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JPG Image Resolution

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  • JPG Image Resolution


    I wrote a print-on-demand book (How to "Ace" Statistics 101: Textbook) which is currently sold by Amazon's Create Space. For the book, I created JPG images at 300 DPI, copied them to a word DOC, and then converted the DOC to a PDF file. They look good!

    For my next print-on-demand book (How to "Ace" Calculus 101: Textbook), the graphing software that I am using creates JPG images at only 72 DPI. I repeated the process and the images also look good.

    Should I be aware of any problems?

    Thank you!

    Dr. T

  • #2
    Seventy-two PPI looks OK on your computer display for the simple reason that the resolution of your display might not be much higher than that. Print, however, has much higher-resolution requirements. If an image is only 72ppi at the physical dimensions it will print, it will be noticeably blurry when printed -- especially on something like a math book, where many of the images might be of various math symbols that need to be razor sharp.


    • #3
      Dear Busy Mod,

      Thank you for your prompt reply. Here is what I did:

      I created a 5 inches by 5 inches JPG image (360 pixels by 360 pixels at 72 PPI). Copied the JPG image to a DOC file. Converted the DOC file to a PDF file. Printed the PDF file. Therefore, I believe there was NO scaling. The printed image looked good. FYI, the books format will be 8-1/2 inches by 11 inches and the book's images will be 5 inches by 5 inches.

      If there is NO scaling, why would the images on the book be any different that the PDF file I printed?

      FYI, CreateSpace uses PDF files as the source for print-on-demand.

      Any comments?

      Dr. T


      • #4
        Originally posted by DrT View Post
        If there is NO scaling, why would the images on the book be any different that the PDF file I printed?
        I'd need to see the image to offer an opinion on why it looked OK at such a low resolution.

        I'm assuming that you're printing the PDF on a desktop printer of some kind. Output resolutions of desktop printers differ, but it's likely printing at around 1,200 dots per inch (DPI), whereas the resolution of your image is only 72 pixels per inch (PPI). A higher-resolution image can take better advantage of the output device's resolution. This translates into increased detail and sharpness. If your image doesn't have lots of detail, you might not notice a significant difference.

        This whole subject gets fairly technical, and it would be easy for me to get off in the weeds about halftone frequencies, rounding errors, the inherent blurring in lossy compression algorithms (JPG), line art vs. continuous tone images, RGB vs CMYK, etc.

        I don't want to get into all that, but the rule of thumb is that continuous tone images (like photographs) should be around 300 ppi to print well on an offset printing press using a halftone frequency of 150 lines per inch (LPI). Your book apparently isn't going to be printed that way, and that 300 ppi rule of thumb might not apply, but I'd be hesitant to use images under 200 ppi, even though some of them might look OK when your print them.

        There's also a difference between line art and continuous-tone imagery (photos). Line art is imagery with sharp, hard edges, like, typography or in your case math symbols. These things are typically handled using vector artwork that will scale to any resolution and will take full advantage of the output device resolution to ensure very sharp definition that isn't needed in a photo. If, for example, you're making 72ppi screen captures of black and white math symbols, they're going to have fuzzy edges due to the anti-aliasing of the edges (grayness along the edges that provide a simulated sharpness at optical resolutions). Ideally these kinds of things would use resolution-independent vector imagery instead.

        Another thing to consider is that JPG uses lossy compression, which means that data in the image is discarded for the sake of making small-sized files. This data is lost and can't be retrieved. In a photo, a small amount of data loss isn't significant, but in non-vector line art, this data loss can result in obvious degradation of the image (JPEG artifacts).

        None of this really answers your question about why your particular image looks OK when printed out on your printer; I'd need to see the image and the hard-copy output for that. What I have done is tried to give you a little background into some of the issues surrounding your question.


        • DrT
          DrT commented
          Editing a comment
          Dear Busy Mod,

          Thank you for your reply.

          I realize the topic is well above my pay grade. My research has shown that somehow I have to convert my JPG images to 300 PPI without scaling the images.

          My problem is that the graphing software I use (GRAPH 4.4.2) creates images at 72 PPI.

          Dr. T

        • B
          B commented
          Editing a comment
          Scaling things up from 72 ppi will not result in any benefits since there's really no good way to add more detail into an image than is already there.

          However, you can reduce 72 ppi images and increase their resolution if you start out with an oversized 72ppi file. For example, let's say you have a 3x3-inch space in your book where an image needs to be placed. If you grab a 72-ppi screen capture with physical dimensions of, say, 6x6 inches, you can reduce that image down to 3x3 inches -- effectively doubling the resolution to 144 ppi. In other words, if you start out with a large, but not entirely razor sharp image, you can reduce its dimensions and cram all the data from the larger image into a smaller space.

          With that in mind, if you can somehow use your graphing software to produce larger than needed images, you can subsequently reduce the dimensions of those images and increase their resolution.

      • #5
        Hi DrT and welcome to GDF.

        We ask all new members to read very important links here and here. These explain the rules, how the forum runs and a few inside jokes. No, you haven't done anything wrong, we ask every new member to read them. Your first few posts will be moderated, so don't panic if they don't show up immediately. Enjoy your stay.
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