9 Replies Latest reply on Oct 7, 2010 11:04 AM by mark_anon

    CMYK to CMYK conversion Dmax warning

    mark_anon Level 1

      Found an error in the way Photoshop converts from one CMYK profile to another CMYK profile....

       

      When converting from RGB or LAB to CMYK, photoshop will follow the Dmax settings of the CMYK profile you are converting to.  However, if you convert from a CMYK file with a high Dmax to another CMYK profile with a lower Dmax setting, your new file will NOT have the lower Dmax.

       

      For example, If I convert a CMYK file with a 340 Dmax to a CMYK profile with a 300 Dmax, the colors will shift but the new file will have a Dmax of 340, NOT 300.

       

      The only solution to this is to convert from CMYK to LAB, then back to your new CMYK profile.  Then photoshop will use the correct Dmax settings.

       

      I know you shouldn't convert from CMYK to CMYK, etc etc.  But sometimes it isn't avoidable for various workflow reasons.  And I understand why photoshop is unable to convert to the new Dmax when you're going from CMYK to CMYK.  But, I feel that this is such a critical color issue that photoshop should warn you that to get a propper Dmax, you can't go from CMYK to CMYK.

       

      It seems that anyone who is working with CMYK should be extremely concerned about Dmax, so for Photoshop to not warn you about this issue is to invite serious color problems which may not be caught until you're going to press... which is a very stressful time to make such discoveries.

       

      Thanks!!!

        • 1. Re: CMYK to CMYK conversion Dmax warning
          Level 7

          Photoshop has no choice but to use the dmax from the destination profile.

           

          For example, If I convert a CMYK file with a 340 Dmax to a CMYK profile with a 300 Dmax, the colors will shift but the new file will have a Dmax of 340, NOT 300.

          That sounds like you re-tagged the file but didn't do a conversion.

           

          Again, Photoshop CANNOT do a conversion that violates the destination profile.

          • 2. Re: CMYK to CMYK conversion Dmax warning
            mark_anon Level 1

            Hey Chris, thanks for the prompt response.

             

            I'm quite sure that we are converting, not assigning or anything else.  So, this is quite puzzeling.  Could it be an error in our CMYK profile?

            • 3. Re: CMYK to CMYK conversion Dmax warning
              Level 7

              Yes, it could be an error in your CMYK profile.

              Again, Photoshop can't give you values beyond the limits in the profile.

              • 4. Re: CMYK to CMYK conversion Dmax warning
                mark_anon Level 1

                Huh.  The thing which lead us to believe this was an issue with photoshop is that images converted from RGB to our CMYK profile all conform to the 300 dmax of that profile.

                 

                We have a different CMYK profile which occationally must be converted to our main 300 Dmax profile... but this lesser used profile has a Dmax of 320.  When we do a straight conversion from the 320 profile to 300 profile , the Dmax does not change. (using the sample tool in areas of dark brown, the Dmax stays about 320)

                 

                But, when we go from from the 320 CMYK to LAB or RGB, then to the 300 dmax CMYK profile, there are no Dmax issues. 

                 

                Huh.

                • 5. Re: CMYK to CMYK conversion Dmax warning
                  Level 7

                  Again, if you do a conversion, the dmax HAS TO CHANGE to correspond to the destination profile.

                  If you just assign the new profile, then the color will change, but the values (and dmax) will stay the same.

                   

                  If you converted to the new profile, from any other profile, then the dmax would have to match the destination profile -- it has no choice, because there are no values in the destination profile that don't have the dmax of the destination profile.

                   

                  Again, it really sounds like you aren't doing CMYK -> CMYK conversions, but just assigning the destination profile (and not changing the values).

                  • 6. Re: CMYK to CMYK conversion Dmax warning
                    mark_anon Level 1

                    Chris-

                     

                    I figured out what is going on... we're both right.  Photoshop does properly change the CMYK profile and dmax when you are converting to a new profile, IF the file you are converting from has an embedded profile.

                     

                    However, it is fairly common in closed CMYK workflows to not embed the icc profile due to conflicts with other parts of the process.  When you try to convert an image that does not have an embeded profile, Photoshop assumes the "mystery" image has the working CMYK profile.

                     

                    Since the assumed profile, and the destination profile were the same, it did no conversion  (or merely assigned the profile) even though you had selected "Convert to ...."

                     

                    This resulted in images which were outside the Dmax of the CMYK profile we had converted to....because it didn't actually convert.

                     

                    There are 3 ways around this.

                    (1) You can include the embeded profile, and Photoshop will do the propper conversion.

                    (2) You can Change your working CMYK profile to the profile of the image you are converting, and photoshop will assume (correctly) that it is the working profile.

                    (3) You can convert the image to LAB or RGB and then to the desired CMYK profile, however this will only work properly if you tell photoshop which profile you are starting from... otherwise it may assume the incorrect profile.  This will end up with an image which conforms to the desired color space, however you may get extremely undesirable color shifts due to the incorrect assumptions about the initial color space.

                     

                    So, the question becomes, is this a case where Photoshop should protect the user from themselves?  Should photoshop be checking when it converts an image with no embeded profile to make sure the resulting file conforms to the destination color space?  It sort of does with the profile warning dialogs which are turned on by default... but everyone I know turns those off as soon as they start using photoshop. 

                     

                    Does this happen in RGB too?  For example, if my working RGB profile is sRBG, is photoshop going to assume an unknown color profile to be sRGB?  So when I output for web use, is it actually converting an unknown color profile to sRGB, or is it merely assigning like it does with CMYK?

                     

                     

                    I'm pretty sure this is what's going on, let me know if I've made any mistakes in my assumptions.

                     

                    Thanks!

                     

                    -Mark

                    • 7. Re: CMYK to CMYK conversion Dmax warning
                      mark_anon Level 1

                      I guess I would prefer that when you're asking photoshop to convert from an unknown color profile, it would warn you and ask you to select both an original color profile and a destination profile before performing the conversion.

                       

                      But again, perhaps you feel that's an issue covered in the profile mismatch warning dialogs in the color preferences.  (which were turned off)

                      • 8. Re: CMYK to CMYK conversion Dmax warning
                        Level 7

                        Yes, that's what the mismatch and missing profile warnings are for.


                        • 9. Re: CMYK to CMYK conversion Dmax warning
                          mark_anon Level 1

                          Chris, I think a slight modification to the "Convert to profile" dialog options would make it much easier for the user to avoid making mistakes when they are converting profiles....  Basically, In the case of a missing profile, I think photoshop should give you a dialog box which is a hybrid of the convert and assign profile dialog boxes.  Here is a sketch of what I think it should look like.  Again, this would only be in the instance of a missing profile... no need for this if the source profile is known.  But it would greatly reduce the risk of making faulty assumptions when converting images with missing profiles.    Thanks again for all you thoughts and help on the issue.

                           

                          Missing_Convert.png

                           

                          -Mark