Fix off-by-one error in \fad, \fade

Typical greater vs. greater-or-equal case. This especially fixes fades
with zero delays. A zero delay in the two-argument form means no fade at
all, but previously this faded over a single frame, since the code used
"greater" semantics, while "greater or equal" is required here.

Notably, this avoids blinking/flickering in some tightly timed karaoke
scripts.

1 files changed, 5 insertions, 4 deletions

diff --git a/libass/ass_parse.c b/libass/ass_parse.c
index 274d84d..92a47b3 100644
--- a/libass/ass_parse.c
+++ b/libass/ass_parse.c
@@ -190,17 +190,18 @@ interpolate_alpha(long long now, long long t1, long long t2, long long t3,
 {
     unsigned a;
     double cf;
-    if (now <= t1) {
+
+    if (now < t1) {
         a = a1;
     } else if (now >= t4) {
         a = a3;
-    } else if (now < t2) {      // and > t1
+    } else if (now < t2 && t2 > t1) {
         cf = ((double) (now - t1)) / (t2 - t1);
         a = a1 * (1 - cf) + a2 * cf;
-    } else if (now > t3) {
+    } else if (now >= t3 && t4 > t3) {
         cf = ((double) (now - t3)) / (t4 - t3);
         a = a2 * (1 - cf) + a3 * cf;
-    } else {                    // t2 <= now <= t3
+    } else {                    // t2 <= now < t3
         a = a2;
     }