File: ICC_RGB_gamut_warning.txt

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PRMG RGB gamut warning profile - sRGB based (beta)

The PRMG_RGB-sRGB_based.icc profile is intended to be used as a
destination profile for checking to see if source image content extends
significantly beyond the ICC v4 Perceptual Reference Medium Gamut.

This profile is identified as being sRGB based because it contains the
perceptual transforms from the sRGB_v4_ICC_preference.icc profile in all
rendering intents. The profile assigns to RGB image data the PCS
colorimetry produced by color re-rendering sRGB values to the ICC v4
Perceptual Reference Medium (PRM). Consequently, the color gamut
represented in the PCS will not exactly match the PRMG, although it
should be close enough for most purposes. (Most gamut warning software
allows colors to go somewhat outside the destination profile gamut
before identifying them as out-of-gamut.)

It is also possible to convert to the PRMG_RGB-sRGB_based.icc profile.
When this is done the result will be RGB data describing image
colorimetry on the PRM (as opposed to on the sRGB reference display).
Such data can be useful when it is desired to exchange PRM colorimetry
in an RGB file where all possible RGB values represent valid PRM colors.
However, it should be noted that the transforms in this profile use
interpolation of three-dimensional look-up tables, and are therefore not
completely invertible. Multiple conversions into and out of PRMG RGB
will cause image degradation.

A better choice for a RGB working space and exchange color encoding for
PRM colorimetry is the ROMM RGB color encoding specified in ISO 22028-2,
and implemented in the ISO22028-2_ROMM-RGB.icc profile. This profile
contains analytical transforms that do not involve interpolation and
therefore round-trip much better than the PRMG_RGB-sRGB_based.icc
profile. The only disadvantage to using ROMM RGB as a working space is
that a significant percentage of the possible RGB values represent
colors that are outside the PRMG, and in some cases outside the spectral
locus. When editing, care must be taken to avoid using these RGB values.
In workflows where this is a concern the PRMG_RGB-sRGB_based.icc profile
offers an alternative, so long as care is taken to keep the number of
conversions to and from the working space at an absolute minimum.

The ICC recommends the use of the ISO22028-2_ROMM-RGB.icc profile as the
PRM working space and exchange encoding, and the PRMG_RGB-sRGB_based.icc
profile as a gamut warning profile to check for the use of ROMM RGB
colors outside the PRMG.

To use the PRMG_RGB-sRGB_based.icc profile for gamut warning purposes in
Adobe Photoshop:

  * Copy the profile to the profiles folder, e.g.
    - Windows: windows/system32/spool/drivers/color
    - Mac: Library/Application Support/Adobe/Color/Profiles
  * Choose View > Proof Setup > Custom command to open Customize Proof
    Condition dialog, then choose the PRMG gamut warning profile as the
    Device to Simulate profile and select the rendering intent to be
    used. NOTE: Custom proof setups can be saved for future use.
  * Click OK in the Customize Proof Condition dialog.
  * Open an image and, if it does not have an embedded profile, assign
    the appropriate source profile using the Edit > Assign Profile
    command.
  * Choose View > Gamut Warning command to identify out-of-gamut colors.

source: <http://www.color.org/prmg_gamutwarning.xalter>