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[base] Improve trigonometric core. 

FreeType used to rely on a 24-step iteration CORDIC algorithm to
calculate trigonometric functions and rotate vectors. It turns out
that once the vector is in the right half-plane, the initial rotation
by 63 degrees is not necessary. The algorithm is perfectly capable
to converge to any angle starting from the second 45 degree rotation.
This patch removes the first rotation and makes it a 23-step CORDIC
algorithm.

* src/base/fttrigon.c (FT_TRIG_SCALE, FT_TRIG_COSCALE): Update macro
values.
(ft_trig_pseudo_rotate, ft_trig_pseudo_polarize): Remove initial
rotation.
This commit is contained in:
Alexei Podtelezhnikov 2012-12-20 01:03:22 -05:00
parent 768590a99d
commit d7383b7b86
2 changed files with 27 additions and 41 deletions
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17
ChangeLog
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@ -1,3 +1,20 @@
2012-12-20 Alexei Podtelezhnikov <apodtele@gmail.com>
[base] Improve trigonometric core.
FreeType used to rely on a 24-step iteration CORDIC algorithm to
calculate trigonometric functions and rotate vectors. It turns out
that once the vector is in the right half-plane, the initial rotation
by 63 degrees is not necessary. The algorithm is perfectly capable
to converge to any angle starting from the second 45 degree rotation.
This patch removes the first rotation and makes it a 23-step CORDIC
algorithm.
* src/base/fttrigon.c (FT_TRIG_SCALE, FT_TRIG_COSCALE): Update macro
values.
(ft_trig_pseudo_rotate, ft_trig_pseudo_polarize): Remove initial
rotation.
2012-12-19 Werner Lemberg <wl@gnu.org>
* src/base/ftobjs.c (ft_property_do): Fix compiler warning.

51
src/base/fttrigon.c
View File

@ -26,23 +26,23 @@
#endif
/* the following is 0.2715717684432231 * 2^30 */
#define FT_TRIG_COSCALE 0x11616E8EUL
/* the Cordic shrink factor 0.607252935008887 * 2^32 */
#define FT_TRIG_SCALE 0x9B74EDA8UL
/* the following is 0.607252935008887 * 2^30 */
#define FT_TRIG_COSCALE 0x26DD3B6AUL
/* this table was generated for FT_PI = 180L << 16, i.e. degrees */
#define FT_TRIG_MAX_ITERS 23
static const FT_Fixed
ft_trig_arctan_table[24] =
ft_trig_arctan_table[23] =
{
4157273L, 2949120L, 1740967L, 919879L, 466945L, 234379L, 117304L,
58666L, 29335L, 14668L, 7334L, 3667L, 1833L, 917L, 458L, 229L, 115L,
2949120L, 1740967L, 919879L, 466945L, 234379L, 117304L, 58666L,
29335L, 14668L, 7334L, 3667L, 1833L, 917L, 458L, 229L, 115L,
57L, 29L, 14L, 7L, 4L, 2L, 1L
};
/* the Cordic shrink factor, multiplied by 2^32 */
#define FT_TRIG_SCALE 1166391864UL /* 0x4585BA38UL */
#ifdef FT_LONG64
@ -214,25 +214,9 @@
theta -= FT_ANGLE_PI;
}
/* Initial pseudorotation, with left shift */
arctanptr = ft_trig_arctan_table;
if ( theta < 0 )
{
xtemp = x + ( y << 1 );
y = y - ( x << 1 );
x = xtemp;
theta += *arctanptr++;
}
else
{
xtemp = x - ( y << 1 );
y = y + ( x << 1 );
x = xtemp;
theta -= *arctanptr++;
}
/* Subsequent pseudorotations, with right shifts */
/* Pseudorotations, with right shifts */
i = 0;
do
{
@ -283,22 +267,7 @@
arctanptr = ft_trig_arctan_table;
if ( y > 0 )
{
xtemp = x + ( y << 1 );
y = y - ( x << 1 );
x = xtemp;
theta += *arctanptr++;
}
else
{
xtemp = x - ( y << 1 );
y = y + ( x << 1 );
x = xtemp;
theta -= *arctanptr++;
}
/* Subsequent pseudorotations, with right shifts */
/* Pseudorotations, with right shifts */
i = 0;
do
{