Why do we use the term DPI for matters involving images on computers

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I'm told that DPI and Points are no longer relevant in terminology involving graphical displays on computer screens and mobile devices yet we use the term "High DPI Aware" and in Windows you can set the various DPI levels (96, 120, 144, 192).

Here is my understanding of the various terms that are used in displaying images on computer monitors and devices:

DPI = number of dots in one linear inch. But DPI refers to printers and printed images.

Resolution = the number of pixels that make up a picture whether it is printed on paper or displayed on a computer screen. Higher resolution provides the capability to display more detail. Higher DPI = Higher resolution, however, resolution does not refer to size, it refers to the number of pixels in each dimension.

DPI Awareness = an app takes the DPI setting into account, making it possible for an application to behave as if it knew the real size of the pixels.

Points and Pixels: (There are 72 points per inch.)

At 300 DPI, there are 300 pixels per inch. So 4.16 Pixels = 1 point.

At 96 DPI there are 1.33 pixels in one point.

Is there a nice way to "crisply" describe the relationship between DPI, PPI, Points, and Resolution?

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DPI is for printed images. Some people work in Photoshop or work on printed media all day and is is much easier if the scale of their monitor matches the scale of what they are working on rather than having everything be gigantic because it's set to a higher DPI than the computers default – Matthew Darnell
Thanks Matthew - that does make sense. So it also makes sense to me to understand how many pixels are in 1 point. I'm just not clear on why some people state that "points" add no value to the discussion and that we should treat DPI as PPI (Pixels per Inch). (This seems wrong to me because DPI and PPI are not the same.) – ChileAddict - Intel

3 Answers

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You are correct that DPI refers to the maximum amount of detail per unit of physical length.

Computer screens are devices that have a physical size, so we speak of the number of pixels per inch they have. Traditionally this value has been around 80 PPI, but now it can be up to 400 PPI.

The notion of "High DPI Aware" (e.g. Retina) is based on the fact that physical screen sizes don't change much over time (for example, there have been 10-inch tablets for more than a decade), but the number of pixels we pack into the screens is increasing. Because the size isn't increasing, it means the density - or the PPI - must be increasing.

Now when we want to display an image on a screen has more pixels than an older screen, we can either:

  • Map the old pixels 1:1 onto the new screen. The physical image is smaller due to the increased density. People start to complain about how small the icons and text are.
  • Stretch the old image and fill in the extra details. The physical image is the same size, but now there are more pixels to represent the content. For example, this results in font curves being smoother and photographs showing more fine details.
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The term DPI (Dots Per Inch) to refer to device or image resolution came into common use well before the invention of printers that could print multiple dots per pixel. I remember using it in the 1970's. The term PPI was invented later to accommodate the difference, but the old usage still lingers in places such as Windows which was developed in the 1980's.

The DPI assigned in Windows rarely corresponds to the actual PPI of the screen. It's merely a way to specify the intended scaling of elements such as fonts.

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DPI vs. resolution – What’s the difference? The acronym dpi stands for dots per inch. Similarly, ppi stands for pixels per inch. So, why have two different acronyms for measuring roughly the same thing? Because there is a key difference between the two and if you don’t understand this difference it can have a negative impact on your digital signage project.

Part of the confusion between the two terms stems from the fact that many people who use them are lazy and tend to use the terms interchangeably. The simplest way of thinking about them is that one is digital (ppi) and represents what you see on the computer screen and the other is physical (dpi) for example, how an image appears when you print it out on a piece of paper.

I suggest you to check this in-depth article talking about the technicality of this topic. https://blog.viewneo.com/blog/72-dpi-resolution-vs-300-dpi-for-digital-solutions/