Expand comment (cont'd).
diff --git a/src/smooth/ftgrays.c b/src/smooth/ftgrays.c
index 5ae7918..c550c33 100644
--- a/src/smooth/ftgrays.c
+++ b/src/smooth/ftgrays.c
@@ -380,6 +380,12 @@
/* each slanted line. Nevertheless, these divisions are noticeable */
/* in the overall performance because flattened curves produce a */
/* very large number of slanted lines. */
+ /* */
+ /* The division results here are always within ONE_PIXEL. Therefore */
+ /* the shift magnitude should be at least PIXEL_BITS wider than the */
+ /* divisors to provide sufficient accuracy of the multiply-shift. */
+ /* It should not exceed (64 - PIXEL_BITS) to prevent overflowing and */
+ /* leave enough room for 64-bit unsigned multiplication however. */
#define FT_UDIVPREP( c, b ) \
FT_Int64 b ## _r = c ? (FT_Int64)( ~(FT_UInt64)0 >> PIXEL_BITS ) / ( b ) \
: 0
