black (K) arise where an original master image is optically-electrically scanned and trichromatic in sampling primary measured color R, G, B are obtained which are converted for printing magenta, cyan, yellow and black in color separation signals the production of color separations used to print control, and wherein in the color separations at boundaries between the color areas in each case the peripheral zone of a region of one color is displaced beneath the marginal zone of an area of ​​another color.

State of the art

At today usual multi-color printing, in which worked with subtractive color mixing, pass through Passer Error overprinting of the color in the printing units, from other causes, such. As extreme color removal in the individual color separations or through inaccurate Overlay of images called "Blitzer" on, the disturbing occur in the form of color fringes or white points at which the paper white is revealed, in appearance and degrade the image quality.

In DE-OS 24 46 761, for example, a method is. For the additive color pressure indicated reduced in the dark color regions by a predetermined width and bright color areas in the direction of and over the dark areas of color addition can be increased to the color bleeding to compensate, but to ensure the connection of the color regions in the actual printing. In this type of printing a color separation is created for each individual color to be printed, and the color separations are colored depending on the desired color scheme with a color predetermined color composition, z. B. pink, purple or brown, etc.. The result according to the submission by the overprinting of the color separations the desired color pattern, either in the colors of the original document or according to the selected color scheme in another deviating from the original color patterns. Both in the preparation of the original work (sample cartridge) and the pressure itself is working with colors precisely defined color composition, on the one hand in the optical-electrical scanning of the originals, the colors can be accurately detected, and on the other hand the pressure the corresponding parts of the image or pattern elements in the desired color appears.

In this printing method, it is simply that by register inaccuracy, ie by registration error flashes, fringing and unwanted overlaps or uneven width of lines in dark color boundaries, as mentioned above occur, to thereby prevent an increase or decrease of the surfaces of the individual color ranges in be made the sense that dark color is reduced in total by a predetermined width and lighter areas also increases overall in the direction and dark color areas beyond. The criterion for this way of working is easy to win in this method, since only two colors meet at the candidate image points precisely defined color composition, and you can easily find and set at the chosen color combination between the two colors, which lighter to or darker color region forms. In addition, the total number of colors used is limited to a small number, making a clear statement regarding this criterion can be made at any time.

When initially described multi-color printing, in which working with subtractive color mixing and no recognition of single colors is performed by the template here, are completely different conditions. An image points at which such errors described above can occur, a number of different colors can occur together. This may be it will be seen that the individual printing inks can assume all intermediate values ​​between 0 and 100% depending on the color of the original document, which can occur all the colors of the color space. It is therefore not as in DE-OS described initially determined here by the color combination on which of two colors on a contour that is lighter or darker, but from the color signals acquired in the scanning of the original document must be determined in each case, which brighter the or darker ink is.

task

The present invention is therefore based on the object to provide a method and means by which artifacts arising in multicolor printing by incorrect overprinting of the individual color separations, can be avoided.

The invention achieves this by the characterizing parts of the claims 1 and 2 specified characteristics.

The invention is illustrated below with reference to Figures 1 to 12th

Show it:<ul><li>1 shows a schematic representation of the process of overcrowding on a contour,</li><li>Figures 2a and 2b a split in pixels contour with a corresponding field for studying the contour,</li><li>Figure 3a is an example of an overfill of "inside" to "outside"</li><li>Figure 3b is an example of the Überfüllvorgang for Überfüllrichtung from "outside" to "inside"</li><li>Figures 4a and 4b examples of the overfilling when pasting a letter,</li><li>Figures 5 to 11 Examples of the direction matrices and Überfüllmatrizen, as used in the overfill according 4a and 4b and Figures</li><li>12 shows an example of a device for carrying out the method.</li></ul>

Based on the figure 1 the principle of Überfüllvorgangs is shown in more detail.

At a contour 1 a visually bright area I encounters a visually darker area 11. An arrow 3 indicates, to be crowded in what direction. By the arrows 4 and 5, an overlap zone is indicated, which is supposed to have a predetermined trap width.

In Figure 2a is also a contour shown, which has a slightly more irregular course as in Figure 1, the corresponding image surface is divided up into individual pixels, which are represented by square, adjacent fields. The size of the individual pixels results in image scanning from the release or the selected resolution, the image data are stored in a storage medium.

The boundary between the bright area and the dark area I II is indicated in the drawing of Figure 2a represented by crossed pixels that belong to the darker color region II and are therefore outline determinative. Furthermore, in Figure 2a, a rectangular box, R, which is shown in Figure 2b again individually drawn, is, as discussed below with the aid of which it is determined whether the to be examined pixel is inside or outside the contour and by the Überfüllrichtung is determined. A circle with a cross means in Figure 2b that the pixel is within the contour, and a circle without cross means that the pixel outside the contour.

In multicolor printing, an image is reproduced by three or four printing inks which are printed over each other in subtractive color mixing. It follows that on each pixel fall three or four ink-metering signals for the color inks of yellow Y, magenta M, cyan C and black K. These ink metering signals, also referred to below as color separation signals are used in the present invention as a criterion for the Überfüllvorgang. As the quantity of ink and therefore the size of the color separation signals of adjacent pixels are different and the contour impression created by the boundary between a light and dark color range, it is assumed that the darker color region is contour-determining.

In a first step the contour determining color is therefore first determined using the corresponding color separation signal to such a transition. First, the color separation signals Y, M, C and K of two adjacent pixels are rated color-specific weighting factors. This is done by multiplying by a factor_l, a₂, a₃ or a₄, Since the individual colors vary greatly involved in the visual contour impression these factors differ from one another, and the following factors were chosen in this case for the individual printing inks, specifically for Y value of 0.3, for M, the value 0.7, for C the value of 0.9 and K value of 0.2. Following that, the maximum belonging to a pixel color separation signals is then determined and compared to the maximum signal of the neighboring pixel. The largest of these two signals then presents the darker color region contour determinative, constitutes, and the maximum signal is the outline determining color separation signal. Thus, the contour runs along between these two pixels.

According to Figure 1, the width of the overlap zone will now be given in which to be crowded. Within the overlapping area is kept unchanged for the individual pixels of the darker region II the contour determining color separation signal. The other color separation signals of these pixels are replaced by the corresponding color separation signals of the adjacent, brighter image point. In light image area I, the color separation signals remain unchanged.

In other Überfüllrichtung reverses this order.

In the figures 3a and 3b will be explained with two examples. Figure 3a shows the overcrowding of "inside" to "outside" using the corresponding direction and Überfüllmatrizen and 3b overcrowding from "outside" to "inside". The direction matrices shown on the left in the figure include the appropriate Überfüllmatrizen, the pixels indicated in the figure points are crowded.

In the figures 4a and 4b, this is illustrated by the Einkopierens a letter "A". Figure 4a shows the Überfüllvorgang in a bright letter "A" in a dark environment and 4b for a dark letter "A" in a bright environment.

5a to show the LLA to the figure 4a belonging direction and Überfüllmatrizen and Figures 5b to llb to the figure 4b belonging matrices.

As the individual steps of the trapping process is explained in detail below with reference to FIG 12th It is assumed in Figure 12 from the fact that the color separation signals Y, M, C and K of the original image is already available to optoelectronic scanning and digitization in stored form. It may, for. Example, a known image processing system for the printing technique, for. Example, the system Chromacom the applicant, be used, in which the image signals are present after all Procreative corrections and assembly in the form of digitized stored color separations before using this information electronic production of individual color separations, the output of proofs or the visualization of the print result to a color monitor take place. The color separations Y, M, C and K are stored in this system, each 8-bit encoding on a magnetic disk memory. In Figure 12, such a disk 10 is shown from which are transmitted the digital image signals of the color separations in a memory 11, which is 4 x 8 bit deep and has quick access.

Furthermore, it is assumed in Figure 12 from the fact that the contours where the Überfüllvorgang to take place, are already known or has been determined and stored in a storage contour 12th Since the statement that a contour is present, little information is needed, it is sufficient to interpret that store 1 bit depth. The contour can be either by opto-electric scanning a contour mask by input from a digitizer or alternatively by an automatic contour finding as such. B. PCT publication no. DE 80/00069 describes, done.

For determining the contour determining color contour are from the memory 12, the coordinates of the contour points in a register 14 and the coordinates of the neighboring point that does not lie within the contour of the mask placed in another coordinate register 15th With the coordinates of these two picture points of the memory is addressed 11 via an address register 16, and the thus-addressed color separation signals belonging to these pixels, are transmitted in corresponding information registers 17 and 18 which are connected to the memory eleventh These registers are connected to a multiplier 19, which again for its part is in connection with a coefficient memory 20 and a buffer 21st be In the multiplier 19 then, as described above, the color separation signals Y, M, C and K of the two pixels by the coefficients a_l, a₂, a₃ and a₄ multiplied, and the result is stored in the memory 21st

With the memory 21, a comparator is also connected which compares the thus evaluated signals to each other and the maximum signal detected. This maximum signal represents the contour determining color separation signal and thus indicates which of the two pixels contour determining what a bit is set to hold this decision. In memory 21 the color separation signals for two pixels are combined according to the Überfüllvorschrift previously described and stored separately according to the comparison. So the color separation signal of the darker color, which is the maximum, and the other 3 color separation signals of the neighboring point is saved to the contour point, while the color separation signals remain unchanged for the neighboring point.

In other Überfüllrichtung is in the non-form-determining color separation signal, the color separation signal corresponding to the contour determining color, acquired by the contour determining pixel, and the other color separation signals in this section are maintained. For the contour determining point the color separation signals remain unchanged in this case.

Regardless of this maximum selection is determined in a further step, to be crowded in what direction. For this purpose, belonging to the coordinates stored in the registers 14 and 15 pixels of information from the memory 12 are read into an environment space 23, which is connected to the memory 12 as well as with a comparator 24th The comparator 24 is connected via a register 25 having a memory 26 in which are stored in Figures 5 to 11 shown direction matrices. And situated in the environment space environment of the contour points is compared with these matrices, and it is found that matches these matrices with the contour. Then this matrix is ​​transferred to a register 27, this matrix is ​​numerically interpreted as a binary number and adding a bit in the register 27, which was obtained in evaluating the contour determining color separation. This bit can also be fixed on the other hand, whereby the Überfüllrichtung is then determined arbitrarily. The memory content of the register 27 is referred to below as the direction operator and is given as an address to an address register 28 which is connected to a memory 29 in which the Überfüllmatrizen, as shown in Figures 4 to 11, are stored. This direction Operator Word then searches the address of the memory 29 the corresponding Überfüllmatrix from belonging to this contour point. Pour the Überfüllmatrizen as from Figures 5 - 11 can be seen, on which pixels crowded or not to be crowded.

Depending on the predetermined trap width is predetermined, the size of these Überfüllmatrizen, and it must be before the Überfüllvorgang generates the corresponding matrices and input to the memory.

After selecting the appropriate Überfüllmatrix is ​​placed in a further memory 30, which is connected to an address counter 31st This address counter counts from the address starting 0/0, continuously, the two coordinate directions in single steps from where in the x- and y-direction^- the individual pixels of Überfüllmatrix be queried whether the individual pixels to be crowded or not. The address counter is simultaneously connected to the address counter 16 of the memory 11 and addressed there the corresponding pixels in the original image. The data obtained in the coordinate generic query the memory 30 values ​​0 or 1, ie overfill or not be provided on a line 32 to the memory 21, and set according to where a bit is decided which to the data stored in this memory color separation signal combinations replace corresponding pixel in the original image. For this purpose, the memory 21 is connected via a line 33 to the input of the memory eleventh Thus, all points of the Überfüllmatrix via the counter 31 and the address input 16 of the memory 11 are addressed and read out the corresponding pixel information from the memory 21st