From b6eef5ae7bc5fd3a07f237c8f609d12dd4084130 Mon Sep 17 00:00:00 2001 From: Werner Lemberg Date: Thu, 10 Sep 2015 07:44:48 +0200 Subject: [PATCH] Better document various TrueType hinting modes. Text based on private communication with Greg Hitchcock and added to the repository with his permission. --- include/freetype/ftttdrv.h | 77 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 77 insertions(+) diff --git a/include/freetype/ftttdrv.h b/include/freetype/ftttdrv.h index f56040b82..dc0081a0b 100644 --- a/include/freetype/ftttdrv.h +++ b/include/freetype/ftttdrv.h @@ -52,6 +52,83 @@ FT_BEGIN_HEADER * * 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. + * */