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javascriptdpidisplay-dpi

How to access screen display’s DPI settings via javascript?

Is there a way to access the screen display's DPI settings in a Javascript function?

I am trying to position a HTML panel on the page and when the user's DPI is set to large (120), it throws the position off. I need to be able to know what the DPI is so I can adjust the position accordingly.

Solution

  • Firstly, to help with the possible (and very common) confusion with the term DPI (dots per inch):

    DPI isn't exactly a part of "display settings". It's (mis)used in two different contexts:

    1. The native pixels per inch of a display (or video). It determines how small the pixels are. You can have the same 1024x768 resolution on both a 14" laptop screen and a 17" LCD monitor. The former would roughly be 1280/14 = 91 DPI and the latter would roughly be 1280/17 = 75 DPI. The DPI of a screen is immutable; it can't be changed with display settings. More...
    2. The dots per inch painted on paper during printing. This is the number of side-by-side dots a printer/photocopier/fax machine can imprint within an inch of paper. Most printers can be set to print at a lower DPI, by just printing each dot as two, four, etc. dots. More...

    When printing an image, there are many things that affect the final dimensions of the image on paper:

    • The dimensions of the source image -- this is the amount of pixels or data there is.
    • The DPI of the source image. This value determines how the dimensions should be interpreted when printing the image.
    • If the source image doesn't have embedded DPI information (a JPEG can have one, a GIF never does), the program that's being used may have settings to specify a DPI. This could be an image viewer/editor or even a web browser.
    • The zoom factor that's typically specified in a print dialog.
    • The current DPI setting of the printer.
    • The physical (max) DPI of the printer.

    The bottom line is, the image that you're printing will effectively get resampled (reduced or enlarged) to match the final DPI that's used in the print process. Any of the parties involed may be causing this (the program, the print driver, the printer).

    Now, coming back to your question. No, you can't determine the DPI of the screen, because it's not in software domain. It's a hardware domain term, describing how large a monitor a user could afford to buy. Update: I initially wrote this answer back in 2009, with my understanding of the current technologies. As @thure pointed out, you can now (since 2012?) use the window.matchMedia function to determine the DPI of the screen.

    If you're trying to achieve precision in printing an HTML layout, as others have suggested, your CSS should use print dimensions like em, pt or pc instead of px. However, the final outcome might still depend on the browser using. If converting your HTML to PDF (or generating PDF from scratch) is an option for you, printing a PDF would give you the truest WYSIWYG both on screen and paper.