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[CSS21] Issue 149 - px vs. pt

From: fantasai <fantasai.lists@inkedblade.net>
Date: Wed, 16 Jun 2010 18:32:37 -0700
Message-ID: <4C197B35.6070508@inkedblade.net>
To: "www-style@w3.org" <www-style@w3.org>
I was given an action item to write proposed wording for CSS2.1 Issue 149
to define a fixed ratio of 4:3 for pt:px and to allow the physical value
of these units to vary. Since it's a multimedia section and a complicated
set of changes, I've posted the wording as HTML here:

The new text of the Absolute Units section is posted below:

  | Absolute length units are only mainly useful when the physical
  | properties of the output medium are output environment is known.
  | The absolute units are:
  |    * in: inches  1 inch is equal to 2.54 centimeters.
  |    * cm: centimeters
  |    * mm: millimeters
  |    * pt: points  the points used by CSS 2.1 are equal to 1/72nd
  |                   of an inch.
  |    * pc: picas  1 pica is equal to 12 points.
  |    * pt: pixel units  1 pixel unit is equal to 0.75 points.
  | At a zoom level of 100% and when the resolution of the output medium
  | is known, the absolute units should approximate their physical values.
  | For print media and similar high-resolution devices, the reference
  | unit should be one of the standard physical units (inches, centimeters,
  | etc). For lower-resolution devices, and devices with unusual viewing
  | distances, it is recommended instead that the reference unit be the
  | pixel unit: for such devices it is recommended that the pixel unit
  | refer to the whole number of device pixels that best approximates
  | the reference pixel.
  | The reference pixel is the visual angle of one pixel on a device with
  | a pixel density of 96dpi and a distance from the reader of an arm's
  | length. For a nominal arm's length of 28 inches, the visual angle is
  | therefore about 0.0213 degrees. For reading at arm's length, 1px thus
  | corresponds to about 0.26 mm (1/96 inch).
  | The image below illustrates the effect of viewing distance on the size
  | of a reference pixel: a reading distance of 71 cm (28 inches) results
  | in a reference pixel of 0.26 mm, while a reading distance of 3.5 m
  | (12 feet) results in a reference pixel of 1.3 mm.
  |                                 [[IMAGE]]
  | This second image illustrates the effect of a device's resolution on
  | the pixel unit: an area of 1px by 1px is covered by a single dot in
  | a low-resolution device (a computer screen), while the same area is
  | covered by 16 dots in a higher resolution device (such as a 400 dpi
  | laser printer).
  |                                 [[IMAGE]]

Received on Thursday, 17 June 2010 01:33:11 UTC

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