DPI, pixels, and inches?!?! help please.
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Thread: DPI, pixels, and inches?!?! help please.

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    DPI, pixels, and inches?!?! help please.

    I need clarification on the relationship between DPI, the pixel dimensions and the inch/cm dimensions of an image.

    For example, why is it if I increase the DPI of an image but keep the dimensions the same it will print no bigger but look gigantic on my screen?

    Yet sometimes if I want to decrease the quality of a hi-res image I can lower the DPI and shrink the image, or lower the dimensions and keep the original DPI?

    In other words, I posses only basic knowledge of photoshop and am thoroughly confused about how DPI and dimensions relate to one another and determine the size of an image.


    Thanks.

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    Questions of this general pattern are probably the most common here.

    Anyhow, DPI establishes a mapping between pixels and physical dimensions of a printed or displayed work. That is, simplistically speaking a 300x300 pixel image printed at 300 dpi will occupy an area of 1 square inch. That same image at 600 dpi will occupy 1/4 of the area. The exact dimensions will depend on the minutiae of the printer and whether there is a divergence between the layout of pixels in the image and dots by the printer - different offset printing methods for instance will have a different relationship between pixels and dots.

    Pedantry aside, DPI is a parameter of your image telling devices how to display something. Your monitor is pretty much locked to a single PPI per resolution (somewhere between 72 and 120 dpi). When you increase the DPI without changing the dimensions of the image the size increases on screen because your screen can't display higher DPI/PPI whilst the image prints at the same size because printers can often manage up to 1200 dpi. In short, you are boosting the resolution of your image whilst keeping it the same physical size, packing more pixels into the same space. On the monitor this is not possible.

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    Anid Maro is offline Psychotic Eldritch Zeppelin Level 9 Gladiator: Hoplomachi
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    Okay, here's the breakdown...

    Pixel Dimensions

    These are the basic measurement on computers. How many pixels are shown on your screen (and therefore, how large an image looks) is dependant upon your computer's resolution. For example, if your resolution is set at 640x480 (307,200 pixels), an image will look four times as large then when viewed at 1280x960 (1,228,800 pixels).

    When it comes to pixels, there isn't any sort of physical size to them, only the quantity matters.

    Dots Per Inch (DPI)

    DPI is strictly a printing term that relates to the amount of detail printed.

    For example, a 4x6 inch image printed at 300 DPI will look far more detailed and just nice in general than the same 4x6 image printed at 72 DPI.

    The reason why the document gets bigger on screen but not in print size is because of this.

    Strictly speaking, DPI is translated into PPI (Pixels Per Inch). So that 4x6 inch document at 72 DPI
    will be 228x432 in pixel dimensions (since 1 inch = 72 pixels). If you upsample to 300 DPI, then that same 4x6 document becomes 1200x1800 (since 1 inch = 300 pixels).

    When you increase the DPI, you're telling Photoshop that when you print this document you want it to put more pixels worth of information into it, so in response Photoshop adds pixels to the image.

    Since Photoshop isn't, ahh... artistically gifted, the resulting image usually doesn't look so great. So if you're printing work, it's generally considered that you should start at 300 DPI.

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    Quote Originally Posted by Anid Maro
    Since Photoshop isn't, ahh... artistically gifted, the resulting image usually doesn't look so great.
    A good explanation but I feel this bit needs some clarification (principally for the benefit of the original poster): when you increase the pixel dimensions of an image photoshop will interpolate values to fill in the gaps created by upscaling. From and information theory perspective the image contains the same amount of meaningful data spread over a wider area. Photoshop cannot add or reveal detail that doesn't exist but it can move a low fidelity image into a higher fidelity space in a graceful manner. Photoshop by default does this through a process called bicubic filtering. PSCS2 and later implement several submodes, as well as the bilinear filtering common in 3d gaming (Used at a video card hardware level to smooth textures) and nearest neighbour filtering.

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    Quote Originally Posted by Snarfevs
    Photoshop cannot add or reveal detail that doesn't exist but it can move a low fidelity image into a higher fidelity space in a graceful manner.
    well, graceful if you don't mind your image looking a bit like it's made out of fuzzy Legos.

    in commercial graphics & printing of mid- to high-quality, upscaling images (to higher resolution & therefore larger printed image size) via resampling (interpolation) is strongly discouraged, as the results are rarely satisfactory. there are some interpolation algorithms that do a better job than others, depending on the subject, but none are what i would consider very useful for up-sizing. for offset printing, if a too-low-resolution image is the only possible option for pinting, it's better to let the printer's RIP software do it, as this has vastly improved in the last few years due to the very common problem of having to deal with digital photography and scans with too few pixels.

    most of what was said above is the straight poop, but one aspect of resolution (DPI) is that it's completely abstract and meaningless unless the image is to be printed in the real world. then it becomes an issue that is device-dependant. desktop inkjet printers can do well with images as low as 72 - 120 DPI. better quality offset printing requires 200-300DPI, and highest quality offset can require 400 DPI depending on the image.

    since the DPI resolution specification can change, and what is fixed in a digital image is the number of pixels, then it's better to describe DPI as "effective resolution," which relates the number of pixels to the printed image size. thus with a 1000x1000 pixels image, you can print it at 10" x 10" at 100 DPI, 5" x 5" at 200 DPI, about 3.3" x 3.3" at 300 DPI, and so on. the formula is:

    number of pixels (width) / printed image size in inches (width) = effective resolution.

    if the effective resolution doesn't match well with what a printing device requires, you can accept a possibly lower-quality printed image, reduce the printed size of the image (increasing the effective res), or (not recommended), resample the image to create more pixels.

    the formula can be used in reverse to plan image sizes. if you know you'll want a full page magazine illustration at 300 DPI, multiply the mag's width & depth x 300 to get the number of pixels you'll need:

    8.5" x 11" page size = 8.5 x 300 (w) & 11 x 300 (D) = 2450 x 3300 pixels

    same for any other specifically-sized image area. if a job only requires 150 DPI, use that as the multiplier, and so on.

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    What I meant was relatively graceful vs. nearest-neighbour, that is.

    And yes, upscaling anything is the devil.

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    Thanks for all the great information! I understand now. Sorry if this is the hundredth time you've answered the same question.

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