From 2b07a5ae3d8342f171f99958225b523758ed35bc Mon Sep 17 00:00:00 2001 From: Alexei Podtelezhnikov Date: Tue, 28 Jul 2015 23:14:30 -0400 Subject: [PATCH] * src/base/ftcalc.c (FT_Vector_NormLen): Clarify. --- ChangeLog | 4 ++++ src/base/ftcalc.c | 12 ++++++++---- 2 files changed, 12 insertions(+), 4 deletions(-) diff --git a/ChangeLog b/ChangeLog index 986563cfa..065a16bd1 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,7 @@ +2015-07-28 Alexei Podtelezhnikov + + * src/base/ftcalc.c (FT_Vector_NormLen): Clarify. + 2015-07-27 Alexei Podtelezhnikov * src/base/ftcalc.c (FT_Vector_NormLen): Explicate type conversions. diff --git a/src/base/ftcalc.c b/src/base/ftcalc.c index e31eed268..5a8003dc2 100644 --- a/src/base/ftcalc.c +++ b/src/base/ftcalc.c @@ -849,13 +849,15 @@ x_ = (FT_Int32)x; y_ = (FT_Int32)y; - /* Newton's iterations */ + /* Newton's iterations */ do { u = (FT_UInt32)( x_ + ( x_ * b >> 16 ) ); v = (FT_UInt32)( y_ + ( y_ * b >> 16 ) ); - /* converting to signed gives difference with 2^32 */ + /* Normalized squared length in the parentheses approaches 2^32. */ + /* On two's complement systems, converting to signed gives the */ + /* difference with 2^32 even if the expression wraps around. */ z = -(FT_Int32)( u * u + v * v ) / 0x200; z = z * ( ( 0x10000 + b ) >> 8 ) / 0x10000; @@ -866,8 +868,10 @@ vector->x = sx < 0 ? -(FT_Pos)u : (FT_Pos)u; vector->y = sy < 0 ? -(FT_Pos)v : (FT_Pos)v; - /* true length, again taking advantage of signed difference with 2^32 */ - l = 0x10000 + (FT_Int32)( u * x + v * y ) / 0x10000; + /* Conversion to signed helps to recover from likely wrap around */ + /* in calculating the prenormalized length, because it gives the */ + /* correct difference with 2^32 on two's complement systems. */ + l = (FT_UInt32)( 0x10000 + (FT_Int32)( u * x + v * y ) / 0x10000 ); if ( shift > 0 ) l = ( l + ( 1 << ( shift - 1 ) ) ) >> shift; else