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What Are Pixels Per Inch?

The number of pixels per inch affects the quality of images shown on computer screens.
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  • Written By: Diane Goettel
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  • Last Modified Date: 12 December 2014
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The term pixels per inch (PPI), which is also sometimes referred to as pixel density, is used to describe the resolution of a digital image. A pixel is the smallest unit that is used to make up a digital image and usually exists as a single colored dot. For this reason, the resolution of an image is sometimes referred to as dots per inch (DPI) instead of pixels per inch or PPI. The number of pixels that exist within a square inch of a digital image define the PPI or DPI. If the image is expanded or shrunk down in size, the number of pixels per inch will change just as the resolution of the image will change.

When creating or choosing an image for publishing online or in print, pixels per inch is important. An image that has a low number of pixels per inch will look fuzzy on a computer screen and when it is printed. This is, of course, unless the image is significantly reduced in size. An image with a high number of pixels per inch, on the other hand, will look rather crisp and clear both on a computer screen or in print.

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In general, a digital image of good quality will have 300 pixels per inch or more. There are cases in which the numbers of pixels per inch is much higher. Images that are created for sites that allow users to zoom in closely on the images are often created at a very high resolution. A website selling fine jewelry, for example, might create images with a high enough resolution to allow users to zoom in closely on details of the image.

The number of pixels per inch affects the quality of images shown on television screens, as well as the quality of images shown on computer screens. Televisions that are capable of displaying shows and events in high definition are able to display more pixels per inch than televisions that do not have this kind of capacity. PPI also affects the display on hand-held devices, such as mobile phones and personal digital assistants. The same is true for mobile devices that are largely used for recreational purposes such as that that play music, games, and television shows and movies.

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Markerrag
Post 3

@Vincenzo -- That works great for simple logs and things, but not so well for complex photos. Mathematical equations can only plot out images so far, so vector graphics are great for geometric shapes and less detailed graphics, but not so much for complex photos.

That being the case, good old raster graphics (those that use DPI to measure size) are with us for at least the time being.

Vincenzo
Post 2

@Logicfest -- And that is the reason vector graphics are becoming more and more popular among publishers. Pixel density doesn't matter a bit when you are talking about vector graphics because they are based on mathematical equations and can be "forced" into whatever resolution is necessary.

So, a vector that looks good on a computer screen can be translated well to print. Those represent the best of both worlds in that regard.

Logicfest
Post 1

A huge problem for publishers has to do with the way images are scaled down to load faster on Internet sites. It is common to convert images to 72 pixels per inch (PPI) so they will load faster on the Web, but those images don't translate well to print as most publishers want to see at least 300 DPI.

The problem is that an image looks OK on a screen to people, so they wonder why they won't look good in print. The science of it is simple enough. It has to do with digital resolution compared to what is necessary for print. The pixel density can be a lot less on a computer screen and look OK than it can in print.

Fortunately, most people who own an image and post it online can get the original and those are typically big enough in terms of pixel density to print well.

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