20. Resolution, DPI and Ratios

DPI

For output devices such as printers or input devices such as scanners, the image resolution is defined by 3 values:

• Width and Height of document in inch or cm
• DPI, dots per inch

100 DPI = 100 dots per 1 inch

50 DPI = 50 dots per 1 inch. Compared to 100 DPI, distance between dots has doubled; print resolution is lower

PPI

Commonly used to describe the pixel density of an output device such as a computer monitor. The user can change the screen resolution settings

Pixel Resolution

In Computer Graphic DPI do not affect how an image is displayed, there is no need to set DPI to 72, the resolution is defined by 2 values only:

• Width and Height of image, X and Y dimension in pixel, i.e. 1920 x 1080

Image Aspect Ratio

The pixel resolution X and Y dimensions divided, i.e. 1920 : 1080 = 1.78 image aspect ratio

Pixel Aspect Ratio

Refers to formats, which use non-square pixels such as anamorphic cinemascope or PAL TV standard

Anamorphic cinemascope uses a pixel aspect ratio of 2:1, the image pixel resolution is 1828 x 1556 appearing squeezed in width but then displayed with a factor 2 for the width

PAL TV standard uses 1.06 aspect ratio. Old cathode ray tube TV sets display the 720 x 576 image correctly. On a computer monitor, displaying square pixel and not compensating for 1.06 aspect ratio, the 720 x 576 image would appear slightly squeezed

Common Resolutions and Aspects