[smooth] Introduce direct oversampling for overlaps.

This implements oversampling to metigate artifacts in pixels partially
covered by overlapping contours.  It turns out that the 4x4
oversampling is sufficient but, at least, quadruples the rendering
time.  The outline has to set FT_OUTLINE_OVERLAP to use this method.

* include/freetype/ftimage.h (FT_OUTLINE_OVERLAP): New flag.
* src/smooth/ftsmooth.c (ft_smooth_render): Check it to...
(ft_smooth_raster_overlap): ... inflate outline and set up direct
rendering for oversampling with...
(ft_smooth_overlap_spans): ... new span function that integrates them.

1 files changed, 98 insertions, 6 deletions

diff --git a/src/smooth/ftsmooth.c b/src/smooth/ftsmooth.c
index 1e0d0a3e3..b32629205 100644
--- a/src/smooth/ftsmooth.c
+++ b/src/smooth/ftsmooth.c
@@ -333,7 +333,94 @@
 
 #endif  /* FT_CONFIG_OPTION_SUBPIXEL_RENDERING */
 
-  /* convert a slot's glyph image into a bitmap */
+/* Oversampling scale to be used in rendering overlaps */
+#define SCALE  ( 1 << 2 )
+
+  /* This function averages inflated spans in direct rendering mode */
+  static void
+  ft_smooth_overlap_spans( int             y,
+                           int             count,
+                           const FT_Span*  spans,
+                           TOrigin*        target )
+  {
+    unsigned char*  dst = target->origin - ( y / SCALE ) * target->pitch;
+    unsigned short  x;
+    unsigned int    cover, sum;
+
+
+    /* When accumulating the oversampled spans we need to assure that  */
+    /* fully covered pixels are equal to 255 and do not overflow.      */
+    /* It is important that the SCALE is a power of 2, each subpixel   */
+    /* cover can also reach a power of 2 after rounding, and the total */
+    /* is clamped to 255 when it adds up to 256.                       */
+    for ( ; count--; spans++ )
+    {
+      cover = ( spans->coverage + SCALE * SCALE / 2 ) / ( SCALE * SCALE );
+      for ( x = 0; x < spans->len; x++ )
+      {
+        sum = dst[ ( spans->x + x ) / SCALE ] + cover;
+        dst[ ( spans->x + x ) / SCALE ] = sum - ( sum >> 8 );
+      }
+    }
+  }
+
+
+  static FT_Error
+  ft_smooth_raster_overlap( FT_Renderer  render,
+                            FT_Outline*  outline,
+                            FT_Bitmap*   bitmap )
+  {
+    FT_Error    error      = FT_Err_Ok;
+    FT_Vector*  points     = outline->points;
+    FT_Vector*  points_end = FT_OFFSET( points, outline->n_points );
+    FT_Vector*  vec;
+
+    FT_Raster_Params   params;
+    TOrigin            target;
+
+
+    /* Set up direct rendering to average oversampled spans. */
+    params.target     = bitmap;
+    params.source     = outline;
+    params.flags      = FT_RASTER_FLAG_AA | FT_RASTER_FLAG_DIRECT;
+    params.gray_spans = (FT_SpanFunc)ft_smooth_overlap_spans;
+    params.user       = &target;
+
+    params.clip_box.xMin = 0;
+    params.clip_box.yMin = 0;
+    params.clip_box.xMax = bitmap->width * SCALE;
+    params.clip_box.yMax = bitmap->rows  * SCALE;
+
+    if ( bitmap->pitch < 0 )
+      target.origin = bitmap->buffer;
+    else
+      target.origin = bitmap->buffer
+                      + ( bitmap->rows - 1 ) * (unsigned int)bitmap->pitch;
+
+    target.pitch = bitmap->pitch;
+
+    /* inflate outline */
+    for ( vec = points; vec < points_end; vec++ )
+    {
+      vec->x *= SCALE;
+      vec->y *= SCALE;
+    }
+
+    /* render outline into the bitmap */
+    error = render->raster_render( render->raster, &params );
+
+    /* deflate outline */
+    for ( vec = points; vec < points_end; vec++ )
+    {
+      vec->x /= SCALE;
+      vec->y /= SCALE;
+    }
+
+    return error;
+  }
+
+#undef SCALE
+
   static FT_Error
   ft_smooth_render( FT_Renderer       render,
                     FT_GlyphSlot      slot,
@@ -407,14 +494,19 @@
     if ( mode == FT_RENDER_MODE_NORMAL ||
          mode == FT_RENDER_MODE_LIGHT  )
     {
-      FT_Raster_Params  params;
+      if ( outline->flags & FT_OUTLINE_OVERLAP )
+        error = ft_smooth_raster_overlap( render, outline, bitmap );
+      else
+      {
+        FT_Raster_Params  params;
 
 
-      params.target = bitmap;
-      params.source = outline;
-      params.flags  = FT_RASTER_FLAG_AA;
+        params.target = bitmap;
+        params.source = outline;
+        params.flags  = FT_RASTER_FLAG_AA;
 
-      error = render->raster_render( render->raster, &params );
+        error = render->raster_render( render->raster, &params );
+      }
     }
     else
     {