[libpng16] Improved DIV65535 algorithm in simpleover.c. By experiment,
(x + (x>>16) + 32769) works as well as the previous form and avoids one shift.
diff --git a/contrib/examples/simpleover.c b/contrib/examples/simpleover.c
index 38bf834..0a760b8 100644
--- a/contrib/examples/simpleover.c
+++ b/contrib/examples/simpleover.c
@@ -60,12 +60,12 @@
int main(void) {
double err = 0;
unsigned int xerr = 0;
- unsigned int r = 32768;
+ unsigned int r = 32769;
{
unsigned int x = 0;
do {
- unsigned int t = x + (x >> 16) + (x >> 31) + r;
+ unsigned int t = x + (x >> 16) + /*(x >> 31)*/ + r;
double v = x, errtest;
if (t < x) {
@@ -149,7 +149,7 @@
* of the input alpha (1-alpha). A division is required but
* it is by the constant 65535. Approximate this as:
*
- * (x + (x >> 16) + (x >> 31) + 32768) >> 16;
+ * (x + (x >> 16) + 32769) >> 16;
*
* This is exact (and does not overflow) for all values of
* x in the range 0..65535*65535. (Note that the calculation
@@ -159,7 +159,7 @@
# define compose(c)\
tmp = out_pixel[c] * in_alpha;\
- tmp = (tmp + (tmp >> 16) + (tmp >> 31) + 32768) >> 16;\
+ tmp = (tmp + (tmp >> 16) + 32769) >> 16;\
out_pixel[c] = tmp + in_pixel[c]
/* The following is very vectorizable... */
@@ -488,7 +488,7 @@
if (strcmp(sprites[isprite].name, name) == 0)
{
if (!add_sprite(output, out_buf, sprites+isprite, &argc, &argv))
- goto out; /* error in addsprite */
+ goto out; /* error in add_sprite */
break;
}
