ref: b6eef5ae7bc5fd3a07f237c8f609d12dd4084130
parent: e5a27a7596684eea63190d51551165eea5d20717
author: Werner Lemberg <[email protected]>
date: Thu Sep 10 03:44:48 EDT 2015
Better document various TrueType hinting modes. Text based on private communication with Greg Hitchcock and added to the repository with his permission.
--- a/include/freetype/ftttdrv.h
+++ b/include/freetype/ftttdrv.h
@@ -52,6 +52,83 @@
*
* The TrueType driver's module name is `truetype'.
*
+ * We start with a list of definitions, kindly provided by Greg
+ * Hitchcock.
+ *
+ * _Bi-Level_ _Rendering_
+ *
+ * Monochromatic rendering, exclusively used in the early days of
+ * TrueType by both Apple and Microsoft. Microsoft's GDI interface
+ * supported hinting of the right-side bearing point, such that the
+ * advance width could be non-linear. Most often this was done to
+ * achieve some level of glyph symmetry. To enable reasonable
+ * performance (e.g., not having to run hinting on all glyphs just to
+ * get the widths) there was a bit in the head table indicating if the
+ * side bearing was hinted, and additional tables, `hdmx' and `LTSH', to
+ * cache hinting widths across multiple sizes and device aspect ratios.
+ *
+ * _Font_ _Smoothing_
+ *
+ * Microsoft's GDI implementation of anti-aliasing. Not traditional
+ * anti-aliasing as the outlines were hinted before the sampling. The
+ * widths matched the bi-level rendering.
+ *
+ * _ClearType_ _Rendering_
+ *
+ * Technique that uses physical subpixels to improve rendering on LCD
+ * (and other) displays. Because of the higher resolution, many methods
+ * of improving symmetry in glyphs through hinting the right-side
+ * bearing were no longer necessary. This lead to what GDI calls
+ * `natural widths' ClearType, see
+ * http://www.beatstamm.com/typography/RTRCh4.htm#Sec21. Since hinting
+ * has extra resolution, most non-linearity went away, but it is still
+ * possible for hints to change the advance widths in this mode.
+ *
+ * _ClearType_ _Compatible_ _Widths_
+ *
+ * One of the earliest challenges with ClearType was allowing the
+ * implementation in GDI to be selected without requiring all UI and
+ * documents to reflow. To address this, a compatible method of
+ * rendering ClearType was added where the font hints are executed once
+ * to determine the width in bi-level rendering, and then re-run in
+ * ClearType, with the difference in widths being absorbed in the font
+ * hints for ClearType (mostly in the white space of hints); see
+ * http://www.beatstamm.com/typography/RTRCh4.htm#Sec20. Somewhat by
+ * definition, compatible width ClearType allows for non-linear widths,
+ * but only when the bi-level version has non-linear widths.
+ *
+ * _ClearType_ _Subpixel_ _Positioning_
+ *
+ * One of the nice benefits of ClearType is the ability to more crisply
+ * display fractional widths; unfortunately, the GDI model of integer
+ * bitmaps did not support this. However, the WPF and Direct Write
+ * frameworks do support fractional widths. DWrite calls this `natural
+ * mode', not to be confused with GDI's `natural widths'. Subpixel
+ * positioning, in the current implementation of Direct Write,
+ * unfortunately does not support hinted advance widths, see
+ * http://www.beatstamm.com/typography/RTRCh4.htm#Sec22. Note that the
+ * TrueType interpreter fully allows the advance width to be adjusted in
+ * this mode, just the DWrite client will ignore those changes.
+ *
+ * _ClearType_ _Backwards_ _Compatibility_
+ *
+ * This is a set of exceptions made in the TrueType interpreter to
+ * minimize hinting techniques that were problematic with the extra
+ * resolution of ClearType; see
+ * http://www.beatstamm.com/typography/RTRCh4.htm#Sec1 and
+ * http://www.microsoft.com/typography/cleartype/truetypecleartype.aspx.
+ * This technique is not to be confused with ClearType compatible
+ * widths. ClearType backwards compatibility has no direct impact on
+ * changing advance widths, but there might be an indirect impact on
+ * disabling some deltas. This could be worked around in backwards
+ * compatibility mode.
+ *
+ * _Native_ _ClearType_ _Mode_
+ *
+ * (Not to be confused with `natural widths'.) This mode removes all
+ * the exceptions in the TrueType interpreter when running with
+ * ClearType. Any issues on widths would still apply, though.
+ *
*/