* src/base/ftsynth.c (FT_GlyphSlot_AdjustWeight): New API.

* include/freetype/ftsynth.h (FT_GlyphSlot_AdjustWeight): Document it.

include/freetype/ftsynth.h

@@ -68,6 +68,18 @@ FT_BEGIN_HEADER
   FT_EXPORT( void )
   FT_GlyphSlot_Embolden( FT_GlyphSlot  slot );
 
+  /* Precisely adjust the glyph weight either horizontally or vertically.  */
+  /* The `xdelta` and `ydelta` values are fractions of the face Em size    */
+  /* (in fixed-point format).  Considering that a regular face would have  */
+  /* stem widths on the order of 0.1 Em, a delta of 0.05 (0x0CCC) should   */
+  /* be very noticeable.  To increase or decrease the weight, use positive */
+  /* or negative values, respectively.                                     */
+  FT_EXPORT( void )
+  FT_GlyphSlot_AdjustWeight( FT_GlyphSlot  slot,
+                             FT_Fixed      xdelta,
+                             FT_Fixed      ydelta );
+
+
   /* Slant an outline glyph to the right by about 12 degrees.              */
   FT_EXPORT( void )
   FT_GlyphSlot_Oblique( FT_GlyphSlot  slot );

src/base/ftsynth.c

@@ -97,9 +97,18 @@
 
   FT_EXPORT_DEF( void )
   FT_GlyphSlot_Embolden( FT_GlyphSlot  slot )
+  {
+    FT_GlyphSlot_AdjustWeight( slot, 0x0AAA, 0x0AAA );
+  }
+
+
+  FT_EXPORT_DEF( void )
+  FT_GlyphSlot_AdjustWeight( FT_GlyphSlot  slot,
+                             FT_Fixed      xdelta,
+                             FT_Fixed      ydelta )
   {
     FT_Library  library;
-    FT_Face     face;
+    FT_Size     size;
     FT_Error    error;
     FT_Pos      xstr, ystr;
 
@@ -108,16 +117,15 @@
       return;
 
     library = slot->library;
-    face    = slot->face;
+    size    = slot->face->size;
 
     if ( slot->format != FT_GLYPH_FORMAT_OUTLINE &&
          slot->format != FT_GLYPH_FORMAT_BITMAP  )
       return;
 
-    /* some reasonable strength */
-    xstr = FT_MulFix( face->units_per_EM,
-                      face->size->metrics.y_scale ) / 24;
-    ystr = xstr;
+    /* express deltas in pixels in 26.6 format */
+    xstr = (FT_Pos)size->metrics.x_ppem * xdelta / 1024;
+    ystr = (FT_Pos)size->metrics.y_ppem * ydelta / 1024;
 
     if ( slot->format == FT_GLYPH_FORMAT_OUTLINE )
       FT_Outline_EmboldenXY( &slot->outline, xstr, ystr );