When Pixel Density (PPI) Matters for Mobile Phones?
You have certainly heard of PPI – pixels per inch – when looking for a new mobile phone. It refers to the number of pixels lines contained within an inch of a screen. The concept is quite straightforward: the higher the PPI count, the higher the image quality. Among all the various ways in which one can analyse this property, the pixel count is closely related to two: resolution and definition. Unfortunately, many people tend to conflate these terms and spread misconceptions about what they are. Thus, before addressing the significance of mobile phone PPI it’s worth understanding what, exactly, is a pixel and how all the other properties and measures are correctly derived from it.
The smallest screen element
The pixel is the most basic element of a digital display and traditionally consists of three light elements. These elements are called sub-pixels (or dots) and they’re responsible for issuing a different colour wavelength within the RGB spectrum. That is, each pixel is formed by three colours – R stands for red, G for green and B for blue – which, when mixed in different proportions, will generate the millions of colours that we can see in displays.
Distances that count
The gap between each light source, known as “dot pitch” also has its share of responsibility in the formation of images because the distance between dots also defines between pixels. Dot pitch refers to the gap between two sub-pixels of the same colour – that is, different pixels. The smaller the dot pitch, the higher the image resolution, as the dark space between pixels is lower. If dots have the same colour distance from each other, it is reasonable to assume that the same distance exists between full pixels. Although it is not always exactly like this, such approach is valid because it allows to evaluate pixel characteristics that influence image quality.
The image size
Resolution is a more versatile way to tell the amount of detail that can be seen. It is common – although not necessarily recommended by any industrial standard – to refer to picture resolutions as 800 x 600 or 2048 x 1536 pixels, for example. Even if these terms do not follow the standards, they partially solve the problem of identifying the quantity and quality of detail that can be reproduced on an image. Using both of the preceding measurements one can determine that the 800 x 600 image has a full resolution of 480,000 pixels – equivalent to 0.4 megapixels – and that the 2048 x 1536 one has 3,145,728 pixels, or 3.1 megapixels. Naturally, the higher the value, the more details there will be.
However, it is important to be aware that resolution is does not equate to the actual physical size of an image. You can have a very high resolution image that covers only a few centimetres when printed or reproduced on a mobile device, just as it is possible to reproduce low resolution images on an area of many square meters such as a billboard or a cinema screen. This is why resolution alone cannot tell how good an image actually looks like to the human eye.
Quantity in space
This refers to the classic definition of density. In the case of digital displays (current monitors, cell phones and televisions), density is measured in the amount of pixels occupying the visible screen area. Screen area meaning the actual screen size in inches or centimetres – traditionally expressed through a diagonal line across the screen. The higher the pixel density, the higher the image quality. An appropriate analogy to better understand what it is and how it is perceived is to compare pixel density to a tissue’s texture.
Weaving quality largely depends on the amount of yarn used. Making soft tissue, for example, requires a large amount of very thin thread to weave a piece of 1 square meter. Another piece with identical area using thicker threads, will need a smaller amount of raw material, but will have a much coarser and less fine detailed texture.
The same logic applies to digital displays: the smaller and the closer to each other the pixels are (smaller dot pitch), the more uniform and of higher quality the displayed image is.
Mixing everything together
By considering resolution and pixel density together one can assess the image definition and how it actually looks. If image quality can be determined by pixel count of the display (resolution), as well as pixel density in the area occupied by the display unit (density), then it follows that the largest possible values for these two features offer the best possible picture. The Retina Display of an iPhone 4, for example, has a resolution of 960 x 640 pixels on a 3.5 inch diagonal screen. Making the math, we arrive at a dizzying value of 326 PPI. Comfortably above the limits of what the human eye can discern.
Keeping a relative distance
The optimal distance between display and observer varies according to pixel density. This is why very large TVs – 50 inches or more – require a considerably large room to provide the expected picture quality: their PPI is lower than that of mobile devices. So when the viewer is too close to the screen, the dot pitch starts to make itself noticed, which generates discontinuity in the image and ends with all the beauty of 1080p resolution. The solution is easy: just move away from the screen to the recommended distance. The considerably higher mobile phone PPI is a necessity by virtue of the proximity between the viewer’s eyes and the screen.
Why does PPI matter for mobile phones?
Higher PPI matters for mobile phones because it provides better native picture quality and because it frees mobile phones from power hungry image processing tasks, which makes the interface all the more responsive and pleasing to use. There are several ways to artificially improve the picture quality of a screen, and none is as widespread as Anti-Aliasing. The process that turns jagged images into smooth curves. This is, however, a very resource-hungry filter which is impractical for mobile devices that have rather limited graphic processing capabilities. The increasing pixel density of displays, renders these artificial improvements obsolete, and makes the task of mobile phone makers easier by allowing them to concentrate their efforts on more important matters for user experience.
300 PPI is considered the limit of human perception in terms of pixel density. Scientists and the market itself have proved that the experience gained from high PPI screens is far superior to that achieved with less dense screens. The time will come when pixel density of most digital displays is so big that it makes no difference to the users anymore. This is much closer to occur than you probably think.