Commit 90be4b6377f47c9e8241beaf0decf0466555ab50
2014-08-08T22:27:10
* src/base/ftbitmap.c (ft_gray_for_premultiplied_srgb_bgra): Refactor.
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diff --git a/ChangeLog b/ChangeLog
index 2be36e1..e89e459 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2014-08-08 Alexei Podtelezhnikov <apodtele@gmail.com>
+
+ * src/base/ftbitmap.c (ft_gray_for_premultiplied_srgb_bgra): Refactor.
+
2014-07-26 Werner Lemberg <wl@gnu.org>
[cff] Fix typo.
diff --git a/src/base/ftbitmap.c b/src/base/ftbitmap.c
index 6542c79..158a428 100644
--- a/src/base/ftbitmap.c
+++ b/src/base/ftbitmap.c
@@ -378,8 +378,8 @@
static FT_Byte
ft_gray_for_premultiplied_srgb_bgra( const FT_Byte* bgra )
{
- FT_Byte a = bgra[3];
- FT_ULong l;
+ FT_UInt a = bgra[3];
+ FT_UInt l;
/* Short-circuit transparent color to avoid div-by-zero. */
@@ -395,14 +395,14 @@
* http://accessibility.kde.org/hsl-adjusted.php
*
* We do the computation with integers only, applying a gamma of 2.0.
- * The following will never overflow 32 bits; it is a scaled-up
- * luminosity with premultiplication not yet undone.
+ * We guarantee 32-bit arithmetic to avoid overflow but the resulting
+ * luminosity fits into 16 bits.
*
*/
- l = 4731UL /* 0.0722 * 65536 */ * bgra[0] * bgra[0] +
- 46871UL /* 0.7152 * 65536 */ * bgra[1] * bgra[1] +
- 13933UL /* 0.2126 * 65536 */ * bgra[2] * bgra[2];
+ l = ( 4732UL /* 0.0722 * 65536 */ * bgra[0] * bgra[0] +
+ 46871UL /* 0.7152 * 65536 */ * bgra[1] * bgra[1] +
+ 13933UL /* 0.2126 * 65536 */ * bgra[2] * bgra[2] ) >> 16;
/*
* Final transparency can be determined as follows.
@@ -413,11 +413,11 @@
*
* So the formula is a * (1 - l) = a - l * a.
*
- * In the actual code, we undo premultiplication and scale down again.
+ * We still need to undo premultiplication by dividing l by a*a.
*
*/
- return a - (FT_Byte)( ( l / a ) >> 16 );
+ return (FT_Byte)( a - l / a );
}