Patent Publication Number: US-7210753-B2

Title: Inkjet printing using protective ink

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Reference is made to commonly assigned U.S. patent application Ser. No. 10/785,835 filed Feb, 24, 2004 by Douglas W. Couwenhoven, et al., entitled “Using Inkjet Printer to Apply Protective Ink”, the disclosure of which is herein incorporated by reference. 
   1. Field of the Invention 
   This invention pertains to the field of digital imaging, and more particularly to a method for computing an amount of protective ink to be used in the process of printing a digital image. 
   2. Background of the Invention 
   In the field of digital printing, a digital printer receives digital data from a computer and places colorant on a receiver to reproduce the image. A digital printer can use a variety of different technologies to transfer colorant to the page. Some common types of digital printers include inkjet, thermal dye transfer, thermal wax, electrophotographic, and silver halide printers. 
   Modern inkjet printers are capable of delivering excellent image quality, but suffer from poor durability with respect to environmental factors such as atmospheric gases and staining fluids. For example, naturally occurring ozone is known to cause fading in inkjet prints, which are exposed to the atmosphere. The degree of fading can become unacceptable in a relatively short time period, often only a few weeks of exposure to the air. Exposure to moisture and/or staining agents can be another source for unacceptable image quality artifacts in an inkjet print. Many inkjet prints will “run” or “bleed” (where the ink begins to run off the page) when exposed to water. When subjected to other fluids such as coffee or mustard, unacceptable stains can form on the surface of the inkjet print, often in the white portions of the page where ink has not been printed. Additionally, there are optical effects that can occur with inkjet prints, which result in a perceived image quality loss. In particular, the gloss difference at the boundary between the inked and non-inked areas of the image can be disturbing to a human observer. Yet another environmental factor that can cause image artifacts in an inkjet print is handling or abrasion. Rubbing an inkjet print with a finger can cause the ink to smear from a printed area into a non-printed area, resulting in poor image quality. 
   The above described image artifacts can occur in inkjet prints because the surface of an inkjet print is not “sealed” or protected from the environment. Several methods to address these undesirable image artifacts are known in the art. One technique known in the art is to laminate the print, but this is typically too time-consuming and costly. Another technique is to apply an additional, substantially clear ink that has protective properties to the image during or shortly after the printing process. For example, U.S. Pat. No. 6,412,935 to Doumaux discloses an inkjet printer in which a “fixer” ink is printed using a separate printhead, which is vertically offset from the colored ink printheads. This technique involves an extra print pass where the paper is not advanced, and the fixer fluid is printed over the image. Similar techniques are described in U.S. Pat. No. 6,503,978. U.S. Pat. No. 6,443,568 to Askeland, et al., describes a method of underprinting and overprinting a clear fixer fluid, and applying heat to provide for improved water fastness. 
   The above mentioned references teach the use of a protective fluid for improving print durability, but do not teach methods of controlling the laydown of the protective fluid in response to the amount of colored ink that will be printed. For example, the use of pigmented inks is known to provide for some increase in durability properties when compared with dye inks. The application of a full layer of protective fluid on top of an area printed with pigmented inks is likely unnecessary to achieve the desired durability, and is wasteful of ink. Also, indiscriminate application of protective fluid leads to a dramatic increase in the total amount of fluid deposited on the page, which is known to cause other negative image quality artifacts. See for example U.S. Pat. No. 6,435,657. 
   Thus, there is a need for a method of computing a protective ink amount to be applied to an image to provide for improved durability, while minimizing the total amount of fluid deposited on the page. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a method for improving the quality of printed images by providing for improved durability of the image when exposed to environmental factors such as atmospheric gases, water, staining agents, or abrasion. 
   It is a further object of the present invention to provide for improved durability of printed images while minimizing the total amount of ink used. 
   Yet another object of the present invention is to provide for improved image quality by reducing optical effects such as differential gloss between inked and non-inked areas. 
   These objects are achieved by a method of determining and applying a protective ink amount to be printed in addition to a plurality of colored ink amounts to make colored pixels in an image, comprising:
         a) determining the protective ink amount such that the sum of the protective ink amount and the colored ink amounts is greater than or equal to a minimum ink amount necessary to provide adequate durability for the image; and   b) applying using an inkjet printer the colored ink amounts and the protective ink amount to make the colored image pixels.       

   ADVANTAGES  
   The present invention has an advantage over the prior art in that it provides for improved durability of inkjet prints to environmental factors such as atmospheric gases, water, staining agents, or abrasion, using a protective ink, while minimizing the amount of protective ink required to achieve satisfactory durability. This results in lower cost per print, or more prints per cartridge, for the end user, which is a significant advantage. Another advantage of the present invention is that optical effects that can result in poor image quality, such as differential gloss, are minimized. A further advantage of the present invention is that it provides a way for applying a different amount of protective ink in response to the colored inks that are being printed, resulting in a more efficient use of the protective ink, with less waste. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flow diagram showing placement of the protective ink processor in an inkjet printer or printer driver; 
       FIG. 2  is a flow diagram showing one embodiment of the protective ink processor; 
       FIG. 3  is a graph showing the protective ink amount and total ink amount as a function of the total colored ink amount according to one embodiment of the present invention; 
       FIG. 4  is a graph showing the protective ink amount and total ink amount as a function of the total colored ink amount according to another embodiment of the present invention; 
       FIG. 5  is a graph showing stain density contours for various overprints of protective ink and colored ink; 
       FIG. 6  is a graph showing the protective ink amount and total ink amount as a function of the total colored ink amount according to another embodiment of the present invention; 
       FIG. 7  is a flow diagram showing another embodiment of the protective ink processor implemented as a multidimensional look-up table; 
       FIG. 8  is a flow diagram showing a raster image processor which implements a protective ink processor as part of an inkjet printer or printer driver; and 
       FIG. 9  is a flow diagram showing composed look-up table which implements color management look-up tables and the protective ink multidimensional look-up table. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   This invention describes a method for computing a protective ink amount to be printed in addition to a plurality of colored ink amounts to provide for improved image quality as set forth in the objects described above. The protective ink provides durability properties, but has no colorant and is substantially clear. The invention is presented hereinafter in the context of an inkjet printer. However, it should be recognized that this method is applicable to other printing technologies as well. 
   An input image is composed of a two dimensional (x,y) array of individual picture elements, or pixels, and can be represented as a function of two spatial coordinates, (x and y), and a color channel coordinate, c. Each unique combination of the spatial coordinates defines the location of a pixel within the image, and each pixel possesses a set of input code values representing input colorant amounts for a number of different inks indexed by the color channel coordinate, c. Each input code value representing the amount of ink in a color channel is generally represented by integer numbers on the range {0,255}. A typical set of inks for an inkjet printer includes cyan (C), magenta (M), yellow (Y), and black (K) inks, hereinafter referred to as CMYK inks. 
   Referring to  FIG. 1 , a generic image processing algorithm chain is shown for an inkjet printer in which a raster image processor  10  receives digital image data in the form of an input image from a digital image data source  20 , which can be a host computer, network, computer memory, or other digital image storage device. The raster image processor  10  applies imaging algorithms to produce a processed digital image signal having input code values i(x,y,c), where x,y are the spatial coordinates of the pixel location, and c is the color channel coordinate. In one embodiment of the present invention, c has values 0, 1, 2, or 3 corresponding to C, M, Y, K, color channels, respectively. The types of imaging algorithms applied in the raster image processor  10  typically include sharpening (sometimes called “unsharp masking” or “edge enhancement”), color conversion (converts from the source image color space, typically RGB, to the CMYK color space of the printer), resizing (or spatial interpolation), and others. The imaging algorithms that are applied in the raster image processor  10  can vary depending on the application, and are not fundamental to the present invention. In a preferred embodiment of the present invention, the color conversion step implemented in the raster image processor  10  includes a multidimensional color transform in the form of an ICC profile as defined by the International Color Consortium&#39;s “File Format for Color Profiles,” Specification ICC. 1:2001-12. The ICC profile specifies the conversion from the source image color space (typically RGB) to an intermediate color space called the profile connection space (or PCS, in the terminology of the ICC specification). This conversion is then followed by a conversion from PCS to CMYK. 
   Following the raster image processor  10  of  FIG. 1  is a protective ink processor  30 , which receives the input code values i(x,y,c) and control parameters from a protective ink amount controller  40 , and produces a modified image signal having output code values o(x,y,c) which includes an additional colorant channel corresponding to a protective ink. The protective ink is simply treated as an additional colorant channel, and is processed through the rest of the image chain (including halftoning) along with the other color channels. The implementation of the protective ink processor  30  is the main subject of the present invention, and will be described hereinafter. 
   Continuing with the image chain of  FIG. 1 , the protective ink processor  30  is followed by a multitone processor  50 , which receives the output code value o(x,y,c) and produces a multitoned image signal h(x,y,c). The multitone processor  50  performs the function of reducing the number of bits used to represent each image pixel to match the number of printing levels available in the printer. Typically, the output code value o(x,y,c) will have 8 bits per pixel (per color), and the multitone processor  50  generally reduces this to 1 to 3 bits per pixel (per color) depending on the number of available printing levels. The multitone processor  50  can use a variety of different methods known to those skilled in the art to perform the multitoning. Such methods typically include error diffusion, clustered-dot dithering, or stochastic (blue noise) dithering. The particular multitoning method used in the multitone processor  50  is not fundamental to the present invention, but it is required that the protective ink processor  30 , which includes the present invention, is implemented prior to the multitone processor  50  in the imaging chain. Finally, an inkjet printer  60  receives the multitoned image signal h(x,y,c), and deposits ink on the page accordingly to produce the desired image. 
   The fundamental aspects of the invention pertain to the protective ink processor  30  of  FIG. 1 , as will now be described. Turning now to  FIG. 2 , the internal processing of the protective ink processor  30  of  FIG. 1  according to a preferred embodiment of the present invention is shown. The incoming CMYK code values, which are typically 8 bit integer values on the range {0,255} representing the amount of each ink, are coupled to an adder  70  which sums the code values producing a colored ink amount sum, S. The colored ink amount is then input to a protective ink amount generator  80 , which outputs the desired amount of protective ink to be applied. In a preferred embodiment of the present invention, the protective ink amount generator  80  is implemented using a look-up table which is indexed by the sum of the colored ink amounts, and outputs the corresponding protective ink amount, stored as an integer value on the same range {0,255} as the CMYK input values. Other forms of the protective ink amount generator  80  are possible within the scope of the invention. For example, the protective ink amount can be computed based on formulas or equations stored in computer memory. Herein below, the protective ink amount generator  80  will be discussed in the look-up table form of the preferred embodiment. In the processing of  FIG. 2 , the CMYK input values are simply passed unmodified through to the output of the protective ink processor  30  of  FIG. 1 . One skilled in the art will realize that the specific data range used here is not fundamental to the invention, and that the invention applies equally well to data spanning a different range. The shape of the protective ink amount look-up table implemented by the protective ink amount generator  80  controls the amount of protective ink that is applied in response to the sum of the colored ink amounts. In this way, a fine degree of control can be obtained by designing the shape of the look-up table to produce optimal image quality. Several variants of the protective ink amount look-up table designed to optimize different image quality aspects will now be described. 
   Turning to  FIG. 3 , a graph of one variant of the protective ink amount look-up table implemented by the protective ink amount generator  80  of  FIG. 2  is shown. In this graph, the sum of the colored ink amounts is shown on the horizontal axis as a percent number. Thus, a value of 100% means that the maximum amount of one ink is placed at each pixel on the printed page (or 50% of two inks, etc). Similarly, a value of 200% indicates full coverage of two inks, and a value of 400% indicates full coverage of all four (CMYK) inks. As will be obvious to one skilled in the art, the invention will apply to printers using a different number of inks, or different colored inks. In these cases, the percent ink values simply scale to the number of inks used. For example, in a six ink printer using the standard CMYK inks plus light cyan (c) and light magenta (m), the sum of the colored ink amounts would vary between 0% and 600%. Still referring to  FIG. 3 , the desired percent protective ink amount (a.k.a. “P-ink”) is shown plotted as a dotted line, and the total ink amount, which is the sum of the colored ink amounts and the protective ink amount, is shown plotted as a solid line. In light of these plots, consider a region of the print intended to be white (i.e., no colored ink is printed), which will have the sum of the colored ink amounts be 0. According to the look-up table of  FIG. 3 , the amount of protective ink applied in this white region will be 100%, indicating that full coverage of the protective ink will be printed by the printer. This completely seals the media from the environmental factors as described above, providing resistance to staining fluids, water, and smearing of ink from printed areas into white areas. 
   Another important aspect of the look-up table of  FIG. 3  is that the amount of protective ink applied is controlled as a function of the sum of the colored inks such that the total ink amount is at least a minimum ink amount of 100%. For example, a 50% coverage region of the image will obtain an additional 50% coverage of protective ink, bringing the total to 100%. This is a significant deviation from the prior art, and is motivated by the fact that a minimum ink amount is required to achieve sufficient environmental protection. As described earlier, the use of pigmented inks will provide for some protection against the environment, as will the protective ink. As long as the total ink amount is at least the minimum ink amount (in this case 100%), satisfactory protection is achieved. The minimum ink amount required for satisfactory protection will vary depending on the chemistry of the inks and media used, and should be determined experimentally, as will be understood by one skilled in the art. 
   An example of another variant of the protective ink amount look-up table implemented by the protective ink amount generator  80  of  FIG. 2  is shown in  FIG. 4 . In this look-up table, the total ink amount is constrained to be less than a threshold ink amount of 150% for regions where the sum of the colored ink amounts is less than 150%. This has the effect of providing for excellent protection by utilizing 100% coverage of protective ink for light density and white portions of the image (up to 50% coverage), and then reducing the amount of protective ink gradually to keep the total ink amount less than the threshold ink amount of 150% to conserve ink. Note that in this case, the total ink amount (and protective ink amount) vary discontinuously with the sum of the colored ink amounts, which is a deviation from the prior art. 
   Even more complicated variants of the protective ink look-up table of  FIG. 2  can be produced advantageously to provide for optimal environmental protection while minimizing the amount of protective ink required. Consider an experiment in which a square image is printed where the amount of protective ink increases from 0% to 100% horizontally, and the amount of colored ink (assume one ink, such as yellow) increases from 0% to 100% vertically. Thus, the lower left corner of the image has no ink printed, the upper right corner has 200% ink printed (=100% Y+100% protective ink), the upper left corner has 100% Y ink only, and the lower right corner has 100% protective ink only. The ink amounts interior to the square are linearly interpolated from the four corners. The density values are measured at a grid of locations throughout the image, and then the printed image is immersed in a liquid staining agent and mildly agitated for 30 seconds, after which it is removed, rinsed off, and dried. The density values are again measured at the same grid of locations throughout the image. The difference between the “unstained” and “stained” density values indicates the stain density, or the amount of staining that was present. A low value for the stain density indicates that little or no stain was measured. A high value for the stain density indicates the opposite. A contour plot of the stain density that was measured for the above experiment is shown in  FIG. 5 . As expected, the upper right portion of the image had no staining, since this region was protected by high percentages of both the Y and protective inks. Moving towards the lower left, the stain density increases, indicating poorer levels of protection. Each of the contour lines in the plot of  FIG. 5  indicates a constant stain density level. As can be seen from  FIG. 5 , the optimal amount of protective ink to apply for colored ink amounts between 0% and 100% is indicated by a path between the points labeled A, B, and C. This path provides for minimal staining and minimal protective ink usage. In actuality, for the particular protective ink used in this experiment, slightly more than 100% of protective ink would be required to produce absolutely no staining on white paper (as indicated by the small amount of stain density present at point A), but this would require an extra print pass over the same location on the page to apply, and would increase the print time undesirably. Also note that 100% coverage of Y ink was insufficient to provide complete stain protection, and an additional 40% or more of protective ink was required to achieve optimal performance. The data from the optimal path of  FIG. 5  is plotted as a look-up table in  FIG. 6 , where the points labeled A, B, and C correspond between the two figures. Note in this case that the optimal protective ink amount is extrapolated beyond point C in  FIG. 6 , corresponding to sum of colored ink amounts greater than 100%. In a preferred embodiment, an additional set of experiments would be conducted to print and measure stain densities for higher ink laydowns to determine the optimal protective ink amount in this region. Also note that the total ink amount shown in  FIG. 6  has an unusual and nonobvious shape, which results from the staining experiment described above. 
   It is common for the different colored inks in an inkjet printer to be formulated from very different chemical agents. Therefore, the protective properties of each ink can be different. This means that to achieve optimal protection while minimizing the protective ink, a different amount of protective ink may be required depending on which inks are being printed along with it. To provide for this case, another embodiment of the present invention will now be described. Turning to  FIG. 7 , another implementation of the protective ink processor  30  of  FIG. 1  is shown. A multidimensional look-up table  90  is addressed with the colored ink amounts (CMYK code values), and outputs CMYKP code values, where P indicates the protective ink channel value. One skilled in the art will recognize that the multidimensional look-up table  90  permits a more sophisticated protective ink function to be implemented, including providing for varying amounts of protective ink depending on which ink colors are being printed at the current pixel. A preferred embodiment of the present invention would still have the CMYK code values that are output from the multidimensional look-up table  90  match the CMYK input values, although this is not necessarily the case. 
   Those skilled in the art will also recognize that the multidimensional look-up table implementation shown in  FIG. 7  is a more general form of the one dimensional look-up table implementation shown in  FIG. 2 . That is, the look-up table behavior of  FIG. 2  can also be implemented using an implementation as shown in  FIG. 7 . This provides for an additional advantage, as will now be discussed. Consider the inkjet printer image chain as shown in  FIG. 8 , in which the raster image processor  140  receives digital image data from a digital image data source  150 , and directly outputs CMYKP data, which includes the protective ink amount, as indicated by the “P”. The CMYKP data is then input to a multitone processor  160 , which processes the data for output on an inkjet printer  170 . The advantage of this image chain comes in terms of computational efficiency. Recall that the raster image processor  140  typically contains at least one multidimensional color transform in the form of an ICC profile, as described above. A gain in computational efficiency can be achieved by composing several multidimensional look-up tables together, as opposed to applying each multidimensional look-up table separately.  FIG. 9  shows a composed look-up table  130 , which is the combination of several multidimensional look-up tables. Multidimensional look-up table  100  provides the color transformation between the input color space, shown here as RGB, to PCS. The PCS used here is the CIE L*a*b* space, which has a luminance signal L*, and two chromatic signals a* and b*. Multidimensional look-up table  110  then converts the PCS data to CMYK. Then, the multidimensional look-up table  120  performs the protective ink processing, and outputs CMYKP. By combining these three tables into a single table, which takes RGB inputs and directly outputs CMYKP, a significant savings in processing time can be realized. 
   For each of the embodiments of the protective ink processor described above, once the code values representing the protective ink amount and the colored ink amounts have been generated according to the present invention, they are passed along to the multitone processor  50  and subsequently the inkjet printer  60  of  FIG. 1 . The inkjet printer  60  receives the multitoned image signal h(x,y,c), and deposits ink on the page at each pixel location according to the value of the multitoned image signal h(x,y,c) to produce the desired image. All of the pixels in the input digital image are sequentially processed through the image chain of  FIG. 1 , and sent to the inkjet printer  60 , which typically prints the pixels in a raster scanned fashion. 
   A computer program product can include one or more storage medium, for example; magnetic storage media such as magnetic disk (such as a floppy disk) or magnetic tape; optical storage media such as optical disk, optical tape, or machine readable bar code; solid-state electronic storage devices such as random access memory (RAM), or read-only memory (ROM); or any other physical device or media employed to store a computer program having instructions for controlling one or more computers to practice the method according to the present invention. 
   The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. In particular, the present invention has been described in the context of an inkjet printer, which prints with CMYK colorants, but in theory the invention should apply to other types of printing technologies also, as well as inkjet printers using different color inks other than CMYK. 
   PARTS LIST 
     10  raster image processor 
     20  digital image data source 
     30  protective ink processor 
     40  protective ink amount controller 
     50  multitone processor 
     60  inkjet printer 
     70  adder 
     80  protective ink amount generator 
     90  multidimensional look-up table 
     100  multidimensional look-up table 
     110  multidimensional look-up table 
     120  multidimensional look-up table 
     130  composed look-up table 
     140  raster image processor 
     150  digital image data source 
     160  multitone processor 
     170  inkjet printer