Reflections

IX.5 September 2002
Page: 64
Digital Citation

A pixel is not a point


Authors:


Here is another irritating site that thinks a pixel is a
  point:

 

p { font-family: Arial, Helvetica, sans-serif;
font-size: 11px}

 

For a brief period at the beginning of the
  Macintosh era, a pixel was indeed a point. A point, a unit of
  typesetting, is 1/72nd of an inch. Fonts are specified in
  points, so a font of 72 points is one inch high.

 

The original Macintosh quite deliberately was designed
  with screens with a "resolution" of 72 dpi.
  (Resolution, now there’s a misued word. Too often it is used
  to mean the number of pixels on a screen. Adverts announce
  screens with "A resolution of 640x480." That would
  be fine if all screens had the same physical size, then
  resolution and number of pixels would be the same. But they
  don’t. I have a screen with 1024x748 pixels, but a resolution
  of 125 dpi.) The Macintosh decision was deliberately chosen
  to make an exact match between pixels and points.
  Unfortunately, it led to a generation of people who thought
  they were the same thing. On my machine, with a resolution of
  125 dpi, a 72 pixel font will not be one inch high. It will
  be 72/125 inches high, or 0.576 inches high. So a 72 pixel
  font on my machine is 72 x (72/125) = 41 points high.

 

And the font sizes specified as 11px on the above Web site
  are (11 x 72/125) points. That is, just over six points. Put
  another way: unreadable.

 

If Moore’s Law works for LCD screens as well (and they’re
  made of transistors, so why not?), we can expect the dpi of
  screens to increase by radic2.gif every 18 months or so (at
  constant price). My next machine might be 175 dpi, and the
  one after that 245 dpi. At present it looks like Moore’s law
  is being used to reduce the price of screens rather than
  increase their dpi, but I anticipate the dpi growing again
  soon. I’ve already seen 200 dpi screens on sale for about
  $3,000.

 

Sooner or later it will get home to Web designers not to
  design in pixels anymore, because each page will fit on a
  postage stamp if they do. But at what point will they realize
  they shouldn’t be designing their sites with fixed sizes at
  all, pixels or points, but using percentages, and letting the
  user decide on the base size, so that users can increase the
  base size to make the fonts larger and therefore more
  readable? Probably when their own sight starts to fade,
  around retirement age, I suppose. Pity.

 

©2002 ACM  1072-5220/02/0900  $5.00

 

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