[ft-devel] Windows default rendering

[Werner LEMBERG]

In private communication, Greg Hitchcock from Microsoft gave some
interesting insights how Windows selects rendering modes, which I post
here (with permission).  The main question was whether a font can
request ClearType rendering.  Below is his answer.  Note that Windows
provides three user-selectable rendering modes: CLEARTYPE_QUALITY,
ANTIALIASED_QUALITY, and NONANTIALIASED_QUALITY.

  For Windows, fonts cannot request ClearType, although they can
  provide data to font rendering to indicate how well they work with
  ClearType.

  Typically what happens in Windows is there is the default rendering
  type for GDI.  When we added anti-aliasing support (which we called
  `font smoothing') to the system we had a system variable that
  indicated what the default should be: bi-level or anti-aliased.
  That variable did not guarantee, though, how text would be rendered.
  Other factors such as the bit depth of the device or bitmap, an
  application making an explicit per font choice on rendering (by
  using the *_QUALITY flags in the LogFont) or the font indicating
  rendering thresholds through the `gasp' table could all impact what
  was displayed.  Note that if there is no gasp table in the font, the
  system uses its own default `gasp' table.

  For Windows 95 the default state of the variable was bi-level
  rendering.  This default changed to anti-aliasing for Windows 2000,
  and ClearType for Windows Vista.  This default is irrespective of
  the actual display surface -- so ClearType will be displayed on
  non-LCD screens.  These defaults are changeable by the user through
  the control panel or a Windows OEM can change the default behavior
  when they ship a version of Windows.

  When a font is rendered in the system, it uses the system default
  setting unless overridden by the application or the gasp table.

  Non-GDI applications in Windows can and do have different behavior;
  these include GDI+, WPF, and the new DirectWrite.

  The `ClearType' bit in the head table is just guidance mostly for
  East Asian complex scripts that have embedded bitmaps for smaller
  sizes, and only impacts WPF rendering.

  [...] here is the rough flow chart for GDI [in Vista].  [...] My
  intent with this flowchart is not to show every detail but instead
  to give the gist of the decision tree.

    00 Display Font
    01 If Application chooses ClearType (CLEARTYPE_QUALITY), goto 11
    02 If Application chooses Font Smoothing (ANTIALIASED_QUALITY), goto 08
    03 If Application chooses Bi-Level (NONANTIALIASED_QUALITY), goto 07
    04 If ClearType is system GDI default, goto 11
    05 If Font Smoothing is system GDI default, goto 09
    06 If Bi-Level is system GDI default, goto 07
    07 Render Font bi-level with hinting, goto 12
    08 If bit-depth < 16bpp, goto 07
    09 If gasp indicates no font smoothing, goto 07
    10 Render Font anti-aliased with hinting, goto 12
    11 Render Font ClearType with hinting, goto 12
    12 END


  Werner