Abstract:
A digital color printing method and system is described which automatically detects when a neutral grey object is commanded to be printed, and changes the composition of the neutral color depending on the previous value of the neutral grey object and on the presence or absence of non-black colorants in the background color at the position on the page at which the neutral object is commanded to be printed. A single-color neutral object which is commanded to be printed at a position in which there is a previously rendered color background which contains non-black colorants may automatically have its neutral color changed from a single-color neutral to a process neutral containing at least one non-black colorant such as cyan, magenta, or yellow, said process neutral chosen to have equivalent visual density and neutrality as the previous single-color neutral value. Conversely, a process neutral grey object which is commanded to be printed at a position in which the background does not contain non-black colorants may automatically have the neutral color changed from process neutral to single-color neutral, said single-color neutral chosen to have the equivalent visual density as the process neutral. For many printing systems the resulting prints exhibit fewer visual defects. Neutral objects printed on top of a color field are printed as process neutral and thereby may exhibit a greater gloss and show fewer problems caused by the misregistration of separations or other printer defects, such as white fringes around the neutral objects. Conversely, neutral objects not on a color background are printed as single-color neutral objects and therefore do not exhibit objectionable process neutral problems such as color fringing, excessive colorant, and excessive gloss contrast with the background.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to digital color printing in which neutral grey objects and color objects are intermixedly printed, and more particularly, in which the composition of colorants of the neutral color for each neutral objects is automatically changed depending on the presence or absence of background color which contains non-black colorants. 
     2. Description of the Related Art 
     Digital color printers use a plurality of colorants, such as cyan, magenta, yellow, and black, to form images which appear to the human visual system to contain a multitude of colors. In particular, a neutral color may be formed in a number of ways. First, it may be formed of equal or nearly equal combinations of cyan, magenta, and yellow. In practice, the black colorant is also often added to the combination of cyan, magenta, and yellow colorants. When a neutral color is formed with a mixture of some of cyan, magenta, yellow, and black, it is a type of neutral known as process neutral. Second, a neutral color may also be formed by only using the black colorant. This type of neutral color is known as single-color neutral. 
     Each type of neutral, process neutral and single-color neutral, have advantages and disadvantages when used in different contexts in the printing process. 
     When single-color neutral is used over a color field, it is often the case that the single-color neutral colorant has a gloss which is different from the gloss of the surrounding color field, and the gloss difference can be objectionable. Also, the ink coverage or toner pile height of single-color neutral may be noticeably less than that of the surrounding color background. Finally, the presence of misregistration between the plurality of separations during the printing process, or the presence of other interactions between the separations, can result in objectionable white or light-colored fringing around a single-color neutral object on a color background. This occurs for example if the single-color neutral colorant is not printed precisely and without distortion into the hole left at that position when printing the other separations. 
     On the other hand, when process neutral is used in a white or grey field which does not contain any non-black colorants, slight misregistrations or interactions among the plurality of separations can result in objectionable color fringing at the edges of the object. Furthermore, the use of multiple colorants to form process neutral can result in blurred edges, thickened objects, and objectionable amounts of ink or toner coverage. 
     These advantages and disadvantages of single-color and process neutral are known in the graphics arts industry, and knowledgable graphic artists attempt to design pages which utilize the advantages and avoid the disadvantages. However, some situations, such as a neutral object placed only partially over a color field, or a color background which changes rapidly, or complex text or graphics over a complex color background, makes it difficult to manually avoid the printing problems associated with mixedly printing neutral colors and other colors. Furthermore, many pages not designed by a knowledgeable graphic artist can benefit from a method and apparatus to automatically create a background dependent neutral image. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a method for determining and changing the type of neutral color to print when such a change would increase the gloss of the neutral color to better conform to the gloss of a color background. 
     Another object of the present invention is to provide a method for determining and changing the type of neutral color to print when such a change would increase the total colorant coverage of the neutral color to better conform to the total colorant coverage of a color background. 
     Another object of the present invention is to provide a method for determining and changing the type of neutral color to print when such a change would decrease the objectionable printing defects caused by misregistration or interactions of the neutral color with a color background. 
     Another object of the present invention is to provide a method for determining and changing the type of neutral color to print when such a change would decrease the blurring, thickening, and color fringing at the edges of a neutral object when printed on a white background or a background with no non-black colorants. 
     Another object of the present invention is to provide a method for determining and changing the type of neutral color to print when such a change would decrease the total colorant coverage of a neutral object when printed on a white background or a background with no non-black colorants. 
     To achieve the foregoing and other objects and to overcome the shortcomings discussed above, a digital color printing method and system is provided which automatically detects when a neutral object is commanded to be printed, and changes the composition of colorants forming the neutral color depending on both the current value of the neutral color and the presence or absence of non-black colorants in the background color at the position on the page at which the neutral object is commanded to be printed. As objects are commanded to be printed, a detection operation detects if the objects are commanded to be printed in either a single-color neutral or a process neutral color. If so, a background determining operation is performed to determine a color which has previously been commanded to be printed at the location at which the neutral object is commanded to be printed. If the background color so determined is either white or contains no non-black colorants, and the type of neutral of the neutral object is process neutral, a color changing step is performed to change the process neutral to a single-color neutral which has the equivalent visual density. If the background determining operation determines that the background color contains non-black colorants, and the type of neutral of the neutral object is single-color neutral, a color changing step is performed to change the single-color neutral to a process neutral which has the equivalent visual density and maintains neutral color. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail in reference to the following drawings in which like reference numerals refer to like elements and wherein: 
     FIGS. 1-4 show various outputs produced by the prior art. 
     FIGS. 5-6 show various outputs produced by the method and system of this invention; 
     FIG. 7 is a block diagram of an apparatus made according to and for practicing the method of the invention, in which the changes to the color of neutral objects takes place within the print data generator. 
     FIG. 8 is a block diagram of an apparatus made according to and for practicing the method of the invention, in which the changes to the color of neutral objects takes place within the page information source. 
     FIG. 9 shows a flowchart for the overall method by which a document is changed to produce a background dependent neutral image. 
     FIG. 10 shows the method by which each object is processed before being merged into the output data. 
     FIG. 11 illustrates an output in which a neutral object is printed over a color boundary and is thereby printed as multiple partial objects, with each partial object having a different type of neutral. 
     FIG. 12 shows the method by which the determination is made to change the neutral color of a neutral object based on the current grey value of the neutral object and its background color. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, and particularly to FIG. 1 thereof, a neutral object is shown being imaged on top of a color field using only single-black colorant. As shown in FIG. 1, the type of neutral known as single-color neutral can be seen to have a gloss level and coverage level (toner height, in the example shown) which may be objectionably different from the color background. A further problem is shown in FIG. 2, which also shows a neutral object being imaged on top of a color field using a single-color type of neutral. In this case, an objectionable white fringe can occur around part of the neutral object due to misregistration of the neutral object in relation to the surrounding color field. 
     The opposite problems are shown in FIGS. 3 and 4. In FIG. 3, a neutral object is shown being imaged on top of a white paper background using the type of neutral known as process neutral, composed of combinations of cyan, magenta, yellow, and black toners. As shown in FIG. 3, in certain printing systems, objectionable height differences can occur between the white background and the total pile height of all toners used in process neutral. Other problems with excess colorant coverage not shown in FIG. 3 can also occur, such as excessive bleeding and spread caused by excess ink in certain ink jet color printing systems. FIG. 4 shows a further problem with using process neutral on a white field. In this case, misregistration between the color separations during printing can cause an objectionable color fringing, blurring, and widening of the edges of a neutral object on a white background. 
     FIG. 5 shows how the current invention solves the problems shown in FIGS. 1 and 2 and other problems not shown which can occur when a single-color neutral object is printed on a color field. By detecting single-color neutral objects and changing the single-color neutral to the equivalent process neutral when it is being printed on a color field, the color remains neutral but the problems of insufficient gloss and colorant coverage, and the problem of white fringes due to misregistration or other interaction between separations are solved. 
     FIG. 6 shows how the current invention solves the problems shown in FIGS. 3 and 4 and other problems not shown which can occur when a process neutral object is printed on a white or single-color grey background. By changing the process neutral object to a single-color neutral object, the color remains neutral but the problems of excessive colorant coverage, and the problem of color fringes due to misregistration or other interaction between separations are solved. 
     Referring now to FIG. 7, a generalized page printing system made according to the present invention is shown. It includes a page information source  100  connected via communication link  150  to a print data generator  200 . Generator  200  is connected to a printer  300  via a communication link  250 . Generator  200  includes an I/O controller  210 , a CPU (central processing unit)  220 , a RAM  230  which can store program and data information, and a storage apparatus  240  for non-volatile storage These devices  210 - 240  may be any of a number of conventional devices generally available. Other hardware that accomplish the same functions can be used. As will be seen, various embodiments are realizable from this general structure. 
     Typically, page information source  100  may be a conventional work station or other computer system such as an IBM PC or Apple Macintosh. Communication link  150  may be a part of a computer network or a dedicated link. Print data generator  200  is usually a dedicated computer attached to a printer  300  such as one of many electromechanical devices responsive to raster data for producing a printed page. Many other configurations are possible. For example, generator  200 , programmed to function as described herein, could also be incorporated together in the same computer running the page source  100  software. In this case of shared computer hardware and separate software, the functions of page information source  100  and print data generator  200  remain distinct. The methods as described herein remain applicable through a wide range of apparatus configurations. 
     FIG. 7 shows an embodiment in which the print data generator  200  is programmed to carry out the methods described herein and shown in FIGS. 9-12. In this case, a page representation is received from conventional page source  100 . The page representation may be a conventional page description language such as the language available from Adobe Systems Incorporated known as PostScript, or an equivalent. In the page representation received from the page source  100 , objects such as text, graphics, or pictures can be created and placed on the page in arbitrary order. The objects are defined by descriptive commands, some of which control the location, shape, orientation, and size of an object. At least one command controls the color of an object. An object&#39;s color may include neutral colors as well. Objects may overlap each other, and a priority method determines which objects are on top of other objects. 
     As further discussed herein, when a page representation is received from a conventional page source  100  by print data generator  200  and generator  200  is programmed to carry out the method of this invention, generator  200  may in certain situations modify the output commanded by the page representation. As described herein, generator  200  may change the type of the color of a neutral object contrary to the command of the page representation before merging it into the generated print data. Thus, the page as sent by a conventional page information source  100  will be printed differently than specified, in order to accomplish the objectives of this invention for improving print quality. The advantage of this configuration is that page representations received from any number and variety of conventional page sources  100  may be automatically modified by generator  200  to print with higher quality. 
     In FIG. 8, a different apparatus and configuration is shown. In this case, the page information source  100  includes resources such as I/O controller  110 , CPU  120 , RAM  130 , and storage  140  which allow page source  100  to be programmed to carry out the methods of the invention described herein. In this apparatus, before a page representation is sent via communication link  150  to a conventional print data generator  200 , the methods of this invention are applied in order to produce a page representation which achieves the objectives of this invention by commanding a background dependent neutral image. In this configuration, the print data generator  200  may be conventional and the page information source  100  is programmed according to the methods of this invention. The advantage of this configuration of apparatus is that the page representation, which has been modified according to this invention to command a background dependent neutral image, will be printed with higher quality by any conventional print data generator  200  and printer  300 . 
     FIG. 9 illustrates a generalized flow diagram of a process or method according to the invention by which a background dependent neutral image is formed. After starting in step S 10 , in step S 20  a page representation is received. This page representation represents a collection of colored objects in a data format such as a conventional page description language or equivalent data format as previously described. 
     In step S 30 , the next object which has not yet been processed is retrieved from the page representation. This retrieved object is further processed in step S 40  as will later be described in greater detail. In step S 50 , it is determined if this is the last object to be retrieved from the page description language. If it is not the last object, then control returns to step S 30  where the next object is retrieved. Otherwise, if this is the last object defined by the page description language, control flows to step S 60  where the document, which has possibly been changed during the object processing step S 40 , is output. 
     The form of the outputted changed document of step S 60  depends on where the changes according to the methods of this invention have been made. For example, in the apparatus as described in FIG. 8, in which the methods of this invention are implemented within the page information source  100 , the outputted changed document can be in the form of a modified page description language or equivalent internal data structure which commands a background dependent neutral image. This page description language would, if sent to a conventional printing system, print an image whose neutral objects are dependent on the background color in accordance with the objects of this invention. In this case, the modifications would take the form of substituting existing neutral color commands for new neutral color commands which command the correct type of neutral. 
     Alternatively, as for example in the apparatus as described in FIG. 7, in which the methods of this invention are implemented within the print data generator  200 , the output step of step S 60  can be in the form of a printable page data structure such as a set of bitmaps, pixel maps, or other intermediate data structure which is in a form acceptable for use by a raster color printer. In this case, the output step S 60  involves outputting the modified print data to a network, a printer or a storage device. The output step S 60  could also be a hybrid output consisting for example of a page description language which incorporates a printable data structure such as a byte map. For the purposes of this invention, the output for the changed document could assume any number of equivalent forms. Finally, control flows to step S 70 , where the process ends. 
     FIG. 10 shows in greater detail the step S 40  of processing an object. In step S 405 , the color of the object to be processed is transformed. In many page description languages, the color of an object can be expressed in many color spaces or systems such as calibrated RGB or L*a*b*. For most efficient processing, the preferred embodiment of this invention transforms these variety of color spaces to only two color spaces: either a single-color grey value represented by values of K (black) only, or a process color space such as C, M, Y, without yet a calculated K value. The generation of a K separation later from C, M, Y input values is know in the art as grey component removal (GCR) or under-color removal (UCR), but is not done at this time. The C, M, Y values are also not yet adjusted through output tone reproduction curves (TRCs), so that it is true at this point, after the transformation step S 405 , that C=M=Y indicates a neutral color. Thus, in step S 410 , it is relatively simple to examine the color of the current object to determine if the object&#39;s color is neutral. Any object whose color is expressed solely in values of K (black) is a neutral color object, and any object whose color is expressed in values of C, M, and Y, where C=M=Y is also a neutral color object. 
     If in step S 410  the object color is not neutral, control flows immediately to step S 470 , in which the object is merged into the output page data. This output page data can take many forms depending on the particular printing system within which this invention is embodied. The output page data, at each moment, represents the collection of objects already processed whose color need not be further modified by the method of this invention, collectively represented in an intermediate form which is preparatory to the already described document outputting step of step S 60 . For example, the output page data could be in the form of a pixel map containing a raster of color data for each pixel on the page to be printed, or an intermediate form such as a display list describing the ordered list of colored objects for each scan line in the final page. The merging of an object into such an output page data is a conventional step which involves adding the object to the intermediate form output page data in such a way that it will be printed in the correct location, with the correct shape, with the correct relation to other objects on the page, and with the correct color. In the preferred embodiment, merged objects have their colors expressed in the CMYK color space. In this case, therefore, part of the merging step is to perform GCR or UCR to generate the K separation from the CMY values, and to use output tone reproduction curves as explained previously to adjust the CMYK values in order to maintain a system which is calibrated despite changes to the environment, materials, ink, paper, or printing mechanisms. 
     Returning to step S 410 , if the object color is found to be neutral, the processing proceeds to step S 420  where the background color which is beneath the object is obtained. If the output page data referred to in step S 470  consists of a pixel map, ie, a raster of pixels for each printable location on the page, then the step of getting the background color beneath the object consists of reading the set of pixels found within the shape defined by the neutral object currently being considered. If the output page data consists instead of a display list such as a list of objects per scan line, ordered by position on each scan line and with information controlling the positional layer of each object, then the step of obtaining the background color consists of finding the object or objects already processed whose position along the scan line determines that they would be beneath the currently considered neutral object, and obtaining the color of said objects. If the object consists of shape information such as a bitmap which defines the on and off portions of the image, and color information which assigns a color to the on bits (foreground) and off bits (background) of the object, then the background color is determined by examining the color associated with the off bits of the shape-defining bitmap. 
     In any of the above cases, if an object is found to have two or more background colors within its shape, the control may flow to step S 440 , where the object is split into multiple objects whose edge boundaries follow the edge boundaries of the multiple background colors. The result of this splitting step is that each object or partial object contains one background color which may be analyzed to determine whether the type of neutral currently being used is correct. Thus as shown in FIG. 11, the type of neutral for an object which extends over the boundary of a color field may be made to change within the boundaries of the object at the edge of the color background beneath the object. The partial object which is over the color field may be imaged using the process neutral type, and the partial object which is for example over white paper may be imaged using the single-color neutral type. 
     Returning to FIG. 10, from step S 440  control flows to step S 450 , where the next partial object from the list of partial objects generated in splitting step S 440  is chosen for further processing. 
     It will be appreciated that if it is determined that the splitting step S 440  is not efficient to perform and it is preferable to treat objects as whole objects regardless of transitions of background color beneath an object, said steps S 430 , S 440 , and S 450  need not be performed. In this case, it is clearly possible to choose one of multiple background colors beneath a whole neutral object and use that background color as the representative background color for the entire object in performing the steps S 460  and following. 
     In step S 460 , it is determined according to the method of the invention whether it is necessary to change the type of neutral color of the object or partial object currently being processed, and if it is determined to be necessary, the composition of colorants used to color the object or partial object is changed to correspond to the appropriate one of single-color neutral or process neutral while maintaining the commanded grey value. Step S 460  will be explained in further detail later. 
     Then, control flows to step S 470  where, as previously described, the processed object or partial object is merged into the output page data in such a way that it will be printed in the correct location, with the correct shape, with the correct relation to other objects on the page, and with the correct color which has possibly been changed in previous step S 460 . After merging, it is determined in step S 480  whether this is the last partial object of the partial object list which was generated in step S 440 . If this is the last partial object, or if the splitting steps S 430 -S 450  were not performed but rather whole objects are being processed, then control flows to step S 50 , which was previously described. If this is not the last partial object in the list of partial objects generated in step S 440 , control returns to step S 450 , where the next partial object is fetched and processing on that partial object continues with steps S 460  and following, as previously described. 
     FIG. 12 shows in greater detail the step S 460  of changing the type of neutral color if required. In step S 461 , the background color or section thereof which was fetched in step S 420  and possibly split into multiple sections by steps S 430 -S 450  is analyzed to determine if it contains any non-black colorant. As previously mentioned, in the preferred embodiment, by the time an object is merged into the output page data in step S 470 , its color coordinates will have been converted to match the color separations of the output device, most generally CMYK (cyan, magenta, yellow, black). In this case, the background color as fetched from the output page data containing objects which have been previously processed for printing will have its color expressed in CMYK coordinates. Therefore, the simple test of C=M=Y=O will determine whether the background contains any non-black (C, M, or Y) components. If, for certain output devices, the color is maintained in a different color space, it is still possible to efficiently determine if the background color is neutral, as described previously. In these cases of color spaces other than CMYK, however, additional system information is needed to determine how such colors will be rendered in CMYK, since the concepts of two types of neutral (process neutral or single-color neutral) only have meaning in the CMYK or similar process space. In the preferred embodiment, therefore, in order to avoid the necessity of additional tagged information with each color, it is preferable that the color of objects be transformed into CMYK as part of the merging step S 470 . In this way, the test in step S 461  of analyzing the background color for non-black colorants is reduced to the simple test of C=M =Y =0. 
     If in step S 461  the background color is found to contain non-black colorants, then in step S 462  it is determined if the neutral colored object being currently processed is a single-color neutral, ie, its color expressed using only K (black) values where K&gt;0. If it is a single-color neutral, control passes to step S 463 , where the single-color neutral is changed to a process neutral. At this point in the process, it is very important to ensure that the process neutral will be visually equivalent in its grey value to the single-color neutral, and that the non-black colorant values are correct so that they do not skew the process neutral off of the neutral grey axis. The process neutral must appear grey, without the appearance of slight coloration in the direction of one of the process colorants, as can often occur if too much or too little of any of the colorants is used. The preferred embodiment for this step is to use a one-dimensional lookup table. A one-dimensional lookup table is relatively small, using few system resources, and provides an efficient way to determine substitute values. The lookup table must be previously filled in with entries for each possible value of single-color grey at a certain step-size. For each such value of single-color grey, values of C, M, and Yor similar process colors are experimentally determined which produce neutral outputs with the same density as the input single-color neutral. For maximum usefulness over time, since color printing systems can drift in their output values according to environmental, ink, material, and machine changes, the lookup table values are expressed as process color values such as CMY, and may later be adjusted by adding a K (black) value through the previously discussed GCR/UCR methods and by further using output tone reproduction curves (TRCs) for each process value. In this way, the output TRCs may be periodically calibrated to maintain the original neutrality and density value output swhich were measured at the time the lookup tables were established. Other calibration methods known in the art may also be used to adjust the values stored in the lookup table. Thus, at step  463 , a previously established lookup table is accessed to substitute process color values for the single-color black value of the current neutral object. Now that the color has been changed to a process neutral, the current object being processed may be merged, in step S 470 , into the output page data so as to print in the correct location, with the correct shape, with the correct relation to other objects on the page, and with the corrected neutral color. If, in step S 462  the neutral object being processed is determined not to be a single-color neutral object, then the color need not be changed and processing may pass immediately to merging step S 470 . 
     Returning to step S 461 , if the background color is not found to contain non-black colorants, ie, it is a background composed of white paper or black colorant only, then in step S 464  it is again determined if the object being processed is a single-color neutral object, using the same test mentioned above. If it is determined that it is not a single-color neutral object, then in step S 465  the color of the object is changed to be a single-color neutral object. The preferred embodiment for this step is to access another one-dimensional lookup table. This one-dimensional lookup table has entries which correspond to the single C, M, Y value that defines each process neutral density level at a certain step-size. As described above in step S 405 , each process neutral object will be described by a single value of C, M, and Y, where C=M=Y. By using these single values as input to a one-dimensional lookup table, the value of K which has experimentally been determined to produce a single-color neutral with the same density value may be accessed. Again, this lookup table will have been established previously experimentally. As described above, for maximum usefulness over time, since color printing systems can drift in their output values according to environmental, ink, material, and machine changes, the lookup table output values are best expressed as single-color K values prior to being adjusted according to an output tone reproduction curve (TRC) that modifies the K output. In this way, the K output tone reproduction curve (TRC) may be periodically calibrated to maintain over time the original density value outputs which were measured at the time the lookup tables were established. Other K calibration methods known in the art may also be substituted. If, in step S 464  it is determined that the current neutral object is already a single color neutral, then no change is needed and control passes immediately to merging step S 470  as described above. 
     It will be appreciated that as an alternative embodiment which may be more efficient for some computing apparatus and output page data, after the test for non-black colorants in the background color in step S 461  the correct form of neutral color could be set for each neutral object without testing in steps S 462  and S 465  whether the neutral color is already correct. Thus, every neutral object would have its color set to the correct type whether it was previously already correct or not. This method is equivalent to the method above described and avoids the testing steps S 462  and S 465  but uses the setting steps S 463  and S 465  for each neutral object. 
     While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.