[FFmpeg-devel] [PATCH v4 2/4] avcodec/{ass, webvttdec}: fix handling of backslashes

[Stefano Sabatini]

On date Monday 2024-02-19 22:42:25 +0100, Oneric wrote:
> Backslashes cannot be escaped by a backslash in any ASS renderer,
> but unless followed by specific characters it is just printed out.
> Insert a word-joiner character after a backslash to break up
> active sequences without changing the visual output.
> ---
>  libavcodec/ass.c       | 9 ++++++++-
>  libavcodec/webvttdec.c | 2 +-
>  2 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/ass.c b/libavcodec/ass.c
> index 5058dc8337..a68d3568b4 100644
> --- a/libavcodec/ass.c
> +++ b/libavcodec/ass.c
> @@ -183,9 +183,16 @@ 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);
>  

> +        /* append word-joiner U+2060 as UTF-8 to break up sequences like \N */

I'm confused by this, what kind of \N sequences might appear in an ASS
file? Can you show an offending sequence?

> +        } else if (!keep_ass_markup && *p == '\\') {
> +            if (p_end - p <= 3 || strncmp(p + 1, "\xe2\x81\xa0", 3))
> +                av_bprintf(buf, "\\\xe2\x81\xa0");
> +            else
> +                av_bprintf(buf, "\\");
> +
>          /* some packets might end abruptly (no \0 at the end, like for example
>           * in some cases of demuxing from a classic video container), some
>           * might be terminated with \n or \r\n which we have to remove (for
> diff --git a/libavcodec/webvttdec.c b/libavcodec/webvttdec.c
> index 990d150f16..6e55bc5499 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
>      {">", ">"}, {"<", "<"},
>      {"‎", "\xe2\x80\x8e"}, {"‏", "\xe2\x80\x8f"},
>      {"&", "&"}, {" ", "\\h"},