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RE: px vs. pt

From: Walter Ian Kaye <walter@natural-innovations.com>
Date: Wed, 2 Aug 2000 07:58:04 -0700
Message-Id: <v04220828b5addaa8da74@[]>
To: www-style@w3.org
At 06:09a -0700 07/27/2000, Rowland Shaw didst inscribe upon an 
electronic papyrus:
>  > > Because your UA was assuming a screen resolution of 96dpi, which is used
>  > > when the "large fonts" option is selected by the user in the Display
>  > > Properties on the windows client.
>  >
>  > No it's not. See http://richinstyle.com/small.png for a disproof.
>Woo... seems your graphics card drivers are better than mine :)
>  > > AFAIK, UAs rely on the same setting that wysiwyg text editors (eg. Word)
>  > > for computing font sizes,
>  >
>  > On which subject, Word is broken: see, for example,
>  > http://www.freetype.org/docs/glyphs/index.html (section IV, part 4), an
>  > excellent document that I recommend reading anyway.
>Found it quite interesting, as I coming from a software development role
>more than a DTP background...

Then you'll also enjoy the following, from the Microsoft KnowledgeBase:

Q62911 - Video Resolution Standards

    The following table summarizes the different standards for monochrome,
    CGA, EGA, and VGA video displays:
    Standard        Resolution Pixels    Total Colors    Colors/Screen
    --------        -----------------    ------------    -------------

    Hercules MDA        720 x 348            N/A          N/A
    CGA                 640 x 200             16            4
    EGA                 640 x 350             64           16
    PGA                 640 x 480           4096          256
      CGA Mode          640 x 200           4096          256
      Text Mode         640 x 480             16            2
      CGA Mode          640 x 200           4096          256
      Text Mode         720 x 400             16            2
                        320 x 200        262,144          256
                        640 x 480        262,144           16

    Super VGA           800 x 600            256           16
      (also called                       262,144          256 (depends
       extended VGA                                            on
       or VGA Plus)                                            display
    IBM 8514/A         1024 x 768       16 million         16
                       (interlaced)                       256 (depends
    TI TIGA            1024 x 768       16 million         16
    (Compaq, NEC)    (noninterlaced)                      256 (depends
                                                               on display
                       1280 x 1024      16 million         16  memory)

Windows NT Raster Fonts

    Table 7.2 Raster Font Sizes

    Font set and output device   Horiz. res.   Vert. res.   Aspect ratio H:V

    E = VGA display                96 dpi        96 dpi       1:1
    F = 8514 display              120 dpi       120 dpi       1:1

[[[Note: I used to have a file which listed A-D and other aspect ratios,
    but it's gone missing over the years. --wik]]]

Chapter 17 -- Text and Fonts: Background on Fonts

    The Logical Inch Problem

    [...] Windows 98 defines the system font as being a 10-point font with
    a 12-point line spacing. Depending on whether you choose Small Fonts or
    Large Fonts from the Display Properties dialog, this font could have a
    tmHeight value of 16 pixels or 20 pixels and a tmHeight minus
    tmInternalLeading value of 13 pixels or 16 pixels. Thus, the choice of
    the font implies a resolution of the device in dots per inch, namely
    96 dpi when Small Fonts are selected and 120 dpi for Large Fonts.

    You can obtain this implied resolution of the device by calling
    GetDeviceCaps with the LOGPIXELSX or LOGPIXELSY arguments. Thus, the
    metrical distance occupied by 96 or 120 pixels on the screen can be
    said to be a "logical inch." If you start measuring your screen with
    a ruler and counting pixels, you'll probably find that a logical inch
    is larger than an actual inch. Why is this?

    [...] When people read print on paper, the distance between the eyes
    and the text is generally about a foot, but a video display is commonly
    viewed from a distance of two feet.

    The logical inch in effect provides a magnification of the screen,
    allowing the display of legible fonts in a size as small as 8 points.
    Also, having 96 dots per logical inch makes the 640-pixel minimum
    display size equal to about 6.5 inches. This is precisely the width of
    text that prints on 8.5-inch-wide paper when you use the standard
    margins of an inch on each side. Thus, the logical inch also takes
    advantage of the width of the screen to allow text to be displayed as
    large as possible.

Basic hinting philosophies

    The ultimate goal of hinting is to best represent the font at all the
    resolutions at which it will be displayed. PCs use several different
    resolutions and aspect ratios11. The most common are the VGA at 96 dots
    per inch, SVGA at 120 dpi and the Macintosh at 72 dpi. All these devices
    have a 1:1 aspect ratio. When a font is hinted, the type engineer
    determines the lowest pixel per em size to which they will hint and
    retain satisfactory results. Commonly the lowest ppem size hinted is 9.
    This is the equivalent of 7 point on a VGA screen, 5.5 point on an SVGA
    and 9 point on a Macintosh.

Q101171 - Determining Video Resolution with Windows Paintbrush

    Common video resolution standards for Windows are EGA, VGA, SVGA, and
    8514 Large and Small fonts. The following are examples of these common
    video resolution standards:
    EGA            72 dpi
    VGA            96 dpi
    SVGA           96 dpi
    8514 Large    120 dpi
    8514 Small     96 dpi
Received on Wednesday, 2 August 2000 11:00:31 UTC

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