2 Replies Latest reply on Mar 13, 2014 11:16 AM by Alp Er Tunga

    Perceptual reference medium gamut

    Alp Er Tunga Level 1

      For using with percetual intents, ICC specification defines a special gamut called perceptual reference medium gamut (PRMG).

       

      When using perceptual intents, source gamut is mapped to PRMG and then from the PRMG, data are mapped to the output gamut.

      It is said that it is optional, but I could not understand why ICC needed to specify such a small gamut for using in perceptual intents in place of defined PCSs (CIEXYZ or CIELAB)

       

      Latest ICC specification explains it, but I could not understand the purpose of it from the specification.

      What is the function of it, a function PCS can not do?

      I would really appreciate any help.

       

      Thanks a lot.

        • 1. Re: Perceptual reference medium gamut
          G.Hoffmann Level 3

          (1)
          http://www.color.org/v4_prmg.xalter
          (2)
          http://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CD8QFjAC&url=http%3A%2F %2Fnicobonnier.free.fr%2Fresearch%2Fpublications%2FCIC17_2009_Bonnier.pdf&ei=vyggU8mTGseAy wPM0YGoBA&usg=AFQjCNGlFw-0D5z5bcGObG48UDsRhn8TTA&bvm=bv.62788935,d.bGQ
          Nicolas Bonnier, Phil Green, Alain Sarlat:
          Evaluating the Use of the Perceptual Reference Medium Gamut in ICC Printing Workflows

          ------

           

          I couldn't understand the purpose of PRGM from the ICC-specs or other official informations (1) either.

          But the article (2) by well-known experts explains the background and practical results very well.

          Instead of applying copy&paste I'll try to explain this now by my words.

           

          Let us assume, we want to apply Perceptual Rendering Intent (PRI), coming from an Input Profile,

          like sRGB, aRGB (AdobeRGB), pRGB (ProPhotoRGB) and going through a printer Output Profile, which

          had been created for accurately defined conditions by the user.

          In this case the PRI is established by software of the manufacturer of the profiling system, for instance

          GretagMacbeth (ProfileMaker). The system knows the blackpoint, the paper white and the gamut,

          estimating geometrical boundaries in CIELab.

          No source profile is assumed for this process. RGB black is printed as the darkest printable black

          (RGB black is mapped to printer black), RGB white is mapped to paper white. Out-of-gamut colors are

          handled by squeezing all colors so that none is outside of the gamut boundary. There a probably no

          tricky hue variations, which would be necessary for rendering vibrant blue, which appears often with

          a magenta tint, and for better color appearance (preferred rendering).

           

          Now comes the PRMG. For a certain input profile (sRGB, aRGB, pRGB) the profile for the conversion

          to PRMG can be created and optimized, which may cost much time and effort, but it has to be done

          only once.

          Some regions in CIELab can be tweaked manually or strategically, for instance the blues. The size and

          the features of the source space can be taken into account. In other words, such a profile can be con-

          sidered as optimal.

          Unfortunately it's not directly usable for printing, because real processes differ from the situation of

          PRMG, but the differences are not very large. Therefore it can be expected, that the final conversion

          from the PRGM to the real printer (characterized by measured targets) does not spoil the previously

          achieved optimality.

           

          The article is some years old. I don't have any information, whether this workflow is really used.

          At least it would require, that the manufacturers of profiling software have built-in profiles sRGB–PRGM,

          aRGB–PRGM and so on.  

           

          Personally I don't use the Perceptual Rendering Intent. This is a result of improved features of

          Photoshop and RIP software:

          a) Shadow and Highlight for improving the levels of dark areas.

          b) Blackpoint Compensation for Relative Colorimetric Rendering Intent.

          c) Soft proofing for the intended printing process. Very important for a partial desaturation of some

          areas which would appear otherwise with a loss of levels, because RelCol maps different out-of-gamut

          source colors to the same output color.

           

          Thanks for your question – hopefully we can see now clearer.

           

          Best regards --Gernot Hoffmann

          • 2. Re: Perceptual reference medium gamut
            Alp Er Tunga Level 1

            Thanks for your explanation, very clear and helped me so much.

             

            I think that it is well defined gate for all printer profiles.

            So, maybe, it can be thought as a workspace profile for printer profiles in this respect.