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Document global size metrics needed for native bytecode hinting (#52165).

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This commit is contained in:
Werner Lemberg 2017-10-08 10:37:50 +02:00
parent 91448669a2
commit 08e2e311ef
1 changed files with 41 additions and 1 deletions
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42
include/freetype/freetype.h
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@ -1063,6 +1063,9 @@ FT_BEGIN_HEADER
/* `descender', `height', `underline_position', and */
/* `underline_thickness'. */
/* */
/* Especially for TrueType fonts see also the documentation for */
/* @FT_Size_Metrics. */
/* */
typedef struct FT_FaceRec_
{
FT_Long num_faces;
@ -1582,7 +1585,7 @@ FT_BEGIN_HEADER
/* to the following. */
/* */
/* { */
/* scaled_ascender = FT_MulFix( face->root.ascender, */
/* scaled_ascender = FT_MulFix( face->ascender, */
/* size_metrics->y_scale ); */
/* } */
/* */
@ -1596,6 +1599,43 @@ FT_BEGIN_HEADER
/* */
/* The `FT_Size_Metrics' structure is valid for bitmap fonts also. */
/* */
/* */
/* *TrueType* *fonts* *with* *native* *bytecode* *hinting* */
/* */
/* All applications that handle TrueType fonts with native hinting */
/* must be aware that TTFs expect different rounding of vertical font */
/* dimensions. The application has to cater for this, especially if */
/* it wants to rely on a TTF's vertical data (for example, to */
/* properly align box characters vertically). */
/* */
/* Only the application knows _in_ _advance_ that it is going to use */
/* native hinting for TTFs! FreeType, on the other hand, selects the */
/* hinting mode not at the time of creating an @FT_Size object but */
/* much later, namely while calling @FT_Load_Glyph. */
/* */
/* Here is some pseudo code that illustrates a possible solution. */
/* */
/* { */
/* font_format = FT_Get_Font_Format( face ); */
/* */
/* if ( !strcmp( font_format, "TrueType" ) && */
/* do_native_bytecode_hinting ) */
/* { */
/* ascender = ROUND( FT_MulFix( face->ascender, */
/* size_metrics->y_scale ) ); */
/* descender = ROUND( FT_MulFix( face->descender, */
/* size_metrics->y_scale ) ); */
/* } */
/* else */
/* { */
/* ascender = size_metrics->ascender; */
/* descender = size_metrics->descender; */
/* } */
/* */
/* height = size_metrics->height; */
/* max_advance = size_metrics->max_advance; */
/* } */
/* */
typedef struct FT_Size_Metrics_
{
FT_UShort x_ppem; /* horizontal pixels per EM */