6 Replies Latest reply on Apr 6, 2014 9:54 AM by oldcompositor

    Printing a scanned B&W halftone as a bitmap?

    oldcompositor

      Hi all,

       

      I have a scan of a black and white photo with a rather course halftone screen (unfortunately, I have no access to the printed original). This is going to printed, small, in a four-color book (on coated paper) that I'm laying out.

       

      It seems that the image is going to become very blurry if I try to remove the screen in Photoshop. So I thought about this: what if I just convert the image to a bitmap? Shouldn't it then print with the "original" halftone pattern (it's not going to be rescreened)? I might lighten it up first, while it's still a grayscale, to compensate for ink spread.

       

      What do members with more experience of editing images for print think?

       

      -oldcomp

        • 1. Re: Printing a scanned B&W halftone as a bitmap?
          D Fosse Adobe Community Professional & MVP

          I don't know how it's going to be printed, but taking your word that it's not going to be rescreened, I don't see why it shouldn't work in principle.

           

          But compensating for dot gain (ink spread) is not as straightforward as it may seem. Remember, dot gain compensation is originally performed in the CMYK profile based on a continuous-tone grayscale. But that's not what you have now - you already have a binary image that is either black or white. "Lightening" simply won't work, it'll still be black or white and nothing will change.

           

          So in order to lighten it you need something else, like the Photoshop "maximum" filter (below). And then you're into territory that is almost impossible to control precisely. And CMYK dot gain, moreover, is not a linear function but a curve.

           

          maximum.png

          • 2. Re: Printing a scanned B&W halftone as a bitmap?
            oldcompositor Level 1

            Thanks, twenty_one.

             

            The image is in the grayscale mode. So I can influence the end result bu applying a curve before converting it to a bitmap.

             

            It is true that I'm not going to be able to precisely control the inks spread, of course. But with this image, that isn't so critical. It's just documentary in nature.

             

            Of course, if I want to play it safe, I can blur the individual channels in the RGB mode and then convert it back to a grayscale. I just tried it, and it looks OK. B ut it would be interesting to try the bitmap approach...

            • 3. Re: Printing a scanned B&W halftone as a bitmap?
              D Fosse Adobe Community Professional & MVP

              It would be interesting, and I'd also be interested in hearing your experiences if you go ahead and do it. I've been in this situation a few times, but I played it safe and took away the halftone screen. BTW a combination of noise removal and blur is more effective than blur only.

              • 4. Re: Printing a scanned B&W halftone as a bitmap?
                oldcompositor Level 1

                OK, I might give noise reduction a try.

                 

                And I will report the results if I end up going the bitmap route. It does begin to look more and more challenging...

                • 5. Re: Printing a scanned B&W halftone as a bitmap?
                  G.Hoffmann Level 3
                  The image is in the grayscale mode. So I can influence the end result bu applying a curve before converting it to a bitmap.

                   

                  The scan may be a grayscale, but it seems to contain visible printed dots (spots, as illustrated by twenty_one).

                  In this case the application of curves doesn't help much – the correct data are recovered by strong contrast.

                   

                  If the image has to be reproduced with approximately the same line screen (lpi) as the original, then it can be

                  re-rasterized (for instance 60 lpi, 2400 dpi). Of course, corrected for dot gain.

                   

                  If the image has to be reproduced much smaller than the original, then the standard methods for de-screening

                  should be applied (blurring, de-noising, Fourier-methods).

                   

                  If the reproduced image's line screen is somewhere between, then neither method is promising, in my opinion.

                   

                  Oldcomp, can you please post a part of the scan with original scan-pixels?

                   

                  Best regards --Gernot Hoffmann

                  • 6. Re: Printing a scanned B&W halftone as a bitmap?
                    oldcompositor Level 1

                    Thank you, G.Hoffmann!

                     

                    Curves do seem to have an effect, but the end result is kind of hard to predict.

                     

                    It's going to be printed much smaller than the original, so I just used what you call "the standard methods for de-screening". Now it's even better, with noise reduction too applied.

                     

                    Anyway, I think I've learned something here - so thank you both. Of course, I would still interested if someone knows a workable way of reproducing the original dots...

                     

                    Best,

                    oldcomp