include/freetype/internal/autohint.h

/****************************************************************************
 *
 * autohint.h
 *
 *   High-level 'autohint' module-specific interface (specification).
 *
 * Copyright (C) 1996-2022 by
 * David Turner, Robert Wilhelm, and Werner Lemberg.
 *
 * This file is part of the FreeType project, and may only be used,
 * modified, and distributed under the terms of the FreeType project
 * license, LICENSE.TXT.  By continuing to use, modify, or distribute
 * this file you indicate that you have read the license and
 * understand and accept it fully.
 *
 */


  /**************************************************************************
   *
   * The auto-hinter is used to load and automatically hint glyphs if a
   * format-specific hinter isn't available.
   *
   */


#ifndef AUTOHINT_H_
#define AUTOHINT_H_


  /**************************************************************************
   *
   * A small technical note regarding automatic hinting in order to clarify
   * this module interface.
   *
   * An automatic hinter might compute two kinds of data for a given face:
   *
   * - global hints: Usually some metrics that describe global properties
   *                 of the face.  It is computed by scanning more or less
   *                 aggressively the glyphs in the face, and thus can be
   *                 very slow to compute (even if the size of global hints
   *                 is really small).
   *
   * - glyph hints: These describe some important features of the glyph
   *                 outline, as well as how to align them.  They are
   *                 generally much faster to compute than global hints.
   *
   * The current FreeType auto-hinter does a pretty good job while performing
   * fast computations for both global and glyph hints.  However, we might be
   * interested in introducing more complex and powerful algorithms in the
   * future, like the one described in the John D. Hobby paper, which
   * unfortunately requires a lot more horsepower.
   *
   * Because a sufficiently sophisticated font management system would
   * typically implement an LRU cache of opened face objects to reduce memory
   * usage, it is a good idea to be able to avoid recomputing global hints
   * every time the same face is re-opened.
   *
   * We thus provide the ability to cache global hints outside of the face
   * object, in order to speed up font re-opening time.  Of course, this
   * feature is purely optional, so most client programs won't even notice
   * it.
   *
   * I initially thought that it would be a good idea to cache the glyph
   * hints too.  However, my general idea now is that if you really need to
   * cache these too, you are simply in need of a new font format, where all
   * this information could be stored within the font file and decoded on the
   * fly.
   *
   */


#include <freetype/freetype.h>


FT_BEGIN_HEADER


  typedef struct FT_AutoHinterRec_  *FT_AutoHinter;


  /**************************************************************************
   *
   * @functype:
   *   FT_AutoHinter_GlobalGetFunc
   *
   * @description:
   *   Retrieve the global hints computed for a given face object.  The
   *   resulting data is dissociated from the face and will survive a call to
   *   FT_Done_Face().  It must be discarded through the API
   *   FT_AutoHinter_GlobalDoneFunc().
   *
   * @input:
   *   hinter ::
   *     A handle to the source auto-hinter.
   *
   *   face ::
   *     A handle to the source face object.
   *
   * @output:
   *   global_hints ::
   *     A typeless pointer to the global hints.
   *
   *   global_len ::
   *     The size in bytes of the global hints.
   */
  typedef void
  (*FT_AutoHinter_GlobalGetFunc)( FT_AutoHinter  hinter,
                                  FT_Face        face,
                                  void**         global_hints,
                                  long*          global_len );


  /**************************************************************************
   *
   * @functype:
   *   FT_AutoHinter_GlobalDoneFunc
   *
   * @description:
   *   Discard the global hints retrieved through
   *   FT_AutoHinter_GlobalGetFunc().  This is the only way these hints are
   *   freed from memory.
   *
   * @input:
   *   hinter ::
   *     A handle to the auto-hinter module.
   *
   *   global ::
   *     A pointer to retrieved global hints to discard.
   */
  typedef void
  (*FT_AutoHinter_GlobalDoneFunc)( FT_AutoHinter  hinter,
                                   void*          global );


  /**************************************************************************
   *
   * @functype:
   *   FT_AutoHinter_GlobalResetFunc
   *
   * @description:
   *   This function is used to recompute the global metrics in a given font.
   *   This is useful when global font data changes (e.g. Multiple Masters
   *   fonts where blend coordinates change).
   *
   * @input:
   *   hinter ::
   *     A handle to the source auto-hinter.
   *
   *   face ::
   *     A handle to the face.
   */
  typedef void
  (*FT_AutoHinter_GlobalResetFunc)( FT_AutoHinter  hinter,
                                    FT_Face        face );


  /**************************************************************************
   *
   * @functype:
   *   FT_AutoHinter_GlyphLoadFunc
   *
   * @description:
   *   This function is used to load, scale, and automatically hint a glyph
   *   from a given face.
   *
   * @input:
   *   face ::
   *     A handle to the face.
   *
   *   glyph_index ::
   *     The glyph index.
   *
   *   load_flags ::
   *     The load flags.
   *
   * @note:
   *   This function is capable of loading composite glyphs by hinting each
   *   sub-glyph independently (which improves quality).
   *
   *   It will call the font driver with @FT_Load_Glyph, with
   *   @FT_LOAD_NO_SCALE set.
   */
  typedef FT_Error
  (*FT_AutoHinter_GlyphLoadFunc)( FT_AutoHinter  hinter,
                                  FT_GlyphSlot   slot,
                                  FT_Size        size,
                                  FT_UInt        glyph_index,
                                  FT_Int32       load_flags );


  /**************************************************************************
   *
   * @struct:
   *   FT_AutoHinter_InterfaceRec
   *
   * @description:
   *   The auto-hinter module's interface.
   */
  typedef struct  FT_AutoHinter_InterfaceRec_
  {
    FT_AutoHinter_GlobalResetFunc  reset_face;
    FT_AutoHinter_GlobalGetFunc    get_global_hints;
    FT_AutoHinter_GlobalDoneFunc   done_global_hints;
    FT_AutoHinter_GlyphLoadFunc    load_glyph;

  } FT_AutoHinter_InterfaceRec, *FT_AutoHinter_Interface;


#define FT_DECLARE_AUTOHINTER_INTERFACE( class_ )            \
  FT_CALLBACK_TABLE const FT_AutoHinter_InterfaceRec  class_;

#define FT_DEFINE_AUTOHINTER_INTERFACE(       \
          class_,                             \
          reset_face_,                        \
          get_global_hints_,                  \
          done_global_hints_,                 \
          load_glyph_ )                       \
  FT_CALLBACK_TABLE_DEF                       \
  const FT_AutoHinter_InterfaceRec  class_ =  \
  {                                           \
    reset_face_,                              \
    get_global_hints_,                        \
    done_global_hints_,                       \
    load_glyph_                               \
  };


FT_END_HEADER

#endif /* AUTOHINT_H_ */


/* END */