Questions about dpi

Discussion in 'Image Processing' started by KillRamsey, Mar 3, 2015.

  1. KillRamsey

    KillRamsey SC Hall of Famer

    Jun 20, 2012
    Cambridge, MA
    Kyle
    Hey folks. I have some dpi questions... The fujis create jpgs that are between 3.5 and 5.5 megs, and are 300 dpi. When I open / edit these jpgs in;

    1. Photoshop -- I get 72 dpi files in return. I'm sure I can change that, I just never noticed it.
    2. MS Office -- I get 72 dpi files. I cannot change it.
    3. Polarr (a new online editor that I actually like a lot): I get 180 dpi files, and cannot change it. I can save at one of 4 preset quality levels, which range from like 460k to 16 megs, but all sizes are 180 dpi, so it's the worst of both worlds -- lowered dpi, and enormous files clogging my hard drive.

    Are these editors lowering my dpi, blowing up my file size, and then leaving me no option to undo all that? Again I get that PS probably allows whatever I want, I just can't use it during the day, which is when I have the most time to peck away at images. How and why do editing programs do this?
     
  2. WoodWorks

    WoodWorks SC Regular

    107
    Dec 21, 2014
    Who knows why they do it? But bottom line: it doesn't matter. Dpi is only relevant when you go to print an image. For everything else, all that really matters is total pixel dimension. So a file that comes out of your camera at, say 6000 x 4000 pixels will stay at those dimensions no matter which program you use to view them, and no matter what dpi they "virtually" assign to your image.
     
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  3. Luckypenguin

    Luckypenguin SC Hall of Famer

    Dec 24, 2010
    Brisbane, Australia
    Nic
    Doesn't the dpi value stored in an image's properties just define a default print size (if you were to print from Photoshop for example)? If you choose to print at any other size then the dpi will be whatever it calculates to be.
     
  4. KillRamsey

    KillRamsey SC Hall of Famer

    Jun 20, 2012
    Cambridge, MA
    Kyle
    What started this was a blurb on dpi guidelines for a banner template the Mrs is using, stating it needed minimum 300 dpi. And that taking a 150 dpi image and placing it (in Illustrator) at 50% size would effectively make it a 300 dpi image.
     
  5. ReD

    ReD SC Hall of Famer

    Mar 27, 2013
    What I find differs using Picasa - I take a crop say 900x900 import to create a collage & the result is an image 5120x5120 pixels - I use this feature a lot as it then allows reduction in size
     
  6. WoodWorks

    WoodWorks SC Regular

    107
    Dec 21, 2014
    "In theory," yes. But I'd resize it in Photoshop to the correct dimensions and dpi first, and then place it in Illustrator. That way there'd be no doubt about it.
     
  7. WoodWorks

    WoodWorks SC Regular

    107
    Dec 21, 2014
    Having never used it, I have no idea what Picasa does to an image. But if you're really taking a 900x900 pixel crop of an image, and once imported into Picasa it somehow becomes a 5120x5120 pixel image, then that image must be undergoing one heck of a lot of interpolation, and therefore degradation.
     
  8. ReD

    ReD SC Hall of Famer

    Mar 27, 2013
    not to my eyes - its very useful when printed too
     
  9. WoodWorks

    WoodWorks SC Regular

    107
    Dec 21, 2014
    My guess, and it's only a guess, is that Picasa is attempting to display a 5120x5120 pixel image at 900x990 dots per inch on your screen. So while the pixel dimension of your crop is actually 5120x5120 pixels all along, Picasa is displaying (or attempting to—most monitors display resolutions of 72 or 96 dpi) a 900x900 dpi version of it. I'm not aware of any software algorithm capable of resampling an image from 900 to 5120 pixels without turning it into mush.
     
  10. ReD

    ReD SC Hall of Famer

    Mar 27, 2013
    the readout says 5120 pixels
    obviously with a tiny size crop you have degradation (I try to use this to advantage) - 5120 looks the same as the 900 x 900 but bigger
    but from the original image all details are preserved just bigger
     
  11. bilzmale

    bilzmale Super Moderator Emeritus

    Jul 17, 2010
    Perth, Western Australia
    Bill Shinnick
    You are comparing how three different pieces of hardware render your image.
    Camera eg 12MP
    Computer screen eg 72 dpi
    Inkjet printer eg 300dpi.
     
  12. bartjeej

    bartjeej SC Hall of Famer

    Nov 12, 2010
    bart
    As said, dpi is completely irrelevant when talking about digital files. If in doubt, just try to answer the question: what's a digital inch? It doesnt exist, as inches are a physical world thing. So long as you're talking about a digital file, you're talking about pixels.

    When talking about prints (or monitors, which are also physical things), inches do come into the equation, but it's best to talk about Pixels Per Inch or ppi (that's also what the 300 number refers to); it indicates how dense the information is; higher density means you can get closer to the image without seeing individual pixels, although a badly upressed image might still look poor.

    Dots Per Inch (dpi) are about printer technology and that's only relevant when you're buying a new printer; there's no direct relation to ppi or what size the ideal digital file should be, as different printers need different numbers of dots to display 1 pixel.

    Upressing 900*900 to 5120*5120, in the true sense of the word, is impossible without severe degradation; if there's 900*900 pieces of information, theres no way that software can turn that into almost six times as many pieces of unique information (it would mean 5 out of 6 pixels are "invented", and that would be very noticeable). ReD, you mentioned a collage; does the 5120 image also include other pictures?
     
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  13. ReD

    ReD SC Hall of Famer

    Mar 27, 2013
    no a straight image to enlarge

    and I do take the point being made but then I am simply reporting on what the file readouts say
     
  14. KillRamsey

    KillRamsey SC Hall of Famer

    Jun 20, 2012
    Cambridge, MA
    Kyle
    Very helpful, folks. Much obliged.