15 Replies Latest reply on Apr 16, 2012 7:13 PM by Lundberg02

    conversion from bitmap to grayscale

    gwhPoster Level 1

      Hi everyone,

       

      I have a 1 bit bitmap file but when I convert to grayscale and then on to CMYK the black CMYK components end up as:

       

      C: 75%

      M: 68%

      Y: 67%

      K: 90%

       

      I need to keep black at 100% but all the other values to be at 0%, so:

       

      C: 0

      M: 0

      Y: 0

      K: 100%

       

      Do anyone know how I can do this?

       

      Appreciate any help.

        • 1. Re: conversion from bitmap to grayscale
          c.pfaffenbichler Level 9

          Copy the grayscale image and paste it into the black Channel of an identically dimensioned CMYK-image.

          • 2. Re: conversion from bitmap to grayscale
            gwhPoster Level 1

            Thanks for the reply,

             

            Yes that worked but the problem has returned as I took the next steps. After pasting into the black channel the black was 100%. After that I inverted the colours (on the layer - not in the channel) so that what was once black is now white and what was once white is now black. I then needed to colour the white parts a certain CMYK value so I created a blank layer at the top and filled it with the colour and then changed the blend mode to multiply. This coloured the white successfully but the black shade has now changed to:

             

            75%

            77%

            100%

            100%

             

            Is there a way to bring the black back to what it was before? Or am I going about the process the wrong way?

             

            Appreciate any further help.

            • 3. Re: conversion from bitmap to grayscale
              c.pfaffenbichler Level 9

              You may have to use a Layer Mask on the multiplying Layer – namely the inverted grayscale image.

              And if you need to trap the color you could use Filter > Other > Maximum to spread it a bit.

               

              You could also use Blend if-settings, but that woud not be ideal for trapping in my opinion.

              • 4. Re: conversion from bitmap to grayscale
                gwhPoster Level 1

                Thanks - a layer mask solved the problem.

                 

                To avoid trapping, do you think I should use a rich black instead. If yes, do you know what the ideal values are?

                • 5. Re: conversion from bitmap to grayscale
                  c.pfaffenbichler Level 9

                  That depends on the image and how large the black areas are.

                  For fine linework I would avoid rich black as registration might become more of an issue, but with large areas of black it might provide a better result (a somewhat deeper black).

                   

                  I don’t know ideal values (which may depend on process and paper used), but we often use something along the lines of 50/40/40/100, with the cyan above magenta and yellow to »cool« the tone.

                  • 6. Re: conversion from bitmap to grayscale
                    gwhPoster Level 1

                    Ok great - thanks again.

                    • 7. Re: conversion from bitmap to grayscale
                      c.pfaffenbichler Level 9

                      Another option to keep in mind would be to use the bitmap image itself in a layout program – if one needs to trap the image one could use a duplicate of the image that has been choked and is set to knock out.

                      This should provide a better result with regard to sharpness.

                      • 8. Re: conversion from bitmap to grayscale
                        gwhPoster Level 1

                        In my case the bitmap is being placed into Illustrator to be used as part of a larger layout. I had originally wanted to colour the bitmap in Illustrator but I couldn't find a way to do it. I'll keep in mind though about placing in Indesign for future jobs.

                         

                        If you happen to know how to colour it in Illustrator, it would be really useful though.

                        • 9. Re: conversion from bitmap to grayscale
                          c.pfaffenbichler Level 9

                          You should be able to set the Fill Color for a bitmap in Illustrator (at least in CS5).

                          • 10. Re: conversion from bitmap to grayscale
                            gwhPoster Level 1

                            Sorry - just tried it and it worked even though it didn't before. I think I must have placed an incorrect file format before.

                             

                            Thanks again

                            • 11. Re: conversion from bitmap to grayscale
                              emil emil Level 4

                              gwhPoster wrote:

                               

                              Thanks - a layer mask solved the problem.

                              You could have also hold Ctrl and click on the icon of K channel in the Channel panel to load it as a selection, then fill (Alt + Delete) with the desired foreground color - no multiplied mode needed.

                              To avoid trapping, do you think I should use a rich black instead. If yes, do you know what the ideal values are?

                              If your other color contains black,  this will reduce the need for trapping especially if it is darker. And you don't need trapping if using rich black. What is the reason you don't want rich black? Pure black always look very dull on print.

                              The ideal values for rich black are set in the color profile of the printer with regard to best possible color match when converting from other color spaces and the maximum ink coverage allowed on the different media. One way to check the conversion values of various color profiles (color spaces) is to fill an RGB image with black color (R=0 G=0 B=0), open the info panel and if you like the color panel and set its sliders to CMYK mode, choose Edit > Convert to profile and in the Convert to profile dialog, check CMYK and the Preview button, then select the various color profiles, position the mouse on the black in the image, and check the converted CMYK values in the Info panel or the Color panel after clicking the black on the image. I'm not sure about macs but if you are using Windows, once you select a profile from the CMYK menu, you can use the down arrow to change them while your mouse stays on the image checking colors.

                               

                              Regarding editing colors of raster images in Illustrator, you have to first embed the image - you can't edit colors of linked images

                              • 12. Re: conversion from bitmap to grayscale
                                c.pfaffenbichler Level 9

                                Regarding editing colors of raster images in Illustrator, you have to first embed the image - you can't edit colors of linked images

                                That does not seem to apply to bitmap (1bit) images.

                                • 13. Re: conversion from bitmap to grayscale
                                  emil emil Level 4

                                  Oh, I didn't pay attention that you guys were talking about filling with color. I was referring to the color editing choices in the Edit > Edit Color menu where you can do things like Invert. Yes, grayscale and bitmap linked or not can be filled with a color and after that the Recolor artwork from the Edit Menu can be used for finding the best spot color match from a library.

                                  • 14. Re: conversion from bitmap to grayscale
                                    gwhPoster Level 1

                                    Thanks emil emil for the alternative explanation. Much appreciated.

                                    • 15. Re: conversion from bitmap to grayscale
                                      Lundberg02 Level 3

                                      Wow, I haven't seen anyone talk about trapping and choking in ten years in these fora.

                                      Shows how much web futility is always on offer here.