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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>About ICC profiles and Gamut Mapping</title>
    <meta http-equiv="content-type" content="text/html;
      charset=windows-1252">
  </head>
  <body>
    <h2><u>About ICC profiles and Gamut Mapping</u></h2>
    <h3>How ICC profiles support different intents</h3>
    cLUT (Color Lookup Table) based ICC profiles support multiple <span
      style="font-weight: bold;">intents</span> by having a table for
    each intent. In a typical device cLUT profile, there are up to 6
    cLUT's, three for input (AtoB tables, that convert from device space
    to PCS (Profile connection space)), and three for output (BtoA
    tables, that convert from PCS to device space). The tables allow the
    use of different color transforms, each transform being tailored for
    a different effect:<br>
    <br>
    AtoB0, BtoA0:&nbsp;&nbsp; Perceptual<br>
    AtoB1, BtoA1:&nbsp;&nbsp; Colorimetric<br>
    AtoB2, BtoA2:&nbsp;&nbsp; Saturation<br>
    <br>
    The colorimetric intent is meant to convey the exact device color
    behaviour, without any gamut mapping. Typically it is used to store
    the devices behaviour (characterization), and is also used where
    exact color reproduction is required, such as for proofing. The
    Colorimetric tables double up for both relative colorimetric and
    absolute colorimetric with the application of a white point
    restoration.<br>
    <br>
    The Perceptual and Saturation tables are meant to contain gamut
    mapping combined with the device characterization. The allowance for
    this in both the AtoB direction, as well as the BtoA direction
    permits a profile to gamut map from the device gamut to some
    intermediate gamut, and then from the intermediate gamut to the
    device gamut.<br>
    <br>
    [Note that Shaper/Matrix profiles are always Colorimetric intent,
    since there is only a single transformation, and it does not have
    the necessary flexibility to accommodate gamut mapping.]<br>
    <h3>ICC Version 2 behaviour<br>
    </h3>
    Apart from defining the general purpose of the different tables, the
    ICC Version 2 specification doesn't specify exactly how they are to
    achieve this, so it is up to the profile maker to make a choice in
    this regard. There is no common gamut boundary specified for the
    PCS, and such an approach limits the achievable intents in any case
    (see ICC Version 4 behaviour for an explanation why).<br>
    <br>
    What I've chosen to do with Argyll profiles, is to make all the AtoB
    tables the same as colorimetric. This means that the conversion used
    for the source profile is always colorimetric, and also means that
    the source gamut seen by the destination profile is the source
    colorspace gamut. This means that the gamut mapping is done solely
    in the BtoA tables, and that their task is to map the source
    colorspace gamut to the destination colorspace gamut. So to
    construct the perceptual and saturation intent mapping tables, a
    source profile or source gamut needs to be specified, so that a
    gamut mapping can be constructed.<br>
    <br>
    The advantages of this approach is that the behaviour is precisely
    defined, a full range of gamut mapping options is available, and
    compatibility with matrix profiles (which do not have gamut mapping
    transforms) and other foreign profiles can be assured, by simply
    using such profiles as colorimetric sources. The main disadvantage
    is that the gamut mapping will only operate exactly as intended when
    the profile is linked with the source profile it was setup for. This
    is really a fundamental limitation of the idea of having
    pre-computed gamut mapping color transforms, that the ICC profile
    format was intended to support.<br>
    <br>
    Some non-Argyll profile have gamut mapping transforms in their
    Perceptual and Saturation A2B tables, and this means that the
    apparent gamut of a source through these tables may be different to
    the actual device gamut. To accommodate using these profiles with
    CMM's (Color Management Modules) that do not permit the separate
    choice of intent tables for the source and destination profiles,
    Argyll will by default use the gamut defined by the source profile
    perceptual table to create the gamut mapping of the destination
    perceptual table, and the source saturation table to make the
    destination saturation table. Note that this can affect the exact
    nature of the gamut mapping, the distortion of the source gamut
    changing the apparent relationship between it and the destination
    gamut - see "ICC Version 4 behavior" for an illustration of the kind
    of changes this causes. [This default can be overridden though using
    the colprof -nP and -nS flags.]<br>
    <h3>ICC Version 4 behaviour</h3>
    (Note that Argyll does not currently support ICC V4)<br>
    <br>
    By default, ICC Version 4 profile operates similarly to the ICC V2
    profile in regard to gamut mapping, with the exception that a
    minimally specified reference medium and reference viewing
    conditions are introduced for perceptual (and presumably saturation)
    tables, allowing at least the luminance range to have a well defined
    behavior when mixing and matching the perceptual A2B and B2A tables
    of different profiles. A slight adjustment was made to the permitted
    tag contents, to allow things like Display profiles to contain the
    full range of AtoB and BtoA tables, so that they could also be gamut
    mapped. An optional part of ICCV4, introduces a more comprehensively
    specified <span style="font-weight: bold;">Profile Reference Medium
      Gamut</span> (PRMG) as an intermediate gamut boundary between the
    source colorspace, and the destination colorspace. If this option is
    used, then an additional tag in the ICCV4 profile indicates that
    this is the case. This then solves the problem of the gamut mapping
    having to know the source and destination gamuts to operate.
    Instead, the gamut mapping is split into two parts, the first where
    the source gamut to RMG is done by the AtoB tables, and then the RMG
    to destination gamut is done by the BtoA tables. Profiles can
    therefore be mix and matches, while retaining true gamut mapping.<br>
    <br>
    This approach has a number of drawbacks though. One is that the
    colors get gamut mapped twice. Gamut mapping is sometimes not very
    precise, and the geometry of the transforms may not cancel out,
    especially since different profile vendors may choose different
    algorithms in their gamut mapping. By "cancel out", I mean that even
    if you were linking the same source colorspace to the same
    destination colorspace, the gamut may be expanded (say) in the
    process of mapping to the PRMG, and then compressed again in mapping
    from the RMG to the device space, and these expansions and
    compressions may not quite match. Given that the PRMG is a
    relatively large gamut, larger than many real devices actual
    behavior, this sort of expansion and re-compression will be the
    normal thing.<br>
    <br>
    The chief drawback, is that only one (non colorimetric) intent can
    really be supported, that of saturation. <br>
    <br>
    The typically expected behavior of perceptual intent gamut mapping,
    is to compress any areas of the source gamut that lie outside the
    destination gamut, but for areas that fall within the destination
    gamut, change them as little as possible, consistent with keeping
    smooth and proportional with respect to the compressed colors. This
    preserves the source "look" as much as possible, while ensuring that
    out of gamut colors are smoothly brought within the destination
    gamut.<br>
    <br>
    Typical behavior of a saturation intent, is (at least), to not only
    compress out of gamut source colors to fit within the destination,
    but to expand any source boundary that falls within the destination
    gamut outwards match the destination gamut. Some practical
    saturation gamut mappings may go further than this, and expand a
    little beyond the destination gamut to ensure fully saturated
    boundary colors, and also enhance the saturation of all colors
    mapped through it.<br>
    <br>
    &nbsp;By mapping the source gamut to the RMG in the A2B, all
    information about what areas of the source gamut are inside or
    outside of the destination gamut are lost, so the destination gamut
    mapping can not known which colors may be left unchanged, and which
    really need compressing. All it can do is map the RMG to match the
    destination gamut, thereby effecting a saturation style intent. <br>
    <br>
    If the source was not expanded out to fill the RMG in some area by
    the A2B, then the resulting output will be over compressed and end
    up looking dull, because the B2A table has no choice but assume that
    there may be colors that do fill the RMG.<br>
    <br>
    Once again, this is all a fundamental limitation of using
    pre-computed gamut mappings. The only effective way of overcoming
    such limitations is to move to a more active color management
    architecture, in which gamut mappings are computed at link time, to
    accommodate the actual source and destination gamuts.<br>
    <br>
    <br>
    <img alt="Illustration of perceptual and saturation gamut mapping."
      src="gamutmapping1.jpg" style="width: 665px; height: 215px;"><br>
    <br>
    <br>
    <br>
    <br>
    <br>
  </body>
</html>