[raster] Handle drop-outs at glyph borders according to Microsoft.

If a drop-out rule would switch on a pixel outside of the glyph's
bounding box, use the right (or top) pixel instead.  This is an
undocumented feature, but some fonts like `Helvetica Neue LT Com 65
Medium' heavily rely on it.

Thanks to Greg Hitchcock who explained this behaviour.

* src/raster/ftraster.c (Vertical_Sweep_Drop,
Horizontal_Sweep_Drop): Implement it.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/raster/ftraster.c b/src/raster/ftraster.c
index 9638dfb0..a6d3179b 100644
--- a/src/raster/ftraster.c
+++ b/src/raster/ftraster.c
@@ -2403,6 +2403,11 @@
           return;  /* no drop-out control */
         }
 
+        /* undocumented but confirmed: If the drop-out would result in a  */
+        /* pixel outside of the bounding box, use the right pixel instead */
+        if ( pxl < 0 )
+          pxl = e1;
+
         /* check that the other pixel isn't set */
         e1 = pxl == e1 ? e2 : e1;
 
@@ -2579,6 +2584,11 @@
           return;  /* no drop-out control */
         }
 
+        /* undocumented but confirmed: If the drop-out would result in a */
+        /* pixel outside of the bounding box, use the top pixel instead  */
+        if ( pxl < 0 )
+          pxl = e1;
+
         /* check that the other pixel isn't set */
         e1 = pxl == e1 ? e2 : e1;