Abstract:
One-dimensional color transforms are automatically calculated by identifying at least one device-independent color space curve, at least one device-dependent color space curve and at least one association amongst the curves. Depending on the motivation for creating the one-dimensional transform, different curves, associations and calculations can be used to generate the transforms without obtaining additional device measurements or iteratively adjusting transform values.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Reference is made to commonly-assigned copending U.S. patent application Ser. No. 12/014,821, filed Jan. 16, 2008, entitled SIMPLIFIED COLOR WORKFLOW, by Fowler et al., the disclosure of which is incorporated herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention pertains to establishing one-dimensional color transforms to adjust the color response of a device. 
       BACKGROUND OF THE INVENTION 
       [0003]    Color transforms modify color data delivered to a reproduction device to modify a nominal response of the reproduction device. Exemplary reasons for doing so include: establishing an intended tonal response for one or more color channels, establishing an intended reproduction of selected colors, and establishing a device response that simulates or matches a different device condition. 
         [0004]    Color transforms can be one-dimensional, as exemplified by tonal correction transforms, which transform values for a single color channel (e.g. C 1 →C 2 ) based on a one-dimensional curve or LUT. Color transforms can also be multi-dimensional, as exemplified by color matching profiles, which transform color coordinate vectors comprising values for multiple color channels (e.g. [C 1 ,M 1 ,Y 1 ,K 1 ]→[C 2 ,M 2 ,Y 2 ,K 2 ]) based on a multi-dimensional curve or LUT. In some cases, multiple sets of transforms can be successively applied for different purposes, as described in U.S. patent application Ser. No. 12/014,821. 
         [0005]    Color transforms can be automatically generated using tools that measure or otherwise acquire information about the nominal and intended response of the reproduction device. However, these methods can be time consuming and may require the measurement of multiple reproductions produced by a device. Multi-dimensional transforms are also conceptually complex and typically require computation of a significant number of data points to yield desirable results. 
         [0006]    One-dimensional transforms, in contrast, are conceptually simpler and computationally easier to produce but can still be time consuming to produce if reproduction measurements must be made. In addition, changes to one-dimensional transforms can affect the color response of a device so that it no longer matches an intended color response. For example, adjusting a tone reproduction curve for one color channel of a device may adversely affect the reproduction of neutral colors by the device by introducing a color cast or by changing the uniformity of a neutral color ramp. 
         [0007]    In some situations, such as during a press run, the actual device response may appear different than intended. This may require an immediate adjustment that precludes the use of measurements to produce the desired transform. For example, a green cast may appear in the neutral color ramp (i.e. the device coordinates that are supposed to produce perceived neutral colors). As another example, reproduction of a neutral color ramp may be compressed in one region so that the perceived gradation in neutral colors reproduced is not as expected (e.g. uniform throughout the ramp). 
         [0008]    If time is of the essence, manual creation or editing of existing transforms may be the only practical method for correcting perceived mismatches between intended and actual device response. Manual adjustment of multi-dimensional transforms is simply too complex to be reliably done. Instead, manual adjustment of one-dimensional transforms is typically the preferred method for handling such problems. Of course, there may be other situations when manual editing of one-dimensional curves is desired, when timeliness is not the prime motive but rather the problem to be resolved is most readily solved by editing one-dimensional transforms. 
         [0009]    However, manually adjusting one or more one-dimensional transforms may affect the overall color response of the device and thus require additional iterations of adjustments to compensate. In many cases, the complexity and/or time required to achieve the ideal adjustment is too great and a compromise is made. For example, a color cast may be reduced but not entirely removed. As another example, a cast may be removed but neutral gradation uniformity is sacrificed. As another example, neutral gradation uniformity is improved but a color cast is introduced. As another example, a color cast may be reduced in one region but a different cast is introduced in another region. 
         [0010]    Automated methods, such as those disclosed in the related application, can predict the impact to color response from changes to one-dimensional transforms. This information can be used to guide a user to iteratively make adjustments that minimize the undesirable effects of the adjustment without making reproductions. However, the number of iterations may require excessive time. Thus, a means for quickly creating and/or editing one-dimensional color transforms to effect a change in a nominal device response is required, without iterative adjustments or additional device response measurements. 
         [0011]    As indicated above, color transforms are also useful for modifying data so that one device condition can emulate the response of another device condition. As an example, image data intended for a first device condition, characterized by a smaller gamut, can be modified for a second device condition, characterized by a wider gamut. An emulation goal can be to match color amongst reproductions made by both device conditions. In this case, multi-dimensional transforms, such as a device link, can be created. 
         [0012]    An alternative goal can be to make the images similar but take advantage of the wider color gamut (e.g. richer saturated colors). In this case, a device link can also be created but a set of one-dimensional transforms may be preferred for a number of reasons. For example, one-dimensional transforms are easier to comprehend, easier to compute and easier to edit. As another example, one-dimensional curves are guaranteed to map the surface of one gamut to another while the use of interpolation in processing multi-dimensional transforms may cause certain portions of one gamut surface to map to the interior of the other gamut. 
         [0013]    Prior art methods exist for device condition emulation by creating one-dimensional transforms, but these methods typically involve iteratively adjusting data used to create the transforms to minimize color errors (e.g. in the interior of the gamut) for a selection of device coordinates. Thus, a similar need exists to create one-dimensional emulation transforms, without the need for iterative adjustments or additional device response measurements. 
       SUMMARY OF THE INVENTION 
       [0014]    Briefly, according to one aspect of the present invention a system and method for establishing one-dimensional transforms using a non-iterative approach based on existing device response information. In particular, these transforms are established using a computerized system. Device independent color space (DIC) curves, obtainable through device response information, are used as the basis of establishing these transforms. One-dimensional transforms for selected colors are identified by first establishing a motive for the transform and then identifying an association between at least one DIC curve and at least one device-dependent color space (DDC) curve. 
         [0015]    According to one aspect of the invention, a nominal set of one-dimensional transforms for a reproduction device can be adjusted to compensate for an undesirable color characteristic of the device. A DIC curve can be selected as the basis of the adjustment. In one preferred embodiment, a neutral DIC curve can be selected as the basis for evaluating the color characteristic and adjusting to compensate. Although device response information predicts what device coordinates make neutral colors, the actual colors visible in a reproduction may not appear correct. A point on the DIC curve, such as one having the greatest variance from the intended color, can be selected for adjustment and an adjustment vector, specifying a change in color (e.g. lightness and/or chromaticity) for the adjustment point, can be specified. An extent of the DIC curve can be determined relative to the adjustment point to determine a range of influence for the color adjustment. 
         [0016]    A corresponding DDC curve can be identified based on the selected DIC curve and an expected model of the device response. A set of DDC adjustment vectors can be computed based on the DIC curve&#39;s adjustment point, vector, and extent. An adjusted DDC curve can be identified and the original and adjusted DDC curves can be projected onto each DDC axis to determine one-dimensional transforms corresponding to the specified DIC color adjustment. The DDC one-dimensional transforms thus approximate the desired DIC color adjustment for a range of influence. 
         [0017]    According to another aspect of the invention, a set of one-dimensional transforms are created so that a destination device condition can emulate a source device condition. A pair of DIC curves can be selected as the basis of the emulation. In one preferred embodiment, neutral DIC curves associated with both device conditions are selected. A set of points on the pair of DIC curves can be associated with each other. For each pair of associated DIC points, corresponding DDC points are computed and used to plot points on associated one-dimensional transforms. For example, neutral source DDC point  1  (S 1 ) and neutral destination DDC point  1  (D 1 ) can be associated (i.e. [C S1 , M S1 , Y S1 ]⇄[C D1 , M D1 , Y D1 ]), so that a point plotted on the cyan one-dimensional transform identifies an output tint of C D1  for an input tint of C S1 . 
         [0018]    Accordingly, the basis colors produced by the destination device will be similar to those that can be produced by the source device and the variation in basis colors will appear to be uniformly spaced even though corresponding points may be calorimetrically different. When those basis colors are neutral, the neutral tone ramp produced by the destination device will appear similar to the one produced by the source device. 
         [0019]    For special device colors, one-dimensional emulation transforms can be determined by identifying DIC curves for the tonal range of each special color in both source and destination space. One-dimensional emulation transforms can be created as above by associating a set of DIC curve points and plotting information derived from the corresponding DDC color values on the one-dimensional transform for that color. 
         [0020]    When emulating source color by a destination device using one-dimensional transforms, all of the colors on the surface of the source gamut will map to colors on the surface of the destination gamut. Further, when the neutral DIC curves are selected as the basis of emulating chromatic process colors, neutral color ramps will appear similar. Since human perception of color difference is very acute for neutral colors, this emulation method can be a simple but effective gamut mapping method. 
         [0021]    These and other aspects of the present invention are illustrated in the detailed description of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a diagram illustrating an exemplary arrangement of color transforms for a reproduction device condition according to the present invention. 
           [0023]      FIGS. 2A-2D  are diagrams illustrating an exemplary set of DIC curves for an exemplary set of device conditions. 
           [0024]      FIG. 3  is a diagram illustrating an exemplary DDC curve corresponding to DIC curve of  FIG. 2A . 
           [0025]      FIG. 4  is a diagram illustrating an exemplary user interface for defining a desired adjustment to a nominal device response according to the present invention. 
           [0026]      FIG. 5  is a diagram illustrating an exemplary undesirable DIC curve according to the present invention. 
           [0027]      FIGS. 6A and 6B  are diagrams illustrating exemplary adjustments made to a DDC curve according to the present invention. 
           [0028]      FIG. 7  is a diagram illustrating exemplary adjustments made to one-dimensional transforms based on an adjusted DDC curve according to the present invention. 
           [0029]      FIG. 8  is a diagram illustrating an exemplary association between source and destination process color coordinates based on DIC curves according to the present invention. 
           [0030]      FIG. 9  is a diagram illustrating an exemplary set of one-dimensional transforms for mapping source process color coordinates to destination process color coordinates according to the present invention. 
           [0031]      FIG. 10  is a diagram illustrating an exemplary association between source and destination DIC curves, derived from single color tone ramps, according to the present invention. 
           [0032]      FIG. 11  is a diagram illustrating an exemplary one-dimensional transform  99 C for mapping source tint values to destination tint values for a single color according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]      FIG. 1  is a diagram illustrating an exemplary arrangement of color transforms ( 105 A- 105 E,  106 , and  108 A- 108 E) for a reproduction device condition according to the present invention. Reproduction device  101  can receive image data  130  and operates to produce a reproduction of an image. Device  101  has an intrinsic device response for a specific device condition which includes a basic tonal response  102  and basic color response  103 . The device response can be identified, for example, by providing known image data  130  including test patches, and measuring portions of the resulting reproduction. 
         [0034]    A set of one-dimensional color transforms  105 A- 105 E can be used to modify image data  120  to produce image data  130  which is then supplied to device  101  so that a corrected tonal response  112  can be achieved for tonally corrected device  111 . Transforms  105 A- 105 E can affect the device&#39;s color response which can be measured as tonally corrected color response  113 . 
         [0035]    Similarly, a multi-dimensional color transform  106  can be used to modify image data  110  to produce image data  120  so that a color corrected color response  123  can be achieved for color corrected device  121 . 
         [0036]    Another set of one-dimensional color transforms  108 A- 108 E can be used to modify image data  100  to produce image data  110  for color corrected device  121  so that color corrected device  121  can achieve some other device response. As an example, transforms  108 A- 108 E can be used to make color corrected device  121  emulate some other device condition. 
         [0037]    Transforms  105 A- 105 E,  106 , and  108 A- 108 E can be computed by one or more computer systems and utilized by the same or different computer systems to modify image data  100 ,  110 , and  120  prior to being received by device  101 . The example of  FIG. 1  represents one typical example of the use of color transforms, and in particular one-dimensional color transforms (e.g.  105 A- 105 E and  108 A- 108 E) that are the subject of the present invention. Of course, many other configurations and uses of one-dimensional transforms are possible. 
         [0038]      FIGS. 2A-2D  are diagrams illustrating an exemplary set of DIC curves for an exemplary set of device conditions.  FIG. 2A  illustrates a neutral tone DIC curve  10 A, plotted in CIELAB coordinates, for a first exemplary device condition. The device condition, for example, may represent an offset printing press operating with standard characteristics. 
         [0039]    DIC curve  10 A represents a series of points starting with an exemplary DIC light point  11  representing the color perceived when no inks have been deposited. DIC light point  11  does not lie on the neutral axis. Rather, its coordinates include a value of zero for b* and a slightly negative value for a*. This may represent a slight color cast in the printing stock. 
         [0040]    DIC dark point  12  represents an exemplary perceived color where maximum ink deposition has occurred. Coordinates for DIC dark point  12  are also off the neutral axis and include slightly positive values for both a* and b*. This may represent a slight color cast in the combination of inks. 
         [0041]    However, the majority of points on neutral DIC curve  10 A lie near the neutral axis (e.g. quarter-tone point  14 , mid-tone point  13 A, and three-quarter tone point  15 ) and others approach the neutral axis from the DIC light point  11  and DIC dark point  12 . Colors on DIC curve  10 A may be generated by a particular set of DDC coordinates on a neutral DDC curve which can be established through a variety of means. One method of establishing the neutral DDC curve may be to use a reverse color model to find DDC coordinates based on a neutral DIC curve. Another exemplary method of establishing the neutral DDC curve can be to follow a particular reproduction standard (e.g. FOGRA) and to first create color transforms (e.g.  105 A- 105 E and  106 ) which enable the DDC coordinates on the standard neutral DDC curve to produce near-neutral coordinates in DIC. 
         [0042]      FIG. 2B  illustrates a neutral tone DIC curve  30  for a second exemplary device condition. This device condition, for example, may represent a newspaper printing press operating with standard characteristics. Note that in comparison to the offset press of  FIG. 2A , the newspaper press of  FIG. 2B  has a different neutral tone DIC curve. In particular, light point  31  has a lower L* value than DIC light point  11  and has positive values for both a* and b*. Similarly, dark point  32  has a higher L* value than DIC dark point  12  and negative values for both a* and b*. The majority of points on neutral tone DIC curve  30  lie near the neutral axis (e.g. mid-tone point  13 A). 
         [0043]    Note that many other types of DIC curves can be identified for use with the present invention. However, experience suggests that neutral tone curves may be preferable due to human sensitivity to these colors. As an example, a DIC curve representing typical flesh tones may be used. As another example, a DIC curve representing a range of colors at constant L* may be used. In general, a DIC curve representing a smooth transition of DIC colors that is important to a particular reproduction can be selected. 
         [0044]      FIGS. 2C and 2D  illustrate exemplary undesirable neutral DIC curves. For example, the neutral DDC curve that is supposed to correspond to neutral DIC curve  10 A in fact may correspond with curves  10 C or  10 D instead. 
         [0045]    If DIC curve  10 C is produced, the mid-tone colors reproduced are in fact neutral but have a non-uniform distribution. For example, mid-tone point  13 C is lighter than expected. In particular, mid-tone point  13 C needs to be increased by lightness adjustment  16 C to match mid-tone point  13 A. Other mid-tone points on DIC curve  10 C likely are likely also in need of adjustment so that steps in the mid-tones appear to have a non-uniform distribution. Quarter-tone neutral point  14  and three-quarter tone point  15  are depicted as being approximately correct neutral color and lightness. 
         [0046]    If DIC curve  10 D is produced, the mid-tone colors have a color cast instead of being neutral. For example, mid-tone color  13 D is depicted as having a distinct color cast (e.g. positive a* and negative b* values). In addition, other mid-tone points on DIC curve  10 D are depicted as also requiring adjustment. 
         [0047]      FIG. 3  is a diagram illustrating an exemplary DDC curve  20 A corresponding to DIC curve  10 A of  FIG. 2A . For illustrative clarity only, the exemplary device condition includes only chromatic process colors: cyan, magenta, and yellow so that a simple DDC geometry can be drawn. For some reproduction devices, other chromatic process colors (e.g. red, green, and blue) can be used. For some devices, more than three process colors may be used. For example, black, which is achromatic, can be used. As another example, additional chromatic process colors (e.g. orange, red, and blue) can be used in some printing devices. When additional colors are used, the DDC curve traverses an N-dimensional geometry. 
         [0048]    DDC light point  21  reproduces DIC light point  11  and represents no ink deposition. DDC dark point  22  reproduces DIC dark point  12  and represents maximum color. For some printing device conditions, especially those with additional inks, DDC dark point  22  may be governed by ink-limiting rules to be less than 100% of one or more inks. Points  23 A,  24 A, and  25 A respectively reproduce points  13 A,  14 , and  15 . 
         [0049]    In other words, reproducing an image with DDC coordinates from DDC curve  20 A should result in an image having colors corresponding to associated points on DIC curve  10 A. The association between these points can be determined based on forward and/or reverse models derived, for example, from measuring reproduced colors. However, in some circumstances, when the nominal device response is not as expected, reproduction colors may be more consistent with curve  10 C,  10 D, or some other deviation from curve  10 A. 
         [0050]      FIG. 4  is a diagram illustrating an exemplary user interface  40  for specifying a desired adjustment to a nominal device response according to the present invention. Such an interface can be useful, for example, when a sample reproduction exists which includes nominal reproductions of a number of points from a DDC curve (e.g. DDC curve  20 A). 
         [0051]    As one alternative, such an interface may also be used without benefit of a reproduction to guide the adjustment. In this case the user simply needs a goal for modifying an expected device response. For example, the user may believe that the neutral DIC curve  10 A can be achieved on a device with a particular paper stock but wants to operate the device with a new colored paper stock. If the device response for this altered device condition has not yet been calculated, the user may wish to adjust the existing device response based on his perception of the color difference in paper stocks. 
         [0052]    User interface  40  includes a mode selector which can be used to select whether to adjust individual one-dimensional color transforms or to adjustment a set of those based on a DIC curve. Button  41 , for example, is depicted as being depressed to select a mode where cyan, magenta and yellow one-dimensional color transforms are adjusted together based on DIC curve  10 A. Since button  41  was pressed, areas  42  and  43  are presented. Area  42  presents the computed adjustments to one-dimensional transforms. Area  43  presents the user with controls for specifying an adjustment and visualizing the adjustment. Specifying an adjustment is described with reference to  FIG. 5 . 
         [0053]      FIG. 5  is a diagram illustrating an exemplary undesirable neutral DIC curve  10 B according to the present invention. In other words, if a sample reproduction of the coordinates on DDC curve  20 A were measured, the measurements would produce DIC curve  10 B. The section of DIC curve  10 B between quarter tone point  14  and three-quarter tone point  15  does not include neutral colors with a uniform distribution of lightness as desired (i.e. DIC curve  10 A). At a mid-tone point  13 B, an adjustment  16 B, comprising a lightness adjustment  17  and a color cast adjustment  18  is required to compensate (e.g. adjusts mid-tone point  13 B to reach mid-tone point  13 A). Note that DIC mid-tone point  13 A, quarter tone point  14 , and three-quarter tone point  15  correspond to DDC points  23 A,  24 A, and  25 A of  FIG. 3 . 
         [0054]    It is important to note, however, that measurements of the neutral DIC curve (e.g.  10 B) likely do not exist. Rather, the expected neutral DIC curve (e.g.  10 A) is what has been previously measured and/or computed for corresponding DDC curve  20 A. Thus, DIC curve  10 B represents what color deviations that a user perceives in a sample reproduction. As one alternative, DIC curve  10 B may represent a perceived deviation from some goal that is different than the previously determined neutral DIC curve  10 A. 
         [0055]    A user can specify an adjustment by first selecting a region of DIC curve  10 A using control  44 . Control  44 , for example, can include a set of buttons specifying adjustment regions in relation to DIC curve  10 A and corresponding DDC curve  20 A.  FIG. 4  depicts the mid-tone button being depressed which corresponds to selection of DIC mid-tone point  13 A. Mid-tone point  13 A can be pre-defined or can be computed based on some characteristic of, for example, DIC curve  10 A or DDC curve  20 A. Mid-tone point  13 A cam also be specified explicitly as one or more DDC or DIC coordinates. 
         [0056]    In the examples of  FIGS. 4 and 5 , specifying an adjustment point involves implicitly specifying an adjustment extent for DIC curve  10 A. The adjustment extent bounds the range of DDC coordinates that are subject to adjustment in the one-dimensional transforms. Quarter tone point  14  and three-quarter tone point  15  are implicitly selected as endpoints of the adjustment extent in this example. Various means for selecting these points can be used. As example, extent endpoints can be pre-defined in relation to mid-tone point  13 A (e.g. 10 L* units darker and lighter than mid-tone point  13 A). As another example, the user can dynamically specify the endpoints in terms of one or more DDC or DIC coordinates. 
         [0057]    Desired adjustment  16 B for adjustment mid-tone point  13 A, can then be input by the user via controls  45 - 47 . Visual feedback on the estimated effect of adjustment  16 B at mid-tone point  13 A can be presented in before patch  48  and after patch  49 . 
         [0058]    In one embodiment, after patch  49  can present the expected color corresponding to mid-tone point  13 A. This appears in  FIG. 4  as a neutral gray color. In the same embodiment, before patch  48  can present the color representing mid-tone point  13 A with the opposite of adjustment  16 B applied (i.e. point  13 B). On a color monitor with reasonable color accuracy, feedback from before patch  48  may be used to assist the user in accurately operating controls  45 - 47  by matching the color of before patch  48  with a sample. 
         [0059]    In another embodiment, before patch  48  can present the expected color corresponding to mid-tone point  13 A while after patch  49  can present the color determined from mid-tone point  13 A with adjustment  16 B applied. This can be useful, for example, in adjusting color to meet a goal that is different than the expected device response. 
         [0060]    In another embodiment, before and after patches  48  and  49  can be configured to present a relative color difference corresponding to adjustment  16 B with additional controls for establishing the absolute color presented by either before patch  48  or after patch  49 . This can be useful, for example, when a display providing user interface  40  is not accurately calibrated and the user is attempting to visually represent a reference color (e.g. expected, sample or desired color) in either before patch  48  or after patch  49 . 
         [0061]    Once adjustment  16 B is input for mid-tone point  13 A, the present invention can compute the corresponding DDC adjustment  33  to DDC point  23 A by using a reverse model of the expected device response to determine the change in DDC coordinates required to produce DIC mid-tone point  13 A offset by adjustment  16 B. In the case where the actual device response is not as expected, the DDC adjustment will be approximate. 
         [0062]    Next, according to one embodiment, the present invention can compute a set of implicit adjustments for other points on the adjustment extent bounded by endpoints (e.g. quarter tone point  14  and three-quarter tone point  15 ). A number of approaches can be used to identify the implicit adjustments. Two exemplary embodiments are discussed here in reference to  FIGS. 6A and 6B . 
         [0063]    In general, however, the present invention calculates adjusted DDC curve (e.g.  20 B or  20 C) and projects the difference between corresponding points on DDC curve  20 B (or  20 C) and DDC curve  20 A on each DDC axis to identify one-dimensional adjustments for the adjustment extent. 
         [0064]      FIG. 6A  is a diagram illustrating one exemplary set of DDC adjustments  33  and  31 A- 34 A (arrows) made to DDC curve  20 A (dashed line) to produce adjusted DDC curve  20 B (solid line) according to the present invention. First, a set of points on DIC curve  10 A between endpoints quarter tone point  14  and three-quarter tone point  15  are selected. For example, the DIC points, not shown in  FIG. 2A , but nominally produced by DDC points  26 A- 29 A, can be selected. DIC points can be selected, for example, by finding equidistant points on curve  10 A between mid-tone point  13 A, quarter tone point  14 , and three-quarter tone point  15 . 
         [0065]    For each selected DIC point, a DIC adjustment can be derived by maintaining the direction of adjustment  16 B used for mid-tone point  13 B, but reducing its magnitude to zero as it approaches quarter tone point  14  and three quarter tone point  15  along DIC curve  10 A. Thus, the selected DIC points will have adjustment magnitudes that vary according to their distance from mid-tone point  13 A. Then, using the device&#39;s reverse model, DDC adjustments  31 A- 34 A can be calculated from DIC points. Note that the direction and magnitude of these DDC adjustments  33  and  31 A- 34 A may vary, as depicted. 
         [0066]    Adjusted DDC points  23 B and  26 B- 29 B can be calculated by adding DDC adjustments  33  and  31 A- 34 A to DDC points  23 A and  26 A- 29 A respectively. Finally adjusted DDC curve  20 B can be derived by interpolation between the DDC adjustment points. 
         [0067]      FIG. 6B  is a diagram illustrating another exemplary set of adjustments made to DDC curve  20 A to produce adjusted DDC curve  20 C according to the present invention. In this example, which is somewhat easier to compute, DDC adjustment points  26 C- 29 C can be selected directly, for example, by finding equidistant points on DDC curve  20 A between DDC points  23 A,  24 A, and  25 A. Further, derived DDC adjustments  31 B- 34 B can, for example, be based on DDC adjustment  33  directly and maintain the direction of DDC adjustment  33  but vary in magnitude according to the distance between DDC adjustment points  26 C- 29 C and  33 . Adjusted DDC point  23 B, which is the same in both methods, and adjusted DDC points  26 D- 29 D can thus be computed along with adjusted DDC curve  20 C in a manner similar to that described above. 
         [0068]    Regardless of the method for deriving adjusted DDC curve  20 B (or  20 C), the present invention can then select a set of device coordinate values for each DDC color axis and compute a smooth one-dimensional transform adjustments for each color. Area  42  of  FIG. 4  depicts such a resulting set of one-dimensional transform adjustments. 
         [0069]    For example, cyan offset  50 , magenta offset  51 , and yellow offset  52  correspond to adjustment DDC point  23 A and DDC adjustment  33 . In particular, the “Tint in” values for offsets  50 - 52  correspond to the coordinates of adjustment DDC point  23 A. Thus, for example, DDC point  23 A has coordinates (38, 40, 35). Similarly, the “Tint out” values for offsets  50 - 52  correspond to the magnitude of DDC adjustment  33 . Thus, for example, DDC adjustment  33  has relative coordinates (−6, 10, 5). Similarly, offsets  53 - 55  correspond to DDC point  24 A and offsets  56 - 58  correspond to DDC point  25 A. 
         [0070]      FIG. 7  is a diagram illustrating exemplary adjustments made to one-dimensional transforms (cyan transform  61 , magenta transform  62 , and yellow transform  63 ) based on adjusted DDC curve  20 B (or  20 C) according to the present invention. The offsets  50 - 58 , depicted in area  42 , are used to create a new one-dimensional transform for each color, as shown. In the case where one-dimensional transforms already exist, the offsets can be added to the output tint values for the corresponding input tint values and then interpolating the points in between. 
         [0071]    The above examples are particularly compelling because they involve mapping coordinates from one three-dimensional space to another, such that a nominal DIC curve has one and only one associated DDC curve. It is not uncommon for a device to have four or more process colors, such that there are a large number of possible DDC curves that can produce DIC curve. 
         [0072]    In one embodiment, when a black process color is added, the black color is ignored in the adjustment process. In other embodiments, rules can be established to govern the relationship between black and the other process colors so that black is adjusted in a coordinated fashion with the other colors. One skilled in the art will realize that there are many prior art techniques for trading black color for combinations of cyan, magenta and yellow color. 
         [0073]    In general, regardless of the number of process colors, rules that enable association of one N-dimensional DDC curve with one three-dimensional DIC curve can be established so that adjustment to each color&#39;s one-dimensional transform can be determined. 
         [0074]    One-dimensional transforms, derived through the use of DIC curves, can also be used to map image data, intended for a source device, into image data for a destination device. The modified image data enables the destination device to emulate the response of the source device. Creating transforms for this purpose can be requested by providing inputs to a computer system through a user or software interface. Exemplary methods for creating transforms for this purpose are described below with reference to  FIGS. 8-11 . 
         [0075]      FIG. 8  is a diagram illustrating an exemplary association between source and destination process color coordinates based on DIC curves  10 A and  70  according to the present invention. In a preferred embodiment, neutral DIC curves can be used for associating DDC coordinates involving process colors. To enable one-to-one mapping between DDC and DIC coordinates, process colors can be limited to three (e.g. cyan, yellow, and magenta), with other colors associated in a manner described below. Where additional process colors are to be mapped using neutral DIC curves  10 A and  30 , rules for consistently mapping one DIC curve to one of many possible DDC curves must be used. 
         [0076]    In  FIG. 8 , neutral colors (e.g. destination DIC points  11 ,  12 , and  73 A- 78 A), corresponding to neutral DIC curve  10 A, can be produced by a destination device for process color coordinates (e.g. DDC points  21 ,  22 , and destination DDC points  83 A- 88 A, respectively) corresponding to destination DDC curve  20 A. Similarly, neutral colors (e.g. source DIC points  71 ,  72  and  73 B- 78 B), corresponding to neutral DIC curve  70 , can be produced by a source device for process colors coordinates (e.g. DDC points  21 ,  22 , and source DDC points  83 B- 88 B, respectively) corresponding to source DDC curve  20 E. 
         [0077]    An association can be made between source and destination DDC points, for example, by first starting with a source DIC point (e.g. source DIC point  77 B) and, using a reverse model of the source device&#39;s response, finding source DDC point  87 B. Next, an association can be made between source DIC point  77 B and destination DIC point  77 A based on a normalized DIC characteristic. 
         [0078]    In one embodiment, this association can be made based on normalized L* values. That is, for both DIC curves  10 A and  70 , find corresponding DIC points (e.g. DIC points  77 A and  77 B) whose L* value, divided by the L* range for the associated DIC curves (e.g. DIC curve  10 A and  70 , respectively), match. In another embodiment, instead of matching based on normalized L* values, matching can be based on normalized Euclidian distance along the corresponding DIC curve. Another exemplary method can incorporate a model of human perception into mapping of a DIC curve (e.g. curve distance) since human perception of change is not uniform, with respect to Euclidian distance between DIC points, throughout device-independent space. 
         [0079]    Once source and destination DIC points  77 A and  77 B are associated, an association between destination DIC point  77 A and destination DDC point  87 A can be made using a reverse model of the destination device&#39;s response. Thus, for example, DDC light point  21  (no color) and DDC dark point  22  (full color) in both source and destination DDCs can be commonly associated so that the source gamut surface maps to the destination gamut surface. Also, for example, source DDC points  83 B- 88 B map respectively to destination DDC points  83 A- 88 A so that mapped neutral colors have similar appearance. 
         [0080]      FIG. 9  is a diagram illustrating an exemplary set of one-dimensional transforms  61 - 63  for mapping source process color coordinates to destination process color coordinates according to the present invention. Transforms  61 - 63  use source coordinate values (e.g. tint in) as input to produce destination coordinate values (e.g. tint out) as output based on smoothly fitting coordinates from a number of associated DDC points. For example, source coordinates c s , y s , and m s , for source DDC point  87 B are plotted in relation to destination coordinates c d , y d , and m d , respectively for associated destination DDC point  87 A. 
         [0081]      FIG. 10  is a diagram illustrating an exemplary association between source and destination DIC curves  91 B and  91 A, derived from single color tone ramps  90 B and  90 A, according to the present invention. Note that tone ramps  90 A and  90 B are essentially DDC curves in a one-dimensional DDC. This approach can be used, according to one embodiment, as a first step in mapping source DDC tint values to destination DDC tint values for other colors (e.g. black, other process colors, and spot colors) that are not mapped according to the methods described in  FIGS. 8 and 9 . 
         [0082]    Accordingly, single color tone ramps  90 A and  90 B, for destination and source devices respectively, can be used in conjunction with their respective device forward models to determine DIC points on DIC curves  91 A and  91 B. In this black color example, the source and destination tints are depicted with slightly different color casts (i.e. different a* and b* offsets). In addition, the source device produces a narrower range of lightness (L*) values than the destination device does. Mapping of various tone values (e.g. 10% increments) to corresponding DIC points is depicted with dashed arrows. 
         [0083]      FIG. 11  is a diagram illustrating an exemplary one-dimensional transform  99  for mapping source tint values to destination tint values for a single color according to the present invention. The first step in determining the one-dimensional transform  99  involves creating an association between points on source and destination DIC curves  91 A and  91 B. Associations between selected points are depicted in  FIG. 10  with solid arrows. 
         [0084]    As described above, associations between points on similar DIC curves  91 A and  91 B can be made using a variety of methods. In this example, DIC points are associated as described below. First, source L* values  93  and destination L* values  95  are normalized as source D* values  94  and destination D* values  96  respectively. In general, however, any DIC attribute applicable to both DIC curves can be used as a basis for a one-to-one mapping between points on the DIC curves. 
         [0085]    D* values, in this example, represent a degree of darkness scaled to the range of L* values for the device. In other words, D*=0 represents no tint and D*=100 full tint for the single color. Thus, the associations depicted in  FIG. 10  are between points on DIC curves  91 A and  91 B having the same D* values. As an alternative, E* values (not shown) could be used instead of D* values. E*, for example, can represent the normalized Euclidian distance along respective DIC curves  91 A and  91 B. 
         [0086]    Next, based on the association between source and destination DIC points, destination tint values producing source D* values  97  can be calculated. This can be done, for example, by plotting input tint values  92  versus destination D* values  96  (plot  98 B) and source D* values  94  (plot  98 A) and interpolating. Output tint values for one-dimensional transform  99  can then be found for an input tint value, for example, by first selecting a source tint value (e.g. 20% tint identified by the arrow in step A). Next, at step B, a source D* value (e.g. 31) is obtained from plot  98 A. Next, at step C, a destination tint value (e.g. 49%) is obtained from plot  98 B based on the D* value found in step B. Finally, at step D, the destination tint value from step C (e.g. 49%) is paired with the source tint value  92  from step A (e.g. 20%) to identify the destination tint that produces the source tint. 
         [0087]    Embodiments of the present invention may comprise any medium which carries a set of computer-readable signals comprising instructions which, when executed by a computer processor, cause the computer processor to execute a method of the invention. Embodiments may be in any of a wide variety of forms. Embodiments may comprise, for example, physical media such as magnetic storage media including floppy diskettes, hard disk drives, optical data storage media including CD ROMs, DVDs, electronic data storage media including ROMs, flash RAM, or the like or transmission-type media such as digital or analog communication links. The instructions may optionally be compressed and/or encrypted on the medium. 
         [0088]    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 scope of the invention. 
       PARTS LIST 
       [0000]    
       
           10 A DIC curve 
           10 B DIC curve 
           10 C DIC curve 
           10 D DIC curve 
           11  DIC light point 
           12  DIC dark point 
           13 A mid-tone point 
           13 B mid-tone point 
           13 C mid-tone point 
           13 D mid-tone color 
           14  quarter tone point 
           15  three-quarter tone point 
           16 A adjustment 
           16 B adjustment 
           16 C adjustment 
           17  lightness adjustment 
           18  color cast adjustment 
           20 A DDC curve 
           20 B DDC curve 
           20 C DDC curve 
           20 E DDC curve 
           21  DDC light point 
           22  DDC dark point 
           23 A DDC point 
           23 B adjusted DDC point 
           24 A DDC point 
           25 A DDC point 
           26 A DDC point 
           26 B adjusted DDC point 
           26 C DDC adjustment point 
           26 D adjusted DDC point 
           27 A DDC point 
           27 B adjusted DDC point 
           27 C DDC adjustment point 
           27 D adjusted DDC point 
           28 A DDC point 
           28 B adjusted DDC point 
           28 C DDC adjustment point 
           28 D adjusted DDC point 
           29 A DDC point 
           29 B adjusted DDC point 
           29 C DDC adjustment point 
           29 D adjusted DDC point 
           30  neutral tone DIC curve 
           31  light point 
           31 A DDC adjustment 
           31 B DDC adjustment 
           32  dark pint 
           32 A DDC adjustment 
           32 B DDC adjustment 
           33 B DDC adjustment 
           34 B DDC adjustment 
           33  DDC adjustment 
           33 A DDC adjustment 
           34 A DDC adjustment 
           40  user interface 
           41  button 
           42  area 
           43  area 
           44  control 
           45  control 
           46  control 
           47  control 
           48  before patch 
           49  after patch 
           50  cyan offset 
           51  magenta offset 
           52  yellow offset 
           53  cyan offset 
           54  magenta offset 
           55  yellow offset 
           56  cyan offset 
           57  magenta offset 
           58  yellow offset 
           61  cyan transform 
           62  magenta transform 
           63  yellow transform 
           70  DIC curve 
           71  DIC light point 
           72  DIC dark point 
           73 A destination DIC point 
           73 B source DIC point 
           74 A destination DIC point 
           74 B source DIC point 
           75 A destination DIC point 
           75 B source DIC point 
           76 A destination DIC point 
           76 B source DIC point 
           77 A destination DIC point 
           77 B source DIC point 
           78 A destination DIC point 
           78 B source DIC point 
           83 A destination DDC point 
           83 B source DDC point 
           84 A destination DDC point 
           84 B source DDC point 
           85 A destination DDC point 
           85 B source DDC point 
           86 A destination DDC point 
           86 B source DDC point 
           87 A destination DDC point 
           87 B source DDC point 
           88 A destination DDC point 
           88 B source DDC point 
           90 A source tone ramp 
           90 B source tone ramp 
           91 A destination DIC curve 
           91 B source DIC curve 
           92  tint values 
           93  source L* values 
           94  source D* values 
           95  destination L* values 
           96  destination D* values 
           97  destination tint values producing source D 
           98 A source D* plot 
           98 B destination D* plot 
           99  one-dimensional transform 
           100  image data 
           101  device 
           102  basic tonal response 
           103  basic color response 
           105 A cyan transform 
           105 B magenta transform 
           105 C yellow transform 
           105 D black transform 
           105 E spot color  1  transform 
           106  multi-dimensional transform 
           108 A cyan transform 
           108 B magenta transform 
           108 C yellow transform 
           108 D black transform 
           108 E spot color  1  transform 
           110  image data 
           111  tonally corrected device 
           112  corrected tonal response 
           113  tonally corrected color response 
           120  image data 
           121  color corrected device 
           123  color corrected color response 
           130  image data