Ignore glyphs[0].pos.y in compute_string_bbox

In most cases, text_info->glyphs[0].pos.y is 0. This makes the bbox
in compute_string_bbox have bbox.y_min = -text_info->lines[0].asc.
Thus, during event positioning, we refer to the text's top and
bottom edge coordinates in two equivalent ways: \an4-6, \pos, \move
and Scroll effects use bbox.y_min and bbox.y_max, while \an1-3
and \an7-9 use -text_info->lines[0].asc and text_info->height -
text_info->lines[0].asc.

However, text->glyphs[0].pos.y is not *actually always* 0.
Specifically, it is nonzero for events that begin with right-to-left
text with nonzero \fay (at least two visible glyphs with the same \fay).

In this case, the anchoring point we use for vertical positioning
is different between \an1-3,7-9 and the other cases. For \an1-3,7-9,
it is the middle point of the text's left edge, which makes sense.
But for \an4-6, \pos, \move and Scroll effects, it is shifted up.
It is easiest to describe for a single-line event: the anchor is
the middle point of the *left edge of the rightmost glyph*. That is,
the anchor point is not the middle of *either* edge of the full text,
and it is hard to understand intuitively, more so in a multiline event.

VSFilter in the same situation places the anchor in the middle of the
whole event's left edge, just as we do for \an1-3 and \an7-9.

This also affects the position of rotation origin,
which is likewise unintuitive and VSFilter-incompatible.

Fix this by making compute_string_bbox ignore text->glyphs[0].pos.y
even in the rare case that it is nonzero. This makes our behavior
obvious and VSFilter-compatible, and our code easier to understand
(as the reader no longer has to wonder whether there is a difference
between the various alignments, wonder *why* there seems to be no
difference after testing common cases, and make the wrong conclusion
that glyphs[0].pos.y is always 0).

1 files changed, 1 insertions, 1 deletions

diff --git a/libass/ass_render.c b/libass/ass_render.c
index 1ed0a14..5cecbb7 100644
--- a/libass/ass_render.c
+++ b/libass/ass_render.c
@@ -845,7 +845,7 @@ static void compute_string_bbox(TextInfo *text, ASS_DRect *bbox)
     if (text->length > 0) {
         bbox->x_min = +32000;
         bbox->x_max = -32000;
-        bbox->y_min = d6_to_double(text->glyphs[0].pos.y) - text->lines[0].asc;
+        bbox->y_min = -text->lines[0].asc;
         bbox->y_max = bbox->y_min + text->height;
 
         for (int i = 0; i < text->length; i++) {