Implement v4++'s \kt tag

\kt allows to set the karaoke timing offset to a value other than the
sum of preceeding karaoke durations. Notably this means multiple karaoke
sequences of one Event can be ative at the same time.

Like in VSFilter, \kt is available regardless of the format version.
Using \kt after a karaoke tag in the same override sequence
always makes the preceeding karaoke act as if already completed.
Using \kt within a run resets timing for the next karaoke run.

Addresses part of https://github.com/libass/libass/issues/461.
Further support for v4++ requires at least an ABI break.

1 files changed, 20 insertions, 1 deletions

diff --git a/libass/ass_parse.c b/libass/ass_parse.c
index f43831e..a7437c6 100644
--- a/libass/ass_parse.c
+++ b/libass/ass_parse.c
@@ -790,6 +790,14 @@ char *parse_tags(ASS_Renderer *render_priv, char *p, char *end, double pwr,
                 val = render_priv->state.style->Italic;
             render_priv->state.italic = val;
             update_font(render_priv);
+        } else if (tag("kt")) {
+            // v4++
+            double val = 0;
+            if (nargs)
+                val = argtod(*args) * 10;
+            render_priv->state.effect_skip_timing = dtoi32(val);
+            render_priv->state.effect_timing = 0;
+            render_priv->state.reset_effect = true;
         } else if (tag("kf") || tag("K")) {
             double val = 100;
             if (nargs)
@@ -975,9 +983,16 @@ void process_karaoke_effects(ASS_Renderer *render_priv)
     int32_t timing = 0, skip_timing = 0;
     Effect effect_type = EF_NONE;
     GlyphInfo *last_boundary = NULL;
+    bool has_reset = false;
     for (int i = 0; i <= render_priv->text_info.length; i++) {
         if (i < render_priv->text_info.length &&
             !render_priv->text_info.glyphs[i].starts_new_run) {
+
+            if (render_priv->text_info.glyphs[i].reset_effect) {
+                has_reset = true;
+                skip_timing = 0;
+            }
+
             // VSFilter compatibility: if we have \k12345\k0 without a run
             // break, subsequent text is still part of the same karaoke word,
             // the current word's starting and ending time stay unchanged,
@@ -997,10 +1012,14 @@ void process_karaoke_effects(ASS_Renderer *render_priv)
         if (effect_type == EF_NONE)
             continue;
 
+        if (start->reset_effect)
+            timing = 0;
+
         long long tm_start = timing + start->effect_skip_timing;
         long long tm_end = tm_start + start->effect_timing;
-        timing = tm_end + skip_timing;
+        timing = !has_reset * tm_end + skip_timing;
         skip_timing = 0;
+        has_reset = false;
 
         if (effect_type != EF_KARAOKE_KF)
             tm_end = tm_start;