ArgyllCMS's 'arts' (Absolute to media Relative Transform
        Space matrix) ICC tag (V1.0)
      
    ArgyllCMS uses a registered (private) ICC tag to solve a long
    standing Display profile compatibility issue. 
    
    Other CMM and profile creation providers are most welcome to
    implement this tag to improve ICC profile color behavior and
    interoperability.
    The problem
    The ICC V2 format originally provided no clear mechanism for Display
    profiles to use a modern chromatic adaptation transform in dealing
    with the D65 white point of typical displays. V2 profile makers
    ended up taking three different approaches :- 
    
      - Use a Bradford chromatic transform for the media (display)
        white to PCS D50 conversion. This was used in creating the
        widely deployed HP & Microsoft sRGB profile, as well as the
        AdobeRGB profile. This provides a good chromatic adaptation, but
        Absolute Colorimetric will have errors if the profile is used
        with a CMM that implements the the ICC specified "Wrong Von
        Kries" chromatic adaptation to map relative colorimetric back to
        absolute colorimetric rather than using the Bradford transform
        it was created with. 
 
- Use the ICC specified "Wrong Von Kries" chromatic transform
        from D65 to D50. This leads to poor relative colorimetric
        behavior (the most important case), but gives correct Absolute
        Colorimetric behavior.
 
- Apply the Bradford transform to the measurement data and then
        disable Absolute Intent for display profiles by setting the
        mediaWhitePoint tag to D50. The latter approach was supplemented
        later on with the introduction of the ChromaticAdapation
        ('chad') tag, but this did not restore the Absolute Colorimetric
        capability to Display profiles, since the 'chad' tag is not
        formally part of the specified mechanism for implementing
        Absolute Colorimetric. Unfortunately, this approach has been
        adopted in the ICC V4 specifications
The ICC standard "Wrong Von Kries" chromatic transform for media
    white to/from PCS D50 has disadvantages in regard to color behavior
    for Output (i.e. print) profiles as well as Display profiles. With
    most normal, white media this is not of great significance, since
    the white point shift is small, but it grows in significance as the
    color of the paper differs from white (i.e. tinted papers).
    
    For all these reasons, and in the pursuit of the best possible color
    quality, ArgyllCMS uses the more color accurate Bradford chromatic
    transform for all profiles types instead of the the ICC standard
    "Wrong Von Kries" transform. This improves compatibility with the
    (extremely common use case) of HP & Microsoft sRGB, and AdobeRGB
    profiles where the color behavior of Bradford vs. "Wrong Von Kries"
    is more critical; provides good color behavior for creating Display
    profiles while giving accurate Absolute Colorimetric Intent
    behavior; slightly improves the quality of Output profiles, most
    noticeable with tinted stock, while having minimum incompatibility
    with Output profiles created for normal, white printed media by
    other profiling programs because the white point shift is typically
    small (But to enable perfect compatibility of ArgyllCMS Output (i.e.
    print)  profiles with other CMM's, the default behavior can be
    changed by using the ARGYLL_CREATE_WRONG_VON_KRIES_OUTPUT_CLASS_REL_WP
    environment variable).
    
    So the problem is that it is impossible to know for sure whether an
    ArgyllCMS create profile, ICC V2 Display profile, or other widely
    used ICC profiles such as sRGB or AdobeRGB has used a Bradford or
    "Wrong Von Kries" chromatic transform in computing Absolute to/from
    Relative white point transform.
    The solution
    The ArgyllCMS SigAbsToRelTransSpace 'arts' tag holds a 3x3 matrix in
    the first 9 elements* of a SigS15Fixed16ArrayType in the
    usual ICC order (i.e. the same element order as the
    ChromaticAdaptationTag 'chad' tag). This matrix holds the Sharpened
    Cone space transform that the Media White point to/from PCS D50
    chromatic Von Kries transform is computed in, rather than the "Wrong
    Von Kries" transform specified by ICC.1:2001-04 (ICCV2.4) Annex A,
    pp 66, equations A.1, A.2 & A.3. So by default ArgyllCMS will
    write an 'arts' tag with the Bradford matrix in it: 
      sig      'arts'
        type     'sf32'
      S15Fixed16Array:
        No. elements = 9
          0:  0.89509583
          1:  0.26640320
          2:  -0.16140747
          3:  -0.75019836
          4:  1.71350098
          5:  0.03669739
          6:  0.03889465
          7:  -0.06849670
          8:  1.02960205
    
    In contrast, ICC standard behavior is the equivalent of a unity
    matrix.
    
    [ Note that ArgyllCMS will write an 'arts' tag containing a Bradford
    matrix even for Display profile written using the 'chad' tag where
    the the MediaWhitePointTag is set to D50, representing the cone
    space used in computing the 'chad' tag. (see ARGYLL_CREATE_DISPLAY_PROFILE_WITH_CHAD)
    i.e. for Display profiles, the 'chad' tag is regarded as just an
    alternate means of representing the true Media White Point. ]
    
    On reading a profile, ArgyllCMS uses the following logic to set its
    Media white to/from PCS D50 chromatic transform:
    
        if 'arts' tag is present:
            use it to compute
    absolute <-> media relative white point transform.
        else:
            if ArgyllCMS created
    profile (ICC header Profile Creator signature = 'argl')
             or V2 Display
    profile && Media W.P. != D50:
               
    use Bradford matrix to compute absolute <-> media relative
    white point transform.
            else:
               
    use unity matrix to compute absolute <-> media relative white
    point transform.
    
    The 'arts' tag need not be written if the profile contains no 'wtpt'
    MediaWhitePointTag tag.
    
    This provides a safe level of backwards compatibility, together with
    the elimination of all ambiguity when the 'arts' tag is present,
    allowing CMM and profile creators to use the preferred Bradford (or
    other Sharpened Cone) space for Von Kries chromatic adaptation of
    the media white point to/from PCS D50, and provide perfect
    compatibility for the widely used sRGB and AdobeRGB profiles and
    similiar.
    Conclusion
    The 'arts' tag allows unambiguous use of the recommended Bradford
    cone space transform in computing the Absolute Colorimetric (Media
    Relative) to/from Relative Colorimetric (PCS D50) chromatic
    transform in ICC profiles, while for the first time also providing
    an unambiguously means of marking the use of the Bradford transform
    in the very widely used standard sRGB and AdobeRGB colorspaces. If
    widely adopted, the use of "Wrong Von Kries" chromatic transform in
    ICC profiles could be safely abandoned.
    
    * If the SigS15Fixed16ArrayType has more than 9 elements, then just
    the first 9 should be interpreted as a 3x3 matrix, and any elements
    beyond that should be ignored.