Digital Image Formats - A Practical Guide to Video and Audio Compression - page 118

6.4 Screen Resolution

The size of the display and the chosen “'GA'” mode will determine the resolution. This is
the number of dots or pixels per unit of distance measured horizontally across the screen.
It does not correlate exactly with the screen sizes that are measured diagonally and the
resolution depends on the aspect ratio of the screen. Vertical and horizontal resolution is
not guaranteed to be identical. Setting up the geometry of a CRT display to get accurate,
and straight/true alignment requires some practice. LCD displays don't require any
geometry adjustment since they are a pixel-based display.
The resolution of a 640 480 display on a 15-inch monitor is computed using a little

Pythagoras to find out the height and width of the display.
Screen display resolutions are described using dots per inch ( DPI ). In the discussions
on film formats we specify sample resolutions as pixels per mm because the film sizes are
expressed in mm. It is easy to convert between them using the 25.4 mm to the inch con-
version ratio, so 10 dots per mm is equivalent to 254 DPI.
We know that the display is 15 inches measured diagonally. We also know that the
aspect ratio is 4:3 (or 16:9). Pythagoras tells us that the diagonal of a triangle whose sides

×

640

480

Figure 6-2 Aspect ratio to size versus resolution.

Viewsonic: http://www.viewsoniceurope.com/
Olympus SH880: http://www.olympus.co.jp/en/news/2004a/nr040414sh880e.cfm
NHK 4000 line display: http://www.nhk.or.jp/strl/publica/dayori-new/en/rd-0210e.html