[FFmpeg-devel] [PATCH] avcodec/webvttdec: strip classes

[Leon Grutters]

On 3/20/25 9:02 AM, Andreas Rheinhardt wrote:
> Leon Grutters:
>> If a supported tag has a class, e.g "<i.bold>" it is ignored entirely;
>> so for example "<i.bold>Hello</i>" would be converted to "Hello{\i0}"
>> instead of the intended "{\i1}Hello{\i0}".
>>
>> Signed-off-by: Leon Grutters<gruttersleonbot2 at gmail.com>
>> ---
>>   libavcodec/webvttdec.c | 51 +++++++++++++++++++++++++++++++++---------
>>   1 file changed, 40 insertions(+), 11 deletions(-)
>>
>> diff --git a/libavcodec/webvttdec.c b/libavcodec/webvttdec.c
>> index 35bdbe805d..4111d138c4 100644
>> --- a/libavcodec/webvttdec.c
>> +++ b/libavcodec/webvttdec.c
>> @@ -29,25 +29,53 @@
>>   #include "ass.h"
>>   #include "codec_internal.h"
>>   #include "libavutil/bprint.h"
>> +#include "libavutil/mem.h"
>>   
>>   static const struct {
>>       const char *from;
>>       const char *to;
>>   } webvtt_tag_replace[] = {
>> -    {"<i>", "{\\i1}"}, {"</i>", "{\\i0}"},
>> -    {"<b>", "{\\b1}"}, {"</b>", "{\\b0}"},
>> -    {"<u>", "{\\u1}"}, {"</u>", "{\\u0}"},
>>       {"{", "\\{{}"}, {"\\", "\\\xe2\x81\xa0"}, // escape to avoid ASS markup conflicts
>>       {">", ">"}, {"<", "<"},
>>       {"‎", "\xe2\x80\x8e"}, {"‏", "\xe2\x80\x8f"},
>>       {"&", "&"}, {" ", "\\h"},
>>   };
>> +static const struct {
>> +    const char *from;
>> +    const char *to;
>> +} webvtt_valid_tags[] = {
>> +    {"i", "{\\i1}"}, {"/i", "{\\i0}"},
>> +    {"b", "{\\b1}"}, {"/b", "{\\b0}"},
>> +    {"u", "{\\u1}"}, {"/u", "{\\u0}"},
>> +};
> These strings are so small that one can avoid the relocations by using
> fixes-size buffers.
What do you mean exactly? I'm not sure I understand.
>>   
>>   static int webvtt_event_to_ass(AVBPrint *buf, const char *p)
>>   {
>> -    int i, again = 0, skip = 0;
>> +    int i, again = 0/*, skip = 0*/;
>>   
>>       while (*p) {
>> +        if (*p == '<') {
>> +            const char *tag_end = strchr(p, '>');
>> +            char *tag_body, *tag_name, *saveptr = NULL;
>> +            ptrdiff_t len;
>> +            if (!tag_end)
>> +                break;
>> +            len = tag_end - p + 1;
>> +            tag_body = av_strndup(p + 1, len - 2);
>> +            if (!tag_body)
>> +                return AVERROR(ENOMEM);
> This allocation seems unnecessary. You can inspect the string without
> modifying it by using strncmp() below and by using a maximum field width
> for the %s directive in a parameter.
My thought process here was that if it's done this way, you have saveptr
for possibly getting the classes (and tag annotation) later on.
>> +            tag_name = av_strtok(tag_body, ".", &saveptr);
>> +            for (i = 0; i < FF_ARRAY_ELEMS(webvtt_valid_tags); i++) {
>> +                const char *from = webvtt_valid_tags[i].from;
>> +                if(!strcmp(tag_name, from)) {
>> +                    av_bprintf(buf, "%s", webvtt_valid_tags[i].to);
>> +                    break;
>> +                }
>> +            }
>> +            p += len;
>> +            again = 1;
>> +            av_freep(&tag_body);
>> +        }
>>   
>>           for (i = 0; i < FF_ARRAY_ELEMS(webvtt_tag_replace); i++) {
>>               const char *from = webvtt_tag_replace[i].from;
>> @@ -59,21 +87,22 @@ static int webvtt_event_to_ass(AVBPrint *buf, const char *p)
>>                   break;
>>               }
>>           }
>> +
>>           if (!*p)
>>               break;
>>   
>>           if (again) {
>>               again = 0;
>> -            skip = 0;
>> +            // skip = 0;
>>               continue;
>>           }
>> -        if (*p == '<')
>> -            skip = 1;
>> -        else if (*p == '>')
>> -            skip = 0;
>> -        else if (p[0] == '\n' && p[1])
>> +        // if (*p == '<')
>> +        //     skip = 1;
>> +        // else if (*p == '>')
>> +        //     skip = 0;
>> +        if (p[0] == '\n' && p[1])
>>               av_bprintf(buf, "\\N");
>> -        else if (!skip && *p != '\r')
>> +        else if (/*!skip && */*p != '\r')
>>               av_bprint_chars(buf, *p, 1);
>>           p++;
>>       }