Display devices have their own PPI.Do these devices respect the DPI information of image file? If not, then what is use of"DPI" of image file?

example: Image file- height: 1200 px width: 900 px DPI:300

Monitor- PPi:100.

What would be size of image when displayed?

Thank you!!

  • On a Microsoft Windows machine, DPI of a monitor is typically 96 pixels per inch. Image files are simply scaled to that resolution when viewed at 100% zoom. So just divide 1200 by 96 and you'll know what the height is in inches. – Robert Harvey Jun 23 '20 at 3:33
  • Thanks.. that i was suspecting. then what about DPI of image file? Any use of this? – sk patra Jun 23 '20 at 4:33
  • Cross-site sort of duplicate and various other questions on Photography with the dpi tag. – Philip Kendall Jun 23 '20 at 12:19

Fundamentally, digitally image files must be measured in pixel counts, such as "1024x768". In the metadata of some image file formats you can store a "DPI" value as well, but this will usually have no effect on the way you would see the image when it is rendered - obviously, if your monitor has a measly 300 DPI, the hardware will not magically get a higher resolution just because you render a particular image on it, no matter how much the designer would like it.

The only purpose that such declarations serve is as a documentation. They essentially mean "this version of the logo is intended to be viewed on a 300dpi monitor", while another version might be intended for a 2000dpi printing press. Essentially, this is a complicated way of expressing e.g. that the logo is supposed to be 10cm wide.

  • thanks..it's clear that , "DPI" info in file is nothing to do with the output. It's just a information, which tells image pixels at this density is good.. thanks – sk patra Jun 23 '20 at 7:23

It really depends on the particular application that is showing the image, but in general the DPI information is just ignored and the display size of the image is depending on its pixel dimensions.

For example a browser will (absent any specific scaling in HTML or CSS) show the image as 1200x900 pixels and the physical dimensions of that will depend on the PPI of the monitor. So with a PPI of 100, the physical size on the monitor will be 12x9 inch. If this is larger than the size of the monitor, the browser might scale it down for your convenience.

DPI only really matters when printing, and even then it may be ignored. E.g. if an image is embedded in a PDF or other layouted document, the effective DPI will depend on the physical dimensions specified in the layout.

  • Typically, viewing software will ignore the DPI information and show 1 image pixel per screen pixel, and document-related software (word processing, typesetting, printing) will initially scale the image to real-world inches respecting its DPI settings. – Ralf Kleberhoff Jun 23 '20 at 10:06
  • @RalfKleberhoff In general, viewing software does not display 1 image pixel per screen pixel, because the resolution of cameras (typically 20 million pixels or higher) is significantly more than even a 4k screen (8 million pixels or so). They do ignore DPI though! – Philip Kendall Jun 23 '20 at 14:26

Software is supposed to use the number of pixels and the dpi to figure out at which size in inches an image should be displayed.

Usually you can somehow override that size. A 600x900 pixel image at 300dpi is supposed to be 2x3 inches, but you can display it at a larger size. Now you can have a look at the quality of your display, or possibly your printer, and check whether the quality will suffer because dpi is not high enough.

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