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  • High Resolution for Print

    when a 600ppi to 1200ppi image is sent to the RIP and outputed to even a larger line screen of 170 lpi the excess resolution is only going to slow down the RIP not improve the printed peice. I was reading some press specs on the web and some specs showed a 300 line screen, so here the 600ppi image may be used, but still a 1200ppi image seems too much. The human eye can only distinguish a little over 200 tonal values-or so I have read and it takes 2.5 of these tones just to change just 1% in screen value. I asked one presenter this question at the Adobe Cretive Suite Seminar from SkillPath. He said that the request for art above the printers max line screen is unnecessary. But that still leaves me wondering why large publications that I have submitted ad work for requested such high res images.

  • #2
    Re: High Resolution for Print

    As far as scanning more contone pixels than you have halftone dots, you can look at it this way: the larger your contone pixels (the lower the resolution), the more likely it is that you'll have a sharp difference between one contone pixel and the next, as each contone pixel covers a larger area of the original image. If a pixel boundary hits inside a dot, you'll get a
    jaggy in the dot itself. The bigger the difference in contone values on either side, the sharper the jaggy corresponding to the edge of the pixel will be. Large sharp features are fairly easy to notice.

    On the other hand, if you have lots of contone pixels, you'll only get small steps going across the halftone dot, and so each step will be less noticeable.

    The upshot is that a halftone screen can actually represent more contone pixels than the stated line frequency, which is why the "scan at 2x line frequency @ 100%" rule of thumb works. In practice, for most subject matter you can actually go to a dpi that's 1x the lpi (i.e. 150 dpi at 150 lpi) if the substrate is uncoated. However on coated substrates you may see the "jaggies" or pixelated edges, if the subject matter has sharp transitions as with architectural subject matter. 2x the lpi (i.e. 300 dpi at 150 lpi) will give you enough contone pixels so that you don't have the jaggies/pixelated edges. There is no real reason to have contone images at a higher resolution.

    Many, if not most, RIPs are set by default to resample contone images to 300 dpi - as if, by default the screening would be 150 lpi. The result is that at 175 lpi the scan ends up slightly less that the 2x rule. But it usually does not cause an issue - except perhaps with FM screens that, being very high frequency will resolve the pixels in the contone image and give you jaggies/pixelation.

    You can try a test on your next printed job that has some unused off-cut area on the sheet. It is much easier to understand if you can see the difference. Just add an image at different resolutions. If it's printed at 175 lpi try 125 dpi, 175 dpi, 250 dpi, 350 dpi, and 600 dpi. Make sure the RIP is set to not resample the images. If you have access to a halftone proofer you can do the same test without going to press.


    best, gordo

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    • #3
      Re: High Resolution for Print

      Gordon, your answer was really above me and I actually cannot tell if you answered my question. I will keep looking at what you wrote but the "contone pixel" language threw me and it seemed that you thought I was referring to scanning an image at one point. What I really wanted to know is why some printers will ask for higher resolution images than their line screen suggest is adequate: ex. a request for 1200ppi image for 150lpi press work. one person said that they just want more to work with but I think there has to be more to it. No doubt you answered my question, but I cannot tell. Thanks in advance for you help.

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      • #4
        Re: High Resolution for Print

        maybe they are using another screening method. hybrid or fm.

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        • #5
          Re: High Resolution for Print

          Sorry Russell,

          I'm assuming that you are referring to printing on a printing press rather than printing to an ink jet proofer (or press)

          I think there are may be two issues in your question.
          1) the resolution of an image that will be halftone screened relative to the halftone screen frequency (lpi)
          2) the resolution of the output device needed to achieve enough gray levels at the requested halftone screen frequency (lpi)

          I tried to answer the first. You wrote: "when a 600ppi to 1200ppi image is sent to the RIP and outputed to even a larger line screen of 170 lpi" "He said that the request for art above the printers max line screen is unnecessary." " request for 1200ppi image for 150lpi press work"
          The bottom line is that you do not need an input image (scan, digital pic) to have a resolution higher than about 2x the lpi of the halftone screen that will be used to print it. The issue is not about gray levels but about the halftone screen being able to resolve the pixels of the original image. If the image resolution is too low the halftone will resolve the pixels which you will see as jagged pixelated edges in the image. Increasing the resolution of the original image makes the pixels smaller relative to the halftone and so you don't see the pixels anymore. A resolution past 2x the lpi (i.e. more than 350 dpi for a 175 lpi screen) is, as you put it, unneeded.

          If you are asking about the resolution of the output device in order to support a particular halftone screen, e.g. 1200 dpi CTP device used to image a 150 lpi halftone, then that is a different matter. Now gray levels become important. But not the gray levels in the image you are reproducing, but the number of gray levels that can be created by the halftone screen given its frequency (lpi) relative to the resolution of the CTP device (1200 or 2400 dpi).

          In general, in a halftone screen, a range of 256 gray levels provides small enough tonal steps to build sharp images, moderate blends and smooth vignettes. The steps from one gray level to the next are virtually indiscernible. However, there is a special relationship between gray levels, output device resolution, and line screen ruling that is summed up in the formula: (dpi/lpi)2 +1= number of gray levels possible. Using this formula would suggest that requesting a 300 lpi screen on an output device with a resolution of 2400 dpi would result in only 65 possible levels of gray – (dpi/lpi)2 +1= 65 gray levels. In this case, with only 65 tones available, we would see visible tone step artifacts such as “shade stepping” or “contouring” where the color steps abruptly from one shade to the next without a smooth transition. When the ratio of dpi to lpi drops below 16, the number of available gray levels drops to below 256 resulting in tonal reproduction that is inaccurate and uneven, and because of this constraint, the number of available gray levels decreases as the screen ruling increases. Fortunately, in reality, the (dpi/lpi)2 +1 calculation only determines the tonal capacity of a single halftone cell and does not reflect modern screening methods.

          Most modern RIPs, instead, use a halftone "supercell", which is a grouping of many halftone cells, to build screens with more accurate angles, screen rulings, and tonal gradations. Supercells contain many more pixels than an individual halftone cell and subsequently can be used to represent many more gray levels than a single halftone cell. By activating pixels in some of the neighboring halftone cells at different gray levels within the supercell, a broader range of gray levels, typically between 1024 and 4096, can be achieved.

          Users of systems without this functionality must resolve gray-level issues by increasing the addressability and resolution of the output device (e.g. going to 4000 dpi), which slows down the imaging speed, or by limiting the screen ruling to a low lpi,

          The optimal performance-to-quality ratio supports the popularity of devices that operate in the range of 2400 dpi. Systems using supercell screening technology to extend gray levels offer all the resolution, all the screening, and all the throughput speed that the majority of users will ever need.

          I hope that's a bit clearer.

          best, gordo

          Comment


          • #6
            Re: High Resolution for Print

            Are we talking about PDF resolution, as set in Distiller, or just the resolution of an image?

            Do the images in question contain type/text?

            rich

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            • #7
              Re: High Resolution for Print

              I once read and then studied and experimented a very simple resolution ratio: 1.2 times printed linescreen... and it works just fine. Why 1.2? Because 1.2 is the 45 degrees measure of a 1 (any unit) square and will therefore account for all angles a dot area might have. This number also makes most RIP's rip faster than funny and exotic numbers. High resolution is handy at time. When there is a need for retouching, manipulating etc. At other times, high resolution becomes a waste of disk space and computing power if not an outright quality-killer. When you proof a 500 ppi file on a 720 dpi inkjet and you really expect to print at 100 lpi, all you are doing is promising a rose garden. Each final dot area will average 25 pixels (5x5) !!!! Do the math.....

              Comment


              • #8
                Re: High Resolution for Print

                I am not familiar with much of the language your using in your responses. I appologise, but could you offer a more laymans explanation to the question "what could be the motivation for publisher's request that images be 1200ppi when their stated line screen is only 150 lpi" As a designer that wants to send better press ready files to print, I'm just wanting to understand this that seems unreasionable. As for all the posts I will continue to study them and I thank everyone for the feedback.

                Comment


                • #9
                  Re: High Resolution for Print

                  Russell;

                  Perhaps the dpi specs you are reading are not making the distinction between 8 bit art files that are halftone screened and 1 bit unscreened files which are used to reproduce line art. It has always been my understanding & experience that bitmap lineart files should be in the 1200 to 2400 dpi range to smoothly reproduce lineart images edge detail. Lineart imaged from 300dpi files always has sawtoothed edges to my eye whereas 2,400dpi files have nice crisp edges.

                  Don Evans

                  Comment


                  • #10
                    Re: High Resolution for Print

                    > {quote:title=russpears wrote:}{quote}
                    > when a 600ppi to 1200ppi image is sent to the RIP and outputed to even a larger line screen of 170 lpi the excess resolution is only going to slow down the RIP not improve the printed peice. I was reading some press specs on the web and some specs showed a 300 line screen, so here the 600ppi image may be used, but still a 1200ppi image seems too much. The human eye can only distinguish a little over 200 tonal values-or so I have read and it takes 2.5 of these tones just to change just 1% in screen value. I asked one presenter this question at the Adobe Cretive Suite Seminar from SkillPath. He said that the request for art above the printers max line screen is unnecessary. But that still leaves me wondering why large publications that I have submitted ad work for requested such high res images.


                    Most printers in their right mind would never request images above 300 ppi. There are certain types of printing that might want images at 600 ppi but not very often. I think you might be confusing what printers are asking for. If you are talking about a PDF then yes they might request it at 1200 or 2400 but that does not mean the images (pixel based) inside of it are going to be 2400 ppi. The images in the PDF will either stay their original or down sample to whatever you specify in your PDF settings but the high res text and vector information will be at 2400 to keep it high res. You definitely do not want type and vector images downsampled to 300 ppi. Once it is screened for output you want everything at 2400 ppi because it is screened as a 1 bit file meaning everything is "like vector". All pixels are either black or white. No in between.
                    Joe
                    OS: Mac OS X 10.10.2 - RIP: Prinergy Connect 6.1 - CTP: Luscher XPose! 160 (2)

                    Comment


                    • #11
                      Re: High Resolution for Print

                      <<why some printers will ask for higher resolution images than their line screen suggest is adequate: ex. a request for 1200ppi image for 150lpi press work.>>

                      Are printers asking specifically for the IMAGES to be 1200 ppi? Or is that the resolution they wanted for the file itself?
                      You might re-read what were were told at about.com - the answer remains the same, irrespective of forum.

                      http://forums.about.com/n/pfx/forum....oppub&tid=5293

                      Edited by: Ted Schulte on Apr 19, 2008 8:57 PM

                      Comment


                      • #12
                        Re: High Resolution for Print

                        Russell,

                        If you want the easiest solution and you are sending them PDF's, ask them for their Distiller settings. Create your art at two times the line screen and use the Distiller settings they send you to make the PDF.

                        If you want to understand why or get a more specific answer, you will have to ask a more specific question. You need to be more specific about what you are sending them. There will be different answers for different situations. Are you sending designs with text or vectors along with digital images? Are you sending just digital images? Are you sending line art?

                        If you are sending only raster stuff, then yes, the publisher is confused, but if it is not just raster stuff, then there are a few possible answers depending on the situation. Any other details would help us in giving you a better answer.

                        Dan R.

                        Comment


                        • #13
                          Re: High Resolution for Print

                          I read and studied these replies. Someone on that forum suggested this forum, but I cannot seem to get what you say is the answer.

                          The only think I read the was this:

                          ""We run a 175 line screen at 2400 dpi. Line screen IS NOT resolution. "" and ""We run a 175 line screen at 2400 dpi. Line screen IS NOT resolution. ""

                          Now is it possiable to do this. Im confused by this answer, because he says that line screen is not the same as resolution. Here the line the 175 lpi seems to only give 350 dpi if you take it 2 times the line screen. Now I know that all of that resolution will not give you anything better than the line screen or printing resolution. But he adds that he runs 2400 dpi? How can this work. This may be at the heart of the issue for me.

                          I just would like to understand better so that I can work better and provide my printed materials without issue.

                          Comment


                          • #14
                            Re: High Resolution for Print

                            2400 dpi is the resolution of the final output device - could be a computer to plate device or film. This represents the pixels that are available to be used to create the halftone dots (each made of many pixels) that will simulate the tones of your original art.
                            175 lpi represents the frequency of the halftone dots in lines per inch (each made up of many pixels on the 2400 dpi device) it is a measure of resolution in the sense that the finer the halftone screen the more detail it can resolve. i.e. a 175 lpi halftone will resolve more detail than a 100 lpi screen.
                            350 dpi at an image reproduction size of 100% is the resolution of the original image that will be halftone screened at 175 lpi using the pixels on the 2400 dpi final output device.

                            best, gordo

                            Comment


                            • #15
                              Re: High Resolution for Print

                              Russell

                              As Ted asks: Are they really requesting 1200dpi for the final effective resolution? Which publications ask for this much resolution? One obvious answer why someone would ask for this much resolution is that they are going to re-purpose the image to a poster or outdoor display where it will be blown up very large. No publication would ask for this much data, and in fact several list a maximum effective resolution of 400dpi because they don't want such big files.

                              The statement "We run a 175 line screen at 2400 dpi" refers to the imagesetter or computer to plate system. When an imagesetter "builds" the dots for 175 lpi, each dot is made up of smaller dots. This has nothing to do with the resolution of the images that will be printed at 175lpi. It is talking about how sharp a resolution each individual 175lpi dot will be built at. Many CTP systems go up to 3600 dpi to make even 'sharper' dots.

                              -Todd Shirley

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