7 Replies Latest reply on Oct 19, 2010 6:47 AM by Rebekkah Angel

    CMYK to RGB to CMYK again...why?

    Rebekkah Angel

      I'll be the first to admit - I am relatively new and inexperienced when it comes to color management.

       

      I am a prepress tech - 6 years now or so. Been in printing for 24 years - was a conventional stripper before that.

       

      Not a complete dummy but I was never exposed to the old ways of electronic prepress -

       

      My question is a "why?"

       

      We have a PDF based workflow running Trueflow.

       

      Because we have an older version of Trueflow that requires specific settings in order to accept PDF files, I have a certain criteria that supplied PDFs must meet.

       

      I don't always get them that way - so I have a preflight routine where I open all PDFs and check them for spots/RGB/Lab color swatches, etc and convert as needed to CMYK. I then flatten them in PitStop extreme and run them through my rip.

       

      We do not have the ability to proof with ripped PDF files - therefore, when a PDF proof is required, I want it to be the print file that will be used to output to plates.

       

      OK - now you have a little background about my methods -

       

      Here is the question.

       

      My client was supplied a CMYK vector PDF ad.

       

      Artist for the client opened ad in photoshop, rasterized it and converted it to RGB, placed it in his Quark document and sent the 16 page PDF file to me

       

      I received it, noted that the file was full of spots and RGB and did a conversion back to CMYK in pitstop

       

      We sent a PDF proof to the client, who approved it (for the record, when we PDF proof, we assume that color is not critical otherwise the client would have asked for a matchprint)

       

      OK - so we print the booklet and client is very unhappy with the color on the back page.

       

      Artist says that we should have let the rip convert the RGB file to CMYK instead of doing it myself - like I trust my rip to do what it should - I don't - it has made a mess of things in the past. I know what condition my files have to be in before they go in the oven. I don't take chances.

       

      My question is, why would he convert it to RGB in the first place? I am completely lost on this one. If I am supplied a CMYK file, what reason is there for converting it to RGB before placing it into the document?

       

      Can anyone explain why this would be necessary?

       

      Thanks in advance!!!! I'm sure there are some gurus out there who can make sense of it in explaining it to me.

        • 1. Re: CMYK to RGB to CMYK again...why?
          thedigitaldog MVP & Adobe Community Professional

          Rebekkah Angel wrote:

          My question is, why would he convert it to RGB in the first place? I am completely lost on this one. If I am supplied a CMYK file, what reason is there for converting it to RGB before placing it into the document?

           

          Can anyone explain why this would be necessary?

          CMYK is a highly device dependent color space, meaning its optimized for one kind of output. CMYK to CMYK conversions using ICC profiles can be very dangerous. The black generation can be totally hosed unless you are working with device link profiles (I can explain further if you wish). So if you don’t have the original RGB data, if all you have is this highly device dependent CMYK and you need to end up with another flavor of CMYK, you end up having to convert the CMYK to RGB, then conducting the CMYK conversion again. Bottom line is, for those that need flexible output workflows, having CMYK sucks! Having RGB means you can produce any number of CMYK conversions from the initial RGB data.

           

          ALL capture devices produce RGB by the way, so at some point, there should have been the source RGB data. Why that’s missing in some workflows is a good question, it should be archived.

          1 person found this helpful
          • 2. Re: CMYK to RGB to CMYK again...why?
            p taz Level 3

            The reason RIP's fail on RGB>CMYK conversion in prepress is that the RGB is not tagged with a profile or that the RGB has out of gamut (cmyk gamut) colours and there is nothing that the RIP can do about that.

             

            The reason that humans do it better is that the human can do a subjective conversion with a choice of source profiles to choose from.

             

            If you have a correct, tagged RGB file and a correct conversion in the RIP and a correct expectation of CMYK outcome compared to RGB then you have a correct workflow.

             

            Miss any one of these steps and you will lose some!

            1 person found this helpful
            • 3. Re: CMYK to RGB to CMYK again...why?
              Rebekkah Angel Level 1

              OK - I am going to try to digest all of that - thanks for the quick responses.

               

               

              My next questrion is naturally going to be what can I do to successfully convert prior to ripping the files?

               

              Or should I only concern myself with spot and lab swatch conversion and leave the RGB images alone? These were images and art - lines going across the bottom of the page that turned muddy - however - viewing the files side by side, the CMYK vector file was vibrant - the faces in the photos were full and had good contrast - the RGB file was washed out and rusty looking on screen. I guess I am not accustomed to trusting machines andf software to blindly undertake tasks when I can't see the result.

               

              For the record, RGB spots generated in newer versions of of InDesign have been known NOT to convert in my rip, generating a fifth plate. That's why I can't take chances with it - that would mean remaking 4 more 30 x 40 plates once that is discovered.

               

              We are upgrading to the newest version soon - so hopefully all of this dreadful preflight will be less of a chore then.

               

              Any further suggestions will be welcomed.

              • 4. Re: CMYK to RGB to CMYK again...why?
                p taz Level 3
                Or should I only concern myself with spot and lab swatch conversion and leave the RGB images alone? These were images and art - lines going across the bottom of the page that turned muddy - however - viewing the files side by side, the CMYK vector file was vibrant - the faces in the photos were full and had good contrast - the RGB file was washed out and rusty looking on screen. I guess I am not accustomed to trusting machines andf software to blindly undertake tasks when I can't see the result.

                Always use Lab references for spot conversions, the other option is rubbish!  This is a common source of trouble that we addressed at my workplace and if the default conversion was used the colours desaturated horribly.

                 

                As for faces and photos, do them manually since the RIP can only do a blind conversion.  You will easily make them look better than that.

                • 5. Re: CMYK to RGB to CMYK again...why?
                  Rick McCleary Level 3

                  My next questrion is naturally going to be what can I do to successfully convert prior to ripping the files?

                   

                  ...the CMYK vector file was vibrant - the faces in the photos were full and had good contrast - the RGB file was washed out and rusty looking on screen...

                   

                  You mention that you're somewhat new to color management. Performing conversions from one color space to another is fairly straightforward, but requires a good working knowledge of what's happening under the color management hood when you hit the "Convert" button.

                   

                  In the your case (face tones are vibrant in CMYK and washed-out in RGB), there is a mis-step somewhere in your conversion process. Skin tones are within the gamut of virtually every color space - RGB or CMYK. You should be able to make a seamless conversion from one color space to another without any perceptible change in skin color. If you walk us through the steps/settings you use to go from one color space to another, we can probably help to troubleshoot the problem.

                   

                  One the spot color issue, p taz is right on the money in recommending L*a*b as the reference space when identifying absolute colors.

                  • 6. Re: CMYK to RGB to CMYK again...why?
                    warzenbeisser Level 1

                    From what you describe in the original post, I get the feeling that the wrong question is being asked. In the particular case, there was probably no need at all to convert (or maybe even rasterize) the PDF you supplied. If the client's artist only needed to make minor changes to an existing layout, he or she would obviously have been better off applying those changes to the original layout document (InDesign, QuarkXPress) to conserve editable vector data and the source colours of any pixel images. Only if the original layout document was not available or had been modified colour-wise at a later production stage would I have used a PDF as the basis for editing the ad. Maybe the artist inadvertently made the RGB conversion when rasterizing the PDF? As you supplied the PDF and your company ultimately did the printing as well, I see no reason why the data would have to be converted to RGB and back to CMYK; the artist could have easily left everything as is and just added the modifications.  What flavour of PDF did you supply? For your CMYK safe workflow, I guess PDF/X-1a would have been the most appropriate one*. Then the artist could work in the device space as you do or convert any new RGB content to the CMYK space as specified in the PDF/X output intent. I would recommend to communicate who does what before the layout work rather than after printing and running across unpleasant surprises. Redundantly converting colours back and forth without a clear and strict process and colour management is a very bad idea in my opinion, and as you saw, it can yield quite unsatisfactory results. It does take more effort to set things up properly, but in the end it tends to cost less and deliver (far) better results than troubleshooting and re-printing a screwed-up mess.

                     

                    * If your Trueflow system requires different, non-standard PDF settings, then things get more complicated. Your company's management may want to consider whether the equipment on hand is really up to the tasks it is supposed to perform ...

                    • 7. Re: CMYK to RGB to CMYK again...why?
                      Rebekkah Angel Level 1

                      "From what you describe in the original post, I get the feeling that the  wrong

                      question is being asked. In the particular case, there was  probably no need at

                      all to convert (or maybe even rasterize) the PDF you  supplied."

                       

                      Thank you - I totally agree - I could not understand why he (the client's

                      artist) took a CMYK vector file, rasterized it and converted to RGB, then sent

                      it to me knowing that it would be converted BACK to CMYK - logic tells me, leave

                      it alone, place it into the document as is - if color is critical, then they

                      should ask for a matchprint - and they did not -

                       

                      Our older version of Trueflow requires a flattened PDF - however, due to

                      transparency flattening issues, we ask for a non-flattened PDF - then I do all

                      flattening myself with pitstop extreme - it seems to eliminate the majority of

                      issues. We are upgrading to the newest version in a matter of weeks -

                       

                       

                      Thank you very much for all of your replies - I am about to do a comparison

                      ripped color proof using the RGB file (which will convert when ripping) and the

                      original CMYK files that the artist was supplied from his advertisers to see

                      what, if any, color differences we come up with. I believe I may even go so far

                      as to leave them unlabeled and let them tell me which color they prefer. Sort of

                      a blind "taste test", if you will.