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diff --git a/doc/iccgamutmapping.html b/doc/iccgamutmapping.html index 8deab44..11994c1 100644 --- a/doc/iccgamutmapping.html +++ b/doc/iccgamutmapping.html @@ -10,13 +10,12 @@ <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> + 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: Perceptual<br> AtoB1, BtoA1: Colorimetric<br> @@ -24,179 +23,144 @@ <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 + 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> + 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> + 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> + 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> + 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> + 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> + 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 + 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> + 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 + 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> + 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> + 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> - 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> + 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> + 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." |