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Paulo_De
Participant
October 31, 2019
Question

RGB value conversion from Dot Gain 20 to Dot Gain 0?

  • October 31, 2019
  • 4 replies
  • 4507 views

Hi,

 

I was wondering if there was a mathematical function for converting between PS' default Dot Gain 20 colour picked RGB values and the "true" colour picked RGB values (Dot Gain 0).

The images I'm working with are GrayScale depth maps.

I've been looking around but I can't really seem to find anything on the PS Dot Gain algorithms.

 

Thanks,

Paulo

 

 

This topic has been closed for replies.

4 replies

rob day
Community Expert
rob dayCommunity Expert
Community Expert
October 31, 2019

I'm not sure, but it seems like you want the gray percentages converting to the same relative percentage on the RGB scale. So, 50% gray converts to 127 (255-(255 x .50)), 75% gray converts to 191 (255-(255 x .25)), or 25% gray to 64 (255-(255 x .75)) etc.

 

If that’s the case, assign sGray to the Grayscale, and set the destination to sRGB—they both use the same gamma:

 

 

 

    Stephen Marsh
    Community Expert
    Stephen MarshCommunity Expert
    Community Expert
    October 31, 2019

    They are both very different tone response curves:

     

     

    I'm not sure of the accuracy of these Apple ColorSync plots, however, the differences are great enough.

     

    The legacy Photoshop gray 20% dot gain pre-dates modern ICC profiles.

     

    I believe that sGray has the same TRC as sRGB, which is sort of based on a 2.2 gamma TRC, however, sRGB is meant to have a custom curve... Then there are "simplified" versions of sRGB that have a "standard" 2.2 gamma TRC.

     

    Just Google for 2.2 gamma vs. sRGB TRC etc.

     

    I am still a little unsure of the practical concerns/issues as these are all device dependent colour modes.

    NB, colourmanagement
    Community Expert
    NB, colourmanagementCommunity Expert
    Community Expert
    October 31, 2019

    Hi Paulo,

    "Dotgain20" and "sGRAY" are for grayscale files only, not for working in RGB

    sGRAY is a greyscale colourspace designed to mimic the curve of sRGB.

     

    You need to get dotgain out of the picture when workign on RGB files.

    https://en.wikipedia.org/wiki/Dot_gain

     

    You mention assigning a different ICC profile, but assigning a different profile to an RGB image does not change the colour numbers - only the file's appearance changes (due to a recalculation as to the "meaning" of the numbers). Converting from one ICC colourspace to another obviously does change the RGB numbers.

    more here: https://www.colourmanagement.net/advice/about-icc-colour-profiles/

     

    Basically you should probably not be working in RGB with an assigned profile of "Dotgain 20" or "sGRAY" - Only RGB colourspaces are applicable to RGB files and it's almost always better to use a recognised standard RGB colourspaces like Adobe RGB or sRGB. If you want RGB values then I recommend you work in an RGB colourspace.

     

    Dotgain is entirely associated with expected gains in density when ink dots are placed on paper and the ink spreads, (maybe a 20% dot becomes 23%).https://en.wikipedia.org/wiki/Dot_gain

    This is not applicable to RGB files.

     

    If we are getting our wires crossed please explain your needs. What are the files you are working on - how will they be used and where did they come from etc? And why do you need the RGB values?

     

     

    I hope this helps

    if so, please "like" my reply

     

    thanks

    neil barstow, colourmanagement.net

     

     

      T
      Test Screen Name
      Legend
      October 31, 2019

      Dot gain isn’t applicable to RGB, this has no real meaning. 

      P
      Paulo_DeAuthor
      Participant
      October 31, 2019

      Hey,

      Sorry maybe I didn't make sense.

      Basically when I colour pick a pixel with Dot Gain 20 as the assigned profile, I get an RGB value of (129,129,129). But when I colour pick the same pixel with sGRAY as the assigned profile I get an RGB value of (106,106,106). 

       

      Is there a mathematical function for converting between those? 

      I took some other data points to check and it's not a linear relationship.

      Any help would be appreciated!

      (Also sorry, I'm not really that familiar with Dot Gain)

       

      Thanks,

      Paulo

      rob day
      Community Expert
      rob dayCommunity Expert
      Community Expert
      October 31, 2019

      Basically when I colour pick a pixel with Dot Gain 20 as the assigned profile, I get an RGB value of (129,129,129)

       

      The RGB numbers you get on a conversion from Grayscale to RGB would depend on both the source Gray space and the destination RGB space.

       

      So here the conversion is from Dot Gain 20% to sRGB:

       

       

      The conversion from Dot Gain 20% to AppleRGB results in different numbers:

       

       

      As Stephen notes, Dot Gain profiles are legacy offset press output profiles. You can also use Black Ink Profiles which are more accurate and match the more modern instrument derived CMYK profiles (which are no longer curve based).

       

      You could make and save a dot gain profile with 0% gain as an .icc profile via Color Settings>Gray>Custom Dot Gain, but it is unlikely that a press would run with 0% gain, and if it did it would surely blow out all of the highlights and turn blacks to gray:

       

       

      My 0% profile assigned to the grayscale. It changes the Grayscale soft proof —the expected appearance on a press if it is really running with no gain—and the conversion to sRGB is much different reflecting the lighter values whn there is no gain:

       

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