[FFmpeg-devel] [PATCH v4 3/4] avcodec/{ass, webvttdec}: more portable curly brace escapes

[Stefano Sabatini]

On date Monday 2024-02-19 22:42:26 +0100, Oneric wrote:
> Unlike what the old comment suggested, standard ASS has no character
> escape mechanism, but a closing curly bracket doesn't even need one.
> 

> For manual authored sub files using a full-width variant of an apropiate

appropriate?

> font and with scaling and psacing modifiers is a common workaround.

spacing

> This is not an option here, but we can still make things much less bad.
> Now the desired opening bracket still shows up in libass and
> standard renders will merely display a backslash in its place
> instead of stripping the following text like before.
> ---
>  libavcodec/ass.c          | 12 ++++++++----
>  libavcodec/webvttdec.c    |  2 +-
>  tests/ref/fate/sub-webvtt |  2 +-
>  3 files changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/libavcodec/ass.c b/libavcodec/ass.c
> index a68d3568b4..e7a1ac0eb5 100644
> --- a/libavcodec/ass.c
> +++ b/libavcodec/ass.c
> @@ -181,10 +181,14 @@ void ff_ass_bprint_text_event(AVBPrint *buf, const char *p, int size,
>          if (linebreaks && strchr(linebreaks, *p)) {
>              av_bprintf(buf, "\\N");
>  
> -        /* standard ASS escaping so random characters don't get mis-interpreted
> -         * as ASS */
> -        } else if (!keep_ass_markup && strchr("{}", *p)) {
> -            av_bprintf(buf, "\\%c", *p);
> +        /* cancel curly brackets to avoid bogus override tag blocks
> +         * hiding text. Standard ASS has no character escapes,
> +         * though (only) libass provides \{ and \}.
> +         * Unpaired closing brackets don't need escaping at all though and
> +         * to make the situation less bad in standard ASS insert an empty block
> +         */
> +        } else if (!keep_ass_markup && *p == '{') {
> +            av_bprintf(buf, "\\{{}");
>  
>          /* append word-joiner U+2060 as UTF-8 to break up sequences like \N */
>          } else if (!keep_ass_markup && *p == '\\') {
> diff --git a/libavcodec/webvttdec.c b/libavcodec/webvttdec.c
> index 6e55bc5499..35bdbe805d 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}"},
> -    {"{", "\\{"}, {"}", "\\}"}, {"\\", "\\\xe2\x81\xa0"}, // escape to avoid ASS markup conflicts
> +    {"{", "\\{{}"}, {"\\", "\\\xe2\x81\xa0"}, // escape to avoid ASS markup conflicts
>      {">", ">"}, {"<", "<"},
>      {"‎", "\xe2\x80\x8e"}, {"‏", "\xe2\x80\x8f"},
>      {"&", "&"}, {" ", "\\h"},
> diff --git a/tests/ref/fate/sub-webvtt b/tests/ref/fate/sub-webvtt
> index ea587b327c..fae50607fb 100644
> --- a/tests/ref/fate/sub-webvtt
> +++ b/tests/ref/fate/sub-webvtt
> @@ -21,7 +21,7 @@ Dialogue: 0,0:00:22.00,0:00:24.00,Default,,0,0,0,,at the AMNH.
>  Dialogue: 0,0:00:24.00,0:00:26.00,Default,,0,0,0,,Thank you for walking down here.
>  Dialogue: 0,0:00:27.00,0:00:30.00,Default,,0,0,0,,And I want to do a follow-up on the last conversation we did.\Nmultiple lines\Nagain
>  Dialogue: 0,0:00:30.00,0:00:31.50,Default,,0,0,0,,When we e-mailed—
> -Dialogue: 0,0:00:30.50,0:00:32.50,Default,,0,0,0,,Didn't we {\b1}talk {\i1}about\N{\i0} enough{\b0} in that conversation? \{I'm not an ASS comment\}
> +Dialogue: 0,0:00:30.50,0:00:32.50,Default,,0,0,0,,Didn't we {\b1}talk {\i1}about\N{\i0} enough{\b0} in that conversation? \{{}I'm not an ASS comment}
>  Dialogue: 0,0:00:32.00,0:00:35.50,Default,,0,0,0,,No! No no no no; 'cos 'cos obviously 'cos
>  Dialogue: 0,0:00:32.50,0:00:33.50,Default,,0,0,0,,{\i1}Laughs{\i0}
>  Dialogue: 0,0:00:35.50,0:00:38.00,Default,,0,0,0,,You know I'm so excited my glasses are falling off here.

Should be good otherwise (but a second look from someone more familiar
with ASS also might be good).