17 Replies Latest reply on Jan 11, 2012 12:09 PM by D Fosse

    Which one is right? The colour picker or the info panel?

    tomaugerdotcom Level 1

      Try this one out at home kids:

       

      Set your colour picker to Greyscale and dial 25% K into your foreground colour swatch.

       

      Now, in a new, RGB document, fill that document with your foreground colour.

       

      Now open the Info palette, set one of your readouts to "Greyscale" and enjoy seeing it read 31% K.

       

      Uh.

       

      WTF?

       

      My RGB readout reads 192 across all channels, which is correct: 192 / 256 = 75% (additive) = 25% (subtractive)

       

      This probably has something to do with colour management, but could someone A) explain to me what is happening, and B) tell me how to set it so I can get mathematically predictable results?

        • 1. Re: Which one is right? The colour picker or the info panel?
          Doug.S Level 3

          conversions between RGB and CMYK are NOT 1:1

          • 2. Re: Which one is right? The colour picker or the info panel?
            tomaugerdotcom Level 1

            Who said anything about CMYK? I suppose if you meant "conversions between additive and subtractive colour models are not 1:1" that might make a little sense. RGB <--> CMYK has never been 1:1 - we can see this by taking an RGB image, converting it to CMYK, inverting it, converting it back to RGB and inverting it back. The colour math doesn't add up.

             

            But here I'm talking channels. Try what I'm suggesting on a single channel of an RGB document. No CMYK conversion is happening here, or at least none should be.

            • 3. Re: Which one is right? The colour picker or the info panel?
              D Fosse Adobe Community Professional & MVP

              All these numbers are relative to whatever profile is used, or what the working space is. They have no significance beyond that.

              • 4. Re: Which one is right? The colour picker or the info panel?
                tomaugerdotcom Level 1

                It depends on what you plan on doing with the numbers. If you're using them to try to gain some type of objective measurement over perceptual data, then I would agree with you, tentatively, that "all is relative" and the numbers are only one factor amongs a number of others such as environmental variables, monitor phosphors/lcd lumens, and a million other things.

                 

                However, if, as in my case, I am looking at these numbers from a mathematical perspective, as inputs into convolution processes, then indeed the numbers are extremely relevant, and furthermore, are not at all subjective. I expect 25% to mean 25% and not 31%. I expect that when I select 192/192/192 I get 25% grey coverage and that's that.

                 

                When Photoshop's own dialog boxes contradict each other, that's where I start to get concerned, hence the intent of this post.

                • 5. Re: Which one is right? The colour picker or the info panel?
                  D Fosse Adobe Community Professional & MVP

                  There is no "RGB". There is only sRGB, Adobe RGB, your monitor profile (which is probably what you're picking up in info) etc.

                   

                  There are no absolutes here. Use the eydropper on a single file, fine. Use it on another file in the same color space, also fine. But it stops there.

                  • 6. Re: Which one is right? The colour picker or the info panel?
                    tomaugerdotcom Level 1

                    We're obviously coming at this from two different worlds. I certainly see your perspective, probably informed by a photographic / prepress background. I wonder whether you can see mine?

                     

                    Regardless, let's not debate an issue that is not aposite to the question at hand. Can someone shed light on why Photoshop is giving two conflicting readings?

                    • 7. Re: Which one is right? The colour picker or the info panel?
                      Noel Carboni Level 8

                      Heh, looks like they're just using different scales in the term grayscale.  Have you looked in the Help for the COLOR panel to see what K means?

                       

                      I have to agree that the controls should probably match.  It's easily reproduced - you can set the color to a particular K % and the info panel shows another K %.  If they mean different things they should not be labeled the same.

                       

                      If you convert your image to a Custom RGB profile with a gamma of 1.8 the numbers will match.  Photoshop was originally developed on Macintosh.  Coincidence? 

                       

                      -Noel

                      • 8. Re: Which one is right? The colour picker or the info panel?
                        D Fosse Adobe Community Professional & MVP

                        I'm not trying to argue...well, I am - but if you could explain precisely what you're after I'm sure it's doable.

                        tomaugerdotcom wrote:

                         

                        But here I'm talking channels. Try what I'm suggesting on a single channel of an RGB document

                        A single channel is a grayscale image and displayed according to what the gray working space is.

                         

                        Here is a screenshot of an Adobe RGB file. The left side is the red channel displayed with working gray gamma 2.2. The right side is the same file with working gray dot gain 10%.

                         

                        grayscale.png

                        To further complicate, all luminosity operations on an RGB image are performed in Lab, not the particular RGB space of the file. Ctrl-click on the RGB thumbnail to load a luminosity selection, and what you get is the L channel of a Lab version. This actually makes sense since color management operations go through Lab (or XYZ) as the reference space.

                         

                        (BTW disregard what I said about the monitor profile above, that's probably not relevant here).

                        • 9. Re: Which one is right? The colour picker or the info panel?
                          Noel Carboni Level 8

                          D Fosse wrote:

                           

                          I'm not trying to argue...well, I am - but if you could explain precisely what you're after I'm sure it's doable.

                           

                          Dag, did you go through the exact sequence he put up in the first post?

                           

                          The issue is that we seem to be able to set the absolute numbers for RGB in the COLOR panel (made visible through the Window - Color) menu via choosing Grayscale and a %K value - in this case 25% sets color (192, 192, 192).  It sets these values no matter what profile the document is using.

                           

                          When that color is used on any old document, the profile is involved with reading back the number in the %K portion of the info panel.

                           

                          No profile is involved with setting the number (because this operation doesn't involve a document), a profile IS involved with reading the number from a document.  That difference is at the root of the issue Tom is sensing, I think.

                           

                          My comment about Photoshop being developed on a Mac... Note that with a profile carrying gamma 1.8, Photoshop DOES make the numbers match., probably owing to a hard-coded gamma correction in the logic of the info panel.  It might be more reasonable for it to just use the gamma derived from the profile, or just do a conversion to linear space first, but then that would change the way Photoshop works, and Adobe appears to have a policy of "keep it very consistent with past releases, so everything still works".

                           

                          -Noel

                          • 10. Re: Which one is right? The colour picker or the info panel?
                            c.pfaffenbichler Level 9

                            The issue is that we seem to be able to set the absolute numbers for RGB in the COLOR panel

                            Ah, I was wondering how the OP set the Color Picker to grayscale.

                            • 11. Re: Which one is right? The colour picker or the info panel?
                              D Fosse Adobe Community Professional & MVP

                              OK, I follow. I agree that no profile comes into play when dialing in the 25%.

                               

                              Here we go (new file adobe RGB, working gray gamma 2.2):

                              25%_gray.png

                               

                              It just has to be the gray working space, or a combination of that and something else. What are you all using for grayscale?

                              • 12. Re: Which one is right? The colour picker or the info panel?
                                Noel Carboni Level 8

                                Aha!  So the grayscale color sampling in the INFO panel uses the working grayscale profile gamma value to read out the K value, instead of trying to derive it using the actual document profile.  That seems wrong, though more likely to match than a hard-coded value.  Thus the difference in numbers is the difference in gamma between the preferred grayscale working profile and the current document profile.  When they match the numbers match.

                                 

                                My working space grayscale profile was set to the default:  Dot Gain 20% which, if I recall correctly, has a 1.8ish gamma.  I virtually never edit in grayscale, so this setting hasn't had attention.

                                 

                                -Noel

                                • 13. Re: Which one is right? The colour picker or the info panel?
                                  D Fosse Adobe Community Professional & MVP

                                  Yes, but the "K" rang a bell. Anyway, it's always nice to get to the bottom of things

                                   

                                  I think if anyone is building something that relies on absolute numbers, those numbers should be Lab. So include something that converts, or somehow extract the numbers from the color management engine (where Lab numbers are always available).

                                  • 14. Re: Which one is right? The colour picker or the info panel?
                                    tomaugerdotcom Level 1

                                    This is interesting. Am I correct in interpreting that when I specify 25% grey, as a result of my colour profile selection, Photoshop is telling me that perceptually what I will see is 31%?

                                     

                                    My understanding of the colour management role in Photoshop is to A) compensate for monitor deviance from the theoretical "mathematical absolute" as represented by the L*a*b colour space that contains the numerical data describing the colour composition of the image and B) to predict or proof for a particular output method (press, print, digital etc).

                                     

                                    If this understanding is more or less correct, then I'm confused as to why Photoshop would give me an "erroneous" dot reading in the Info palette. Even with dot gain on press, a 25% dot is a 25% dot. It may LOOK like a 31% dot because of dot gain (ie: the ink dot has spread so it covers more area, hence the result is a darker dot) but I've still specified a 25% dot in my digital file.

                                    • 15. Re: Which one is right? The colour picker or the info panel?
                                      tomaugerdotcom Level 1

                                      Actually, now that I think aboout it further, I disagree with myself!!!

                                       

                                      Whenever the value of a dot is expressed as a percentage, we're intrinsically talking about printing dots / subtractive colour model. So to me it DOES make sense now, that it would take dot gain into account when taking a sample reading, as confusing as that may be.

                                       

                                      What bothers me is that the "Actual Color" setting also takes this dot gain into account in the case of Greyscale documents. Even worse (to me) is that the luninosity channel of the "Lab" setting in the Info palette ALSO seems to not be accurate, unless the document is in L*a*b mode when you create the colour.

                                       

                                      Try this out:

                                       

                                      Create a new RGB document and fill it with exactly 50% grey (using the 50% grey fill option - SHIFT + BACKSPACE)

                                       

                                      Now look at the info palette with one of the readouts set to "Lab". Depending on your profile settings, you will probably not see 50 as your result.

                                      • 16. Re: Which one is right? The colour picker or the info panel?
                                        D Fosse Adobe Community Professional & MVP

                                        tomaugerdotcom wrote:


                                        Even with dot gain on press, a 25% dot is a 25% dot. It may LOOK like a 31% dot because of dot gain (ie: the ink dot has spread so it covers more area, hence the result is a darker dot) but I've still specified a 25% dot in my digital file.

                                         

                                        Dot gain is the difference between those 25% and 31%.

                                         

                                        Am I correct in interpreting that when I specify 25% grey, as a result of my colour profile selection, Photoshop is telling me that perceptually what I will see is 31%?

                                         

                                        Nothing so complicated. Perceptually you will see the same of course. But, as I said in my first post, the numbers are relative to whichever profile is describing that color. And here you have two profiles with clearly different gamma. The moment you specify grayscale (K), it is interpreted in the gray working space.

                                         

                                        (edited because of a wildly inaccurate dot gain calculation...)

                                        • 17. Re: Which one is right? The colour picker or the info panel?
                                          D Fosse Adobe Community Professional & MVP

                                          tomaugerdotcom wrote:

                                           

                                          Try this out:

                                           

                                          Create a new RGB document and fill it with exactly 50% grey (using the 50% grey fill option - SHIFT + BACKSPACE)

                                           

                                          Now look at the info palette with one of the readouts set to "Lab". Depending on your profile settings, you will probably not see 50 as your result.

                                          You're still not getting it. This is as expected, because the same color is then interpreted through a different profile.

                                           

                                          For the same color, numbers will change with different profiles. This is at the very core of color management. The constant in all this is the reference space, the profile connection space. This is Lab or XYZ, specified in the profile. This is what all the different numbers relate to.