[ft-devel] suggested improvement to FT_GlyphSlot_Embolden

[Graham Asher]

Dear FreeTypers,

I am using FT_Glyphslot_Embolden and so far it seems to work quite well.
However, the default extra boldness (1/24 em) is too great for my purposes,
and ought to be settable via an argument, because different values are
needed at different times. For example, 1/32 em is adequate for making
DejaVu Sans Bold extra-bold.

I therefore suggest the following changes.

The function declaration in ftsynth,h becomes:


  /*
  Make a glyph bolder. The argument "boldness" is a
  fixed-point value in ems. Strokes are thickened
  by this value.
  */
  FT_EXPORT( void )
  FT_GlyphSlot_Embolden( FT_GlyphSlot  slot,
                         FT_Fixed      boldness );


The function itself in ftsynth.c becomes:


  FT_EXPORT_DEF( void )
  FT_GlyphSlot_Embolden( FT_GlyphSlot  slot,
                         FT_Fixed      boldness )
  {
    FT_Library  library = slot->library;
    FT_Face     face    = FT_SLOT_FACE( slot );
    FT_Error    error;
    FT_Pos      xstr, ystr;


    if ( slot->format != FT_GLYPH_FORMAT_OUTLINE &&
         slot->format != FT_GLYPH_FORMAT_BITMAP )
      return;

    /* convert boldness to pixels */
    xstr = FT_MulFix( face->units_per_EM,
                      face->size->metrics.y_scale );
    xstr = FT_MulFix( xstr, boldness );

    ystr = xstr;

    if ( slot->format == FT_GLYPH_FORMAT_OUTLINE )
    {
      error = FT_Outline_Embolden( &slot->outline, xstr );
      /* ignore error */

      /* this is more than enough for most glyphs; if you need accurate */
      /* values, you have to call FT_Outline_Get_CBox                   */
      xstr = xstr * 2;
      ystr = xstr;
    }
    else if ( slot->format == FT_GLYPH_FORMAT_BITMAP )
    {
      xstr = FT_PIX_FLOOR( xstr );
      if ( xstr == 0 )
        xstr = 1 << 6;
      ystr = FT_PIX_FLOOR( ystr );

      error = FT_GlyphSlot_Own_Bitmap( slot );
      if ( error )
        return;

      error = FT_Bitmap_Embolden( library, &slot->bitmap, xstr, ystr );
      if ( error )
        return;
    }

    if ( slot->advance.x )
      slot->advance.x += xstr;

    if ( slot->advance.y )
      slot->advance.y += ystr;

    slot->metrics.width        += xstr;
    slot->metrics.height       += ystr;
    slot->metrics.horiBearingY += ystr;
    slot->metrics.horiAdvance  += xstr;
    slot->metrics.vertBearingX -= xstr / 2;
    slot->metrics.vertBearingY += ystr;
    slot->metrics.vertAdvance  += ystr;

    if ( slot->format == FT_GLYPH_FORMAT_BITMAP )
      slot->bitmap_top += ystr >> 6;
  }


Using the new function I can then pass the boldness value 65536 / 32 = 2048
to get extra boldness of 1/32 em. I have tested these changes and they work
well.

Best wishes,

Graham Asher