9 Replies Latest reply on Mar 25, 2009 12:11 AM by Lou Dina

    RGB to CMYK to commercial printer

      We are using CS2 InDesign and Photoshop 7 on a PC. I am putting together two
      large books. One insert section of the first book has color photos. I have
      to prepare these color photos for a printer located in China. They want all
      images to be CMYK .tiff.

      I have an assortment of about 100 images from various sources around the
      world, there are .tif, .jpg. .bmp, and .psd. All of them appear to be RGB.
      There are images with: 1) no color profile, 2) sRGB IE60966-2.1, and Adobe
      RGB 1998. Do all images need to have the same RGB color profile before
      converting to CMYK?

      I have been converting all of these to .tif (and using a single dpi/pixel
      resolution for
      all images).

      Do I just select/save as color mode CMYK? Or are there special
      settings/profiles I need to know about. Is there anything special for China?
      The printer's English is not that clear . I was hoping that someone in this
      group would be familiar with RGB to CMYK conversion.

      Also, it appears that the final PDF pages (with embedded Tiff images) will
      be very large. The first complete book for this project (there are two
      books) looks like it will be about 20GB. The second book looks like it could
      be 12GB. Both books together equal about 1,000 pages and have about 1,000
      images. 90% of the images are grayscale. What are the current technologies
      for sending large projects like this to a printer?


      ps, the Photoshop group suggested that I write to this group.
        • 1. Re: RGB to CMYK to commercial printer
          Lou Dina Level 3
          Tech Illustrator....Conversion to CMYK can be a complicated subject, especially for the uninitiated.

          First, whenever a conversion takes place, there is ALWAYS a source profile and a destination profile. If the original file arrives "untagged" with a profile, the one is "assumed" in Photoshop (or InDesign). It's like assuming someone is speaking French, Italian, or Spanish...you need a reference point. You can look at the color settings in these programs to see what the default setting is for both RGB and CMYK. If your defaults are Adobe RGB and US Web Coated SWOP v2, the your untagged RGB file will be assigned Adobe RGB for the purposes of conversion. If this is a correct guess, then you are half way there. If it is an incorrect guess, your color will be off. These days, sRGB or Adobe RGB are the most likely guesses, since they are the most widely used. When you bring your files into PS, try assigning sRGB, Adobe RGB, Apple RGB and ColorMatch RGB, and tag each image with the one that looks best to you. Images can have different RGB profiles, and depending on the source, that is very possible. As long as they all end up with the same (correct) flavor of CMYK, you should be okay, but they need to look correct in RGB before conversion.

          The destination profile (press CMYK) is another matter. It would be best if the printer can give you a profile to use. Just saying "give me CMYK files" is a bit like talking and not know what language is being spoken. If they do not have a custom profile, then ask for an "industry standard" profile to use as a target, such as US Web Coated SWOPv2, ISOCoated, etc. If you are printing onto coated stock, you want a coated profile. If you are printing onto UNcoated stock, you want an uncoated profile. If printing in the US on coated stock, US Web Coated SWOP v2 will get you in the ballpark, but it depends on how close the press approximates that standard. In Europe, ISO Coated or Euroscale Coated are decent bets. In Asia, I really don't know.

          If you use the "Mode" command to convert to CMYK, it will be converted to the CMYK profile that is set as the default in Color Settings. It is more flexible to use "Convert to Profile", then you can select the profile and rendering intent you want. Sorry, this stuff gets complicated, especially at first.

          Depending on the line screen (ie, dots per inch) the printer is using, you may need different resolution files. Assuming high quality files, and a line screen between 133 and 175, you probably want to deliver 300 PPI (pixel per inch) files, which is the normally requested resolution most of the time.

          I'd do the conversions to CMYK in Photoshop, then place those converted files into Indesign. There is a lot to know and a number of places you can trip up, so this is a cursory overview.

          I'd start by getting some guidance from your printer, the specs they use, or whatever. They may or may not have any. If not, then ask if they try to adhere to North American, European, or Japan standards. This may be a helpful clue. The more info you can get the better.

          If they don't give you anything, then you may want to run a short press test first (before doing an entire book run) by converting a few images to different profiles. See which gives you the best results and then use that.

          We've only just touched the surface of this complex subject. If you are overwhelmed, see if you can work with a prepress service.

          • 2. Re: RGB to CMYK to commercial printer
            Lou Dina Level 3
            Sorry, I neglected to address your grayscale images. You can print grayscale a number of ways....

            1. As pure grayscale using only a single black ink.

            2. As CMYK using a "Rich black", ie, black boosted with CMY inks for greater depth and smoothness.

            3. As duotones, tritones, etc, using multiple black/gray inks, or even adding a warm or cool ink for slight toning.

            If printing straight grayscale, you can convert to grayscale in Photoshop. Most common is 20% dot gain, but that is no guarantee that you images won't come out too dark or light. It depends on the paper, press, ink, process control, etc. Ask the printer if they know the dot gain for their press. Hopefully, they can give you an accurate, honest answer. Straight grayscale won't normally be as smooth as a rich black, but it should be neutral, since you are using only black ink.

            For B&W images that need to be neutral, you Rich Black should have a heavy dose of black inks, with lesser quantities of CMY inks. Again, a heavy subject unless you are familiar with CMYK, separations, etc. Again, I'd try to find out the standards they strive to meet, then come back with additional information.

            • 3. Re: RGB to CMYK to commercial printer
              (Marco_Ugolini) Level 1
              To what Lou writes, I would add that if you use CMYK inks for grayscale or duotone images, you will achieve more-stable results if you use a profile with a high GCR (Gray Component Replacement) in your conversion. A higher GCR means that, by reducing the amounts of chromatic inks (C, M, Y) and substituting them with black ink (K) in areas of the image that ought to look neutral on press (or that contain a neutral - 'gray' - component), it will become far less likely that any slight imbalance in the printing process would cause an unwanted color cast in the final printed results.

              You can build higher-GCR custom versions of standard ICC CMYK profiles by means of a professional profiling application like XRite's ProfileMaker.

              If you do not have the profiling application, or access to it, then use the standard profile, but remember to watch out for color casts in your grayscale output on press.

              Whatever you do, use ICC profiles and the 'Convert to Profile' command in Photoshop, including an appropriate choice of rendering intent and black point compensation -- and stay away from Photoshop's Custom CMYK engine, which I do *not* endorse in any way, shape or form.
              • 4. Re: RGB to CMYK to commercial printer
                Level 1

                Thank you so much.You have given me so much information. I don't feel
                completely lost now. I am in a much better position to talk with the

                • 5. Re: RGB to CMYK to commercial printer
                  Level 1
                  Thanks for more info. The book(s) will have one
                  section (a complete signature) in color. The rest of the book will be black


                  I did some sample CMYK Convert to Profile
                  using U.S. Web Coated (SWOP) v2? I tried U.S. sheetcoated v2, but the
                  images seem to me to be a bit dull with a bit too much magenta.


                  I am having a problem getting rendering
                  intent info from printers. Is there a basic rendering intent that I
                  can select that will be useable by most printers?
                  Do I
                  need to set the Intent in Photoshop, or can this be set when the PDF export is
                  made from InDesign?
                  • 6. Re: RGB to CMYK to commercial printer
                    Lou Dina Level 3

                    In the conversions, are you referring to conversions of B&W or color images? I'll assume color for the moment.

                    Assuming your monitor is properly calibrated, and assuming you are viewing your color images on your monitor, your conversions should keep "in gamut" colors the same. You will, however, probably see some loss of dynamic range, color, and saturation as you enter CMYK. If the color and dynamic range of the original is all within the destination CMYK gamut, you should see very little, if any, shift. The sad fact is that CMYK on press has limited dynamic range and color, especially compared to a bright monitor. This is generally true, though CMYK can print some colors that lie outside the typical monitor's gamut.

                    If you have a brightly colored original in RGB mode, then you should experiment with both perceptual and relative colorimetric rendering to see which one looks best. This is an image by image thing. Relative colorimetric will reproduce all "in gamut colors" as accurately as possible, then will take out of gamut colors and bring them to the closest printable color. This can cause loss of detail and 'piling up of colors' as you near the boundaries of the image's color gamut in the destination CMYK space. Perceptual scales ALL colors and preserves the relationships between colors, which sacrifices color accuracy and saturation, but sometimes looks more natural, especially if there are a LOT of out of gamut colors. Pick whichever ones looks best overall.

                    Then, once in CMYK, make minor curves adjustments to tweak if for CMYK. Just be careful not to push it too far, especially in the deep shadows, since you may push beyond the ink limit of the press. For example, if you convert to US Web Coated SWOP v2, the ink limits are set at 300 total. If you drastically darken the shadow areas, you could end up with an ink limit of 320, 330, 350, etc. Knowing the ink limit of the press will help you stay within those limits. Final tweaking in CMYK is usually a good thing.

                    How did you determine that the images were dull with too much magenta? Did you print a proof on a calibrated printer, or is this just your screen display? Or is this from a printed image off the press? I have a well calibrated monitor and accurate custom profiles for my inkjet. If I am sending a job to a press that supposedly prints to US Web Coated SWOP, I can proof that file on my inkjet and get a very good match. We need more information to know exactly what you are doing and how you are assessing your images.

                    Normally, I do the conversion in Photoshop, using the rendering intent that looks best. Then I tweak the image in CMYK to get it looking its best. I usually have the press profile, but sometimes work with a standard profile if no custom profile is available. I leave the image tagged with my conversion profile. Then, I usually place these images into InDesign, with the profile intact (color management turned on in InDesign). I right click on the placed image to be sure that the profile and rendering intent are set to what I want. Then, I can either supply the InDesign file to the printer, or convert to PDF (leave color unchanged) and make sure that all profiles are included. InDesign should export each image to the PDF properly, along with profile and intent. I have never sent to job to press using Photoshop...only InDesign or Illustrator.

                    BTW, rendering intent is only used when getting an image from one color space to another, such as a conversion from RGB to CMYK. You do that, choosing the one that looks the best. Once it is in the destination CMYK space, rendering intent is no longer needed, since all the colors and tones have already been remapped into the final space. Rendering intent is just used to help us handle those out of gamut colors.

                    Hope this helps.

                    • 7. Re: RGB to CMYK to commercial printer
                      Level 1
                      Hi Lou,

                      I ended up making all the RGB images Adobe RGB (relative intent) and
                      converting the profile to CMYK US Web Coated SWOP v2. I sent several images
                      (both the original RGB and the converted Tiff) to the printer and he said I
                      did good and everything is usable.

                      Lou, you are a wonder. Thank you. The information you provided to me was
                      complex, but it was 100% clear and easy to follow. If you are not a writer,
                      you should be.

                      • 8. Re: RGB to CMYK to commercial printer
                        (Marco_Ugolini) Level 1
                        >I ended up making all the RGB images Adobe RGB (relative intent)

                        Any conversion between RGB working spaces (RGB to RGB) only uses the Relative Colorimetric intent, since they are matrix-based profiles (as distinguished from profiles that are based on Look Up Tables - LUTs - which, in turn, support all intents).

                        The only RGB working space that I know of which uses LUTs is PhotoGamut RGB ( ) -- otherwise AdobeRGB, ProPhoto RGB, ColorMatch RGB, sRGB and all standard RGB working spaces are matrix-based, and because of that are restricted to using just the Relative Colorimetric intent.
                        • 9. Re: RGB to CMYK to commercial printer
                          Lou Dina Level 3

                          Glad it was clear and helpful. One last thing.

                          If possible, get your commercial printer to print and mail you color proofs to sign off on before the job is run. These are called "contract proofs" for a reason. It shows you what the job should look like when it comes off their press, and it is their job to match the press run to their proof, within tolerances allowed by the standard they are following (if any). They should have some statement on their website. The press run is rarely exactly the same and you may see some differences in density, color, contrast, etc, but it should be close. When possible, I attend the "press check" right before the job is run and I work with the printer to make final adjustments to get it as close as possible. This usually involves a few compromises. If the final printed job migrates too far from their proof, they will have to reprint the job at their expense, but it has to be beyond the leeway allowed. I am assuming a press check will not be possible due to distance, so color proofs supplied by the printer are even more important, especially on a job of this size.

                          Ask for their tolerances, criteria for accepting or rejecting the job, etc. They may say they adhere to SWOP standards, or whatever. Hopefully, you will not have any need to worry about this, but it could save your butt if there are any disagreements over the finished job.

                          Since it is a book with lots of pages, they should also provide you with a mockup of the finished book to be sure images, page order, text and other elements are as they should be. This may be a low rez, B&W or blueline document to keep cost down. It just helps to make sure all elements are laid out properly, with proper bleed, etc.

                          Best of luck. Let us know how it turns out.