DPI DooDoo

Intro

Varying resolutions of output devices (such as CRTs) and varying OS standards coupled with the fact that output devices do not report their geometries back to the OS, have resulted in a situation where a 9-point font does not always appear as a 9-point font.

The DooDoo

There are two different places where "dots per inch" or "pixels per inch" has meaning: the physical display, and the OS. Unfortunately the OS has no way of querying the physical device to find out its actual display geometry, since this is usually adjusted via mechanical and visual means (existing CRTs have no consciousness of how many pixels per inch they are beaming onto their phosphors). Thus, an OS must make certain assumptions about the display. MacOS's imaging engine QuickDraw assumes a nominal 72dpi display regardless of monitor adjustment, CRT screen dimensions, video resolution, or VRAM; the "Precision Bitmap Alignment" option in the LaserWriter's Page Setup dialog reduces bitmap graphics to 96% when sending to a 300dpi printer, as 288 (96% of 300) is an integral multiple of 72. Microsoft Windows has a variety of video resolution "standards" and under VGA it thinks itself to be at 96dpi (see WW0528: Application Note Discussing Fonts, Windows Raster Fonts); however, if you put a tape measure to a standard 13" PC monitor at 640x480 and do some math, you'll arrive at a very different number (possibly even below 72dpi).

Screen Shots

Here are some screen captures of Arial on Win3.1 and MacOS:

Notice how the metrics for Arial 9 Italic under Windows correspond pixel for pixel with Arial 12 Italic under MacOS.

If your browser supports CSS and you have Arial installed, here is how Arial Italic is rendered on your system:

9pt: A QUICK BROWN FOX JUMPS OVER THE LAZY DOG
9pt: a quick brown fox jumps over the lazy dog 0123456789

10pt: A QUICK BROWN FOX JUMPS OVER THE LAZY DOG
10pt: a quick brown fox jumps over the lazy dog 0123456789

11pt: A QUICK BROWN FOX JUMPS OVER THE LAZY DOG
11pt: a quick brown fox jumps over the lazy dog 0123456789

12pt: A QUICK BROWN FOX JUMPS OVER THE LAZY DOG
12pt: a quick brown fox jumps over the lazy dog 0123456789

Glossary

For the purposes of this document, I make the following definitions:

CRT: Cathode Ray Tube; a vacuum tube whose cathode emits a beam of electrons towards a phosphorescent screen. Almost all video display systems use raster imaging; see Raster Display and Vector Display.
Raster Display: An imaging device where graphics are drawn by scanning the electron beam in horizontal lines which progress from the top of the display to the bottom, while adjusting the "color" and intensity of the beam in accordance with a bitmap representation of the data. Opposite of vector display.
Vector Display: An imaging device where graphics are drawn by aiming the electron beam at specific parts of the display, such that a diagonal line would be drawn directly from location A to location B. I have only seen this type of system in dedicated monochrome CAD/CAE hardware from IBM (and that was a long time ago).
Dot: The smallest individually addressable unit in a raster imaging device.
Pixel: The smallest individually manipulable unit in a bitmapped image.
Point: A unit of typography measure; there are approximately 72 points to an inch.
Aspect Ratio: The ratio of width to height. CRT screens for televisions (NTSC, PAL, and SECAM) and for most computer monitors have a 4:3 aspect ratio; Panavision movies have a 16:9 aspect ratio (I think), and MacOS pixels have a 1:1 (square) aspect ratio.
Resolution: For a raster imaging device, the total number of horizontally addressable dots by the total number of vertically addressable dots.
Some resolutions widely deployed: 512x342 (Mac Plus, SE, and SE/30), 512x384 (Mac Color Classic, LC w/12" RGB monitor), 640x400 (some laptops), 640x480, 640x870 (portrait?), 800x600 (SVGA), 832x624, 1024x768, 1152x870, 1280x1024, 1600x1200.
Hi-Res: High Resolution. This is an ambiguous term which causes confusion, because no one knows whether it refers to total pixels, density of pixels, sharpness of picture, or some combination thereof.
Density: The number of linear bits which fill a specific unit of physical distance.
DPI (Dots Per Inch): The horizontal and/or vertical density of an output device, arrived at by dividing the resolution of the device by the physical dimensions of its imaging region.
PPI (Pixels Per Inch): The horizontal and/or vertical density of an OS's graphical display, from the point of view of the OS (and in turn, of the applications running on it).
Dot Pitch: For shadow-mask CRTs, the distance between triplets of RGB phosphor dots. Trinitron tubes get the term "stripe pitch" instead because their phosphors are deposited as vertical stripes rather than discrete dots.

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(wik, August 26, 1997) 3344 visits since 97.08.26