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authorJörg Frings-Fürst <debian@jff-webhosting.net>2014-09-01 13:56:46 +0200
committerJörg Frings-Fürst <debian@jff-webhosting.net>2014-09-01 13:56:46 +0200
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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=ISO-8859-1">
+</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 operate exactly the same as the ICC
+V2 profile in regard to gamut mapping. 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. But an optional part of ICCV4, is to use the <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 behaviour 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>
+Once again, this is 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>