link/icclink

Summary

Link two ICC device profiles to create an ICC device link profile.

icclink takes two device ICC profiles, and links them together, either in a simple fashion using the standard ICC forward and reverse tables of the specified intent, or using color appearance space and true gamut mapping, together with possibly inverting the forward profile,  to allow black ink regeneration or to retain the input black characteristic from the input profile.

Usage Summary

icclink [-options] inprofile outprofile linkedprofile
 -v                  Verbose
 -V                 Verify existing profile, rather than link (Debug option)
 -q [lmhu]       Quality - Low, Medium (def), High, Ultra
 -r res             Override clut res. set by -q
 -n                  Don't preserve device curves in result
 -f                  Special :- Force neutral colors to be K only output.
 -F                 Special :- Force all colors to be K only output.
 -p aprof.icm   Include abstract profile in link
 -s                  Simple Mode (default)
 -g [src.gam]   Gamut Mapping Mode [optional source image gamut]
 -G [src.gam]  Gamut Mapping Mode using inverse outprofile A2B [optional source gamut]
     Simple Mode Options:
 -i in_intent       p = perceptual, r = relative colorimetric,
                       s = saturation, a = absolute colorimetric
 -o out_intent    p = perceptual, r = relative colorimetric,
                       s = saturation, a = absolute colorimetric
     Mapping Mode Options:
 -i intent     set linking intent from the following choice:
                       0: Absolute Colorimetric (in Jab) [ICC Absolute Colorimetric]
                       1: Absolute Appearance
                       2: White Point Matched Appearance [ICC Relative Colorimetric]
                       3: Luminance matched Appearance
                       4: Perceptual (Preferred) [ICC Perceptual]
                       5: Saturation
                       6: Enhanced Saturation [ICC Saturation]
                       7: Absolute Colorimetric (Lab)
                        (Can also use -ia, -ir, -ip or -is as aliases.)
 -w [J,a,b]        Use forced whitepoint hack [optional color to map the white to]
 -c viewcond    set input viewing conditions for CIECAM02,
                       either an enumerated choice, or a parameter
 -d viewcond    set output viewing conditions for CIECAM02,
                       either an enumerated choice, or a parameter
                       Enumerated Viewing Conditions:
                       0: Practical Reflection Print
                       1: Print evaluation environment
                       2: Monitor in typical work environment
                       3: Monitor in darkened work environment
                       4: Projector in dim environment
                       5: Projector in dark environment
                       6: Original scene - Outdoors
                       7: Photo CD - original scene
                       8: Transparencies on a viewing box
         s:surround  a = average, m = dim, d = dark,
                          c = transparency (default average)
         w:X:Y:Z         Adapted white point as XYZ (default media white)
         w:x:y             Adapted white point as x, y
         a:adaptation       Adaptatation luminance in cd.m^2 (default 50.0)
         b:background  Background % of image luminance (default 20)
         f:flare             Flare light % of image luminance (default 1)
         f:X:Y:Z           Flare color as XYZ (default media white)
         f:x:y               Flare color as x, y
       Inverse outprofile A2B Options:
 -k [tzhxr]    CMYK Black generation
                       t = transfer K from input to output
                       z = zero K, h = 0.5 K (def), x = maximum K, r = ramp K
 -k p stle stpo enpo enle shape
                       p = black locus generation curve parameters
 -k q stle0 stpo0 enpo0 enle0 shape0 stle2 stpo2 enpo2 enle2 shape2
                       q = transfer input K to dual curve limits
 -l tlimit           set total ink limit, 0 - 400% (default none)
 -L klimit         set total ink limit, 0 - 100% (default none)

Usage Details and Discussion

-v Turns on verbose mode. Gives progress information as the profile is created. Since gamut map mode inverse profile linking can take a long time to perform, this is often useful.

-V Verifies an existing profile. This is really a debugging option. It is only useful if all the linking parameters are identical to those used during the creation of the profile being verified.

-q [lmhu]       Quality - Low, Medium (def), High, Ultra
-r res             Override clut res. set by -q

This sets the basic quality of the resulting link, by choosing the resolution of various tables in the resulting profile, as well as the resolution of other temporary tables used in creating the link. The -r flag allows overriding the resolution set by the -q option, for the ICC profile CLUT multi-dimensional interpolation table.

Normally the per channel device curves in the input and output profiles are preserved in the resulting device link profile, but the -n option disables this. This can be useful if the device linearisation curves are inappropriate in nature.

The -f option is a special purpose flag useful only for 3 component (RGB or CMY) input to CMYK output linking, that causes the output to be K only for neutral axis input. Neutral axis input is assumed for R=G=B or C=M=Y input values. If the -F flag is used, then all colors will be converted to K only output.

The -p option alows specifying an abstract profile be applied between the input and output profiles. An abstract profile is a way of specifying a color adjustment in a device independent way. The abstract profile might have been created using one of the tweak tools, such as refine.

The basic linking style is chosen by using the -s (default), -g or -G flags. The three behaviors are:

 -s   Simple mode. No gamut mapping is performed, the selected intent AtoB and BtoA tables are simply concatenated to create the output link, with the gamut mapping behavior being determined solely by the BtoA table. The -i and -o options allow selection of the input and output ICC intents. This is typically how other CMS do ICC linking. Details.

 -g   Gamut mapping mode. In this mode, the absolute colorimetric AtoB and BtoA tables are used to perform the link, and the intermediate linking color space is (generally) the CIECAM02 Jab appearance space. The input and output viewing conditions can be selected using the -c and -d options. A gamut mapping is performed between the two spaces, using  the intent selected by the -i option. There is an optional argument, which is a source gamut to use instead of that of the source profile. This is to allow optimizing the gamut mapping to a source gamut of  a particular image, which can give slightly better results that gamut mapping from the gamut of the source colorspace. Such a source image gamut can be created using the tiffgamut utility. More  details about gamut mapping mode.

 -G  Use the gamut mapped, inverse AtoB table linking method. This is generally the most accurate, smooth and flexible linking method, but takes the longest to perform. The gamut mapping mode  (-g) options -i, -c, -d, -k and -l are effective when this method is selected. There is an optional argument, which is a source gamut to use instead of that of the source profile. This is to allow optimizing the gamut mapping to a source gamut of  a particular image, which can give slightly better results that gamut mapping from the gamut of the source colorspace. Such a source image gamut can be created using the tiffgamut utility. More details about the gamut mapping, inverse AtoB mode.

The gamut provided to the -g or -G flag should be in the same colorspace that icclink is using internally to connect the two profiles. For all intents except the last one (no. 7), the space should be Jab appearance space, with the viewing conditions generally being those of the input profile viewing conditions. The input profile will normally be the one used to create a source image gamut using tiffgamut.
 
Simple mode gamut mapping options:

      -i in_intent       p = perceptual, r = relative colorimetric,
                            s = saturation, a = absolute colorimetric
      -o out_intent    p = perceptual, r = relative colorimetric,
                            s = saturation, a = absolute colorimetric

These two options simply select the appropriate ICC table, according to desired intent. Generally, it is a good idea to use the same intent for both source and destination. Not all ICC profiles support all four intents.

Gamut mapping mode options:

 -i intent

Select the gamut mapping intent. In gamut mapping mode there is only a single overall intent. As well as a number selection, the letters a, r, p and s can be used as aliases for the intents absolute colorimetric, relative colorimetric, perceptual and saturation.

The No.0 (or a) intent, Absolute Colorimetric is intended to reproduce colors exactly, irrespective of the white point of the each medium. This is done using CIECAM02 Jab appearance colorspace by forcing the source and destination to have a common white point (but other aspects of the individual viewing conditions are active), and colors are mapped directly from input to output, clipping any out of gamut colors to the closest match. This is equivalent to the ICC Absolute Colorimetric intent.

The No. 1 intent, Absolute Appearance, simply maps the Jab colors directly from input to output, clipping any out of gamut colors to the closest match. This attempts to match the exact appearance of colors as closely as possible, but may not exactly map the white point of the source to the destination, depending on how different the viewing conditions are.

The No. 2 (or r) intent is like Absolute Appearance mode, but maps the white point from input to output precisely, and otherwise maps the Jab colors directly from input to output, clipping any out of gamut colors to the closest match. This is equivalent to the ICC Relative Colorimetric intent.

The No. 3 intent, Luminance matched appearance, linearly compresses or expands the the luminance axis to match the input to the output space, while not otherwise altering the gamut, clipping any out of gamut colors to the closest match.

The No. 4 (or p) intent, Perceptual, uses "knee" type compression to make the input gamut fit within the output gamut. As much as possible, clipping is avoided, hues and the overall appearance is maintained. This is equivalent to the ICC Perceptual intent.

The No. 5 intent, Saturation, uses compression and expansion to try and make the input gamut exactly match the output gamut, and also favors higher saturation over hue or lightness preservation.

The No. 6 (or s) intent, Enhanced Saturation, uses the same basic gamut mapping as No. 5, Saturation, but increases saturation slightly in highly saturated areas of the gamut. This is equivalent to the ICC Saturation intent.

The No. 7 intent, Absolute Appearance (Lab), is similar to intent 0, but L*a*b* colorspace is used rather than CIECAM02 Jab appearance space. This often leads to poor reproduction of blue and red hues.

The -w flag forces the white points to be mapped from source to destination, irrespective of the intent chosen. This is useful if absolute intent is being used, and the two media white points should match, but don't quite due to measurement error.
The -w flag can optionally be followed by three numbers, that specify a color that white should be mapped to. This will be in the colorspace used during linking (typically Jab space, which has similar characteristics to L*a*b* space). This options can be useful in fine tuning paper emulation is absolute colorimetric mapping mode.

The -c and -d options allow specification of the viewing conditions for the source and destination colorspaces respectively. The viewing condition information is used to map the profile PCS (Profile Connection Space, which us either XYZ or L*a*b*) color into appearance space (CIECAM02), which is a better colorspace to do gamut mapping in. The viewing conditions allow the conversion into appearance space to take account of how color will be seen under particular viewing conditions.
Viewing conditions can be specified in two basic ways. One is to select from the list of "pre canned", enumerated viewing conditions, choosing one that is closest to the conditions that are appropriate for the media type and situation. Alternatively, the viewing conditions parameters can be specified in detail individually. If both methods are used, them the chosen enumerated condition will be used as a base, and its parameters will then be individually overridden.

Inverse outprofile A2B Options:

When the -G flag is used, the A2B table is inverted "on the fly", allowing various additional choices as to what device values are used to reproduce a particular color. (If the -G flag is not used, then such decisions are encoded in the B2A table in the profile, and cannot be altered during linking).

-k flag sets the amount of Under Color Removal, or black generation level when inverting a CMYK output profile. The black levels specified by the flag are all in relation to the possible black values for each color being looked up (the possible black locus), NOT the absolute amount of black that will be used. For instance, at the white point, no black will be used in the output, even if the black generation specifies a maximum (since the maximum amount of black that can be used to print white is actually zero). Similarly, at the black point, black may well be used, even if the black generation specifies zero black (since a certain amount of black is needed to achieve the desired density of color). Possible arguments to the -k option are:

-kz selects minimum black (0.0)
-kh selects a black locus value of 0.5
-kx selects the maximum possible black (1.0)
-kr selects a linear locus ramp, starting at minimum black for highlight, and maximum black for shadow (equivalent to -kp 0 0 1 1 1)
-kt, will preserve the black amount from the input (CMYK) profile to the output  (CMYK) profile as much as possible. This may be most useful in creating a CMYK to CMYK conversion between two different press conditions, while preserving as much as possible the  black only use for text etc. in anything converted.
-k p stle stpo enpo enle shape  allows an arbitrary black locus ramp to be defined, consisting of a starting value (stle) for highlights, a breakpoint L value (stpo) where it starts to transition to the shadow level, an end breakpoint L (enpo) where it flattens out again, and the finishing black level (enle) for the shadows. There is also a curve parameter, that modifies the transition from stle to enle to either be concave (ie.  the transition starts gradually and and finished more abruptly) using values 0.0-1.0, with 0.0 being most concave, or convex (the transition starts more abruptly but finishes gradually), using values 1.0-2.0, with 2.0 being the most convex.
-k q stle0 stpo0 enpo0 enle0 shape0 stle2 stpo2 enpo2 enle2 shape2 is a combination of the -kt and -kp functionality, with the black being preserved in CMYK to CMYK linking, with the output black constrained to be between the first and second set of curve parameters.

Typical black locus generation curve with parameters something like: -kp .1 .3 .5 .8 1

         1.0 K   |          enpo
                 |            _______  enle
                 |           /
                 |          /
                 |         /
                 |        /
           stle  | ------/
                 +-------------------
         0.0 K  0.0    stpo        1.0
               White              Black


For minimum sensitivity of printed output to the lighting spectrum, it currently seems best to use the maximum possible black, but other black generation levels (ie. 0.3 to 0.5) may well be preferred if one wants to minimize the noisy appearance of black on an inkjet device, or if the banding behavior or other rendering flaws of the printer is to be minimized.
 
A set of values for an inkjet printer might be something like -kp 0.0 0.2 0.7 0.7 1.0

The xicclu utility can be used to plot out the resulting black level for a given set of parameters, by using the -g flag.

The -l tlimit parameter sets the total ink limit (TAC, Total Area Coverage) for the CMYK separation, as a total percentage from 0% to 400%. The limit value should generally be set a little below the value used in the test chart generation, to avoid the very edges of the gamut. If the test chart ink limit has been chosen to be a little beyond an acceptable level, then this number should be the acceptable level. Although limits can be set below 200%, this will generally restrict the color gamut noticeably, as fully saturated secondary colors will not be reproduced. Values are between 220% and 300% for typical printing devices.

The -L klimit parameter sets the black channel ink limit for the CMYK separation, as a total percentage from 0% to 100%. For printing press like devices, this can be used to prevent the black channel screening pattern "filling in". Typical values might be from 95% to 99%.

The inprofile argument specifies the input profile. This is the color space/device we are attempting to emulate in the overall conversion.

The outprofile argument specifies the output profile. This is the device we are actually displaying on or printing to.

The linkedprofile argument specifies the device link profile. This profile will contain the color transform from the input space to output space.

For information on typical usage, see the Typical Usage Scenarios page.