[FFmpeg-devel] [PATCH 1/2] avcodec/{ass, webvttdec}: fix handling of backslashes

[Oneric <oneric@oneric.de>]

Backslashes cannot be escaped by backslashes in any ASS renderer,
but unless followed by a few specific characters it is just printed
as a regular character. Insert a word-joiner character after a backslash
to break up the active sequences without changing the visual output.
Also the existing \{ and \} escapes are specific to libass only.
---
The  patch assumes UTF-8 encoding in ff_ass_bprint_text_event
(WebVTT requires UTF-8 per sepc). If we cannot assume a particular
encoding, please advise how to best insert a word-joiner character in 
the correct encoding.
---
 libavcodec/ass.c       | 5 ++++-
 libavcodec/webvttdec.c | 2 +-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/libavcodec/ass.c b/libavcodec/ass.c
index 725e4d42ba..461e110ca4 100644
--- a/libavcodec/ass.c
+++ b/libavcodec/ass.c
@@ -157,8 +157,11 @@ void ff_ass_bprint_text_event(AVBPrint *buf, const char *p, int size,
 
         /* standard ASS escaping so random characters don't get mis-interpreted
          * as ASS */
-        } else if (!keep_ass_markup && strchr("{}\\", *p)) {
+        } else if (!keep_ass_markup && strchr("{}", *p)) {
             av_bprintf(buf, "\\%c", *p);
+        } else if (!keep_ass_markup && *p == '\\') {
+           // append word-joiner U+2060 as UTF-8 to break up sequences like \N
+           av_bprintf(buf, "\\\xe2\x81\xa0");
 
         /* some packets might end abruptly (no \0 at the end, like for example
          * in some cases of demuxing from a classic video container), some
diff --git a/libavcodec/webvttdec.c b/libavcodec/webvttdec.c
index 0093f328fa..8cb739697a 100644
--- a/libavcodec/webvttdec.c
+++ b/libavcodec/webvttdec.c
@@ -37,7 +37,7 @@ static const struct {
     {"<i>", "{\\i1}"}, {"</i>", "{\\i0}"},
     {"<b>", "{\\b1}"}, {"</b>", "{\\b0}"},
     {"<u>", "{\\u1}"}, {"</u>", "{\\u0}"},
-    {"{", "\\{"}, {"}", "\\}"}, // escape to avoid ASS markup conflicts
+    {"{", "\\{"}, {"}", "\\}"}, {"\\", "\\\xe2\x81\xa0"}, // escape to avoid ASS markup conflicts
     {"&gt;", ">"}, {"&lt;", "<"},
     {"&lrm;", ""}, {"&rlm;", ""}, // FIXME: properly honor bidi marks
     {"&amp;", "&"}, {"&nbsp;", "\\h"},
-- 
2.30.2

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