Fix \kf fill positioning

Our effect_timing is currently used as a coordinate relative to a weird origin:
take each glyph's center within the run, apply 3D transforms, pick the leftmost
one, and round it down a little. This makes no sense and is the result of an
unrelated code change. But if I recall correctly, \kf positioning was already
incorrect before that last change (but in a different manner).

To fix this and hopefully to prevent this kind of error from occurring again,
convert effect_timing to absolute screen coordinate.

Start the fill at the glyph run's leftmost post-transform control point.
This matches VSFilter and allows karaoke to work in vertical text (unlike,
for example, starting the fill at the first glyph's pre-transform origin).

Fixes https://github.com/libass/libass/issues/216.
Fixes https://github.com/libass/libass/issues/357.

1 files changed, 8 insertions, 3 deletions

diff --git a/libass/ass_render.h b/libass/ass_render.h
index eb4124e..9796841 100644
--- a/libass/ass_render.h
+++ b/libass/ass_render.h
@@ -99,9 +99,14 @@ typedef struct {
     FilterDesc filter;
     uint32_t c[4];              // colors
     Effect effect_type;
-    int effect_timing;          // time duration of current karaoke word
-    // after process_karaoke_effects: distance in pixels from the glyph origin.
+
+    // during render_and_combine_glyphs: distance in pixels from the karaoke origin.
+    // after render_and_combine_glyphs: screen coordinate in pixels.
     // part of the glyph to the left of it is displayed in a different color.
+    int effect_timing;
+
+    // karaoke origin: screen coordinate of leftmost post-transform control point x in subpixels
+    int32_t leftmost_x;
 
     size_t bitmap_count, max_bitmap_count;
     BitmapRef *bitmaps;
@@ -142,7 +147,7 @@ typedef struct glyph_info {
     ASS_Vector cluster_advance;
     Effect effect_type;
     int effect_timing;          // time duration of current karaoke word
-    // after process_karaoke_effects: distance in pixels from the glyph origin.
+    // after process_karaoke_effects: distance in pixels from the karaoke origin.
     // part of the glyph to the left of it is displayed in a different color.
     int effect_skip_timing;     // delay after the end of last karaoke word
     int asc, desc;              // font max ascender and descender