I was wondering what happens when I change display resolution (i.e. 1280x1024 pixel) in my OS (Windows XP) does it affect the DPI (PPI) setting of my OS too?

Or when I change the DPI (PPI) setting of my OS does it affect display resolution?

So when resolution changes to lower or higher, according to the formula, something should change to get different value (i.e. 800x600 pixel).

Formula for calculating display resolution:

  • Display Resolution (Horizontally) = DPI (PPI) x width of display
  • Display Resolution (Vertically) = DPI (PPI) x height of display

Or may be I am missing something? Please enlighten me.

For example:

Technical specs of a PC monitor:

  • Diagonal Size = 17" (inch)
  • Width = 13.3" (inch)
  • Height = 10.64" (inch)
  • Screen DPI = 96dpi (The maximum number of pixel (dots) per linear inch this monitor supports.)
  • System DPI = 96dpi (The maximum number of pixels (dots) per linear inch Windows XP runs on.)
  • Display Resolution or Pixel-Dimension (max or physical) = 1280x1024 pixel
  • Display Resolution or Pixel-Dimension (systems or current) = 1024x768

If I change the resolution of above monitor from 1024x768 pixel to 800x600 pixel technically (according to the equation) change happens at either DPI (PPI) or width or height, right? So how are the display settings processed?

  • 1
    This is an abandoned question, but for People from the Future: if something is X per Y (dots per inch; dpi) , the formula is X divided by Y (dots[aka pixels] divided by inches). Per means divided by. – horatio Oct 8 '13 at 16:29

Generically speaking: pixel = dot = point. They are different physical elements, depending on the medium you're working in. On computer monitors, pixels matter. In printing, dots are what count. Points are more generic and could refer to pixels or dots. The terms are commonly interchanged and often confused.

"Resolution" is the total number of [pixels, points or dots] wide, by total number of [pixels, points or dots] high. So a printer could have a resolution of 1200x1200 dots per inch, while a monitor could have a resolution of 1280x1024.

DPI and PPI are simply ratios. DPI is "dots per inch," PPI is "points per inch" or "pixels per inch." Those ratios increase and decrease based on the resolution (width x height, in pixels) and size (in inches) of a given medium.

To calculate the DPI, you need to determine the actual physical widths and heights of the medium. A common example is the Apple iPhone 4 screen:

Physical Width = 1.94 inches Physical Height = 2.91 inches

Width (in pixels) = 640 Height (in pixels) = 960

The assumption is that all pixels, dots, or points occupy a square space. Therefore, the simple equation to determine PPI / DPI is to divide pixel height by physical height, yielding roughly 329 DPI.

This information helps to answer your question. Windows does not have any idea what the DPI of your display is, because it has no concept of what the physical dimensions of the display are. You can buy 20" monitors with 1920x1080 resolution, as well as 70" monitors with the same 1920x1080 resolution. Both have signficantly different DPI's, yet Windows has no idea and nothing to do with it.

While Windows offers the option of increasing or decreasing the DPI, all it will really do is adjust system font sizes and default icon / UI sizes of things. Many other apps, graphics, websites and emails will actually get very poorly distorted if you make changes to the DPI settings.

Apple Mac OS (especially iOS) has significantly better support for DPI, and knows, based on the devices it is installed on, which DPI setting to use.

  • thanks for the reply! the technical formula for display resolution or pixel-dimension, as i mentioned in my question, works when i change resolution in my os.Right. lets say i have two 17" pc monitor with 96 dpi(both screen and sys) 1 showing 800x600 and other showing 1280x1024 now what changes in formula that made the display different? – bkdubey May 9 '12 at 13:37
  • @bkdubey if you have two 17" monitors with different resolutions, then they have different screen DPI values. – Breakthrough May 9 '12 at 14:19
  • @Breakthrough the interesting thing is screen dpi never changes (as mentioned above both of them has 96 dpi as screen and sys dpi), what changes is system dpi but even this is unaffected after lowering the res in xp which made me doubt myself. Anyway i think there is some kind of conversion going on between os(XP) and monitor and this is what i am looking for exactly which will also explain the relation between dpi and res. – bkdubey May 9 '12 at 16:54
  • If you have a monitor with 1280x1024 pixels, then any image that fills the screen will be a 1280x1024 image. It is a lie to call a 800x600 image that has been scaled up to 1280x1024 pixels a 800x600 image, no matter how many people repeat the lie. – kreemoweet Nov 14 '15 at 3:07

Your understanding of what DPI should be is better than Window's! The problem is that Windows has no idea of the physical size of a display and so just uses some baked-in assumptions by default.

You can change the DPI setting to be the correct one in the display properties. While it may give you a warm and fuzzy feeling, its effect on display is minimal. Most applications ignore the setting entirely and simply draw with pixels or scaled according to the window.

The major exception is with fonts when rendered through standard Windows API. In this case, the DPI setting is used to render fonts at the approximate size according to the set DPI. In practice this can make things look horribly wrong because fonts scale while other sizes do not follow on some applications and you end up with clipped or overflowing text.

To answer your question: No. Windows does not change the DPI when you change resolution nor does it change the resolution when you change the DPI setting. Note that Windows can even replicate across a number of displays of the same resolution and it will necesarily show the same pixels although the actual DPI of the screen differs.

  • 2
    If Windows is ignorant of display size, then it is willful ignorance. The EDID which every display device is supposed to provide has fields for H. and V. physical dimensions of the screen, from which actual DPI is readily derived. Setting Windows DPI to the physical reality will often simply break the displayed image in many ways. Window's treatment of DPI is a linguistic and logical abomination. – kreemoweet Nov 14 '15 at 2:56
  • There's the extreme case of a projector, where the size changes as you move the projector towards the wall it projects to - impossible to know for the hardware what the real size is. – gnasher729 Jan 12 at 15:02

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