Re: [FFmpeg-devel] [PATCH] avformat/flvenc: allow to write qualifying metadata as number

[Gyan Doshi <ffmpeg@gyani.pro>]

On 2023-02-11 04:32 pm, Gyan Doshi wrote:
> The FLV format can store metadata as numbers which are used and handled
> by many streaming platforms.
>
> Now, metadata values can be sent as AMF Number type if
> 1) tag key starts with "num_"
> 2) value is scanned and can be represented as a double.
>
> Written tag will have "num_" prefix removed if written as AMF Number.

Comments?

> ---
>   doc/muxers.texi      |  2 ++
>   libavformat/flvenc.c | 33 ++++++++++++++++++++++++++++-----
>   2 files changed, 30 insertions(+), 5 deletions(-)
>
> diff --git a/doc/muxers.texi b/doc/muxers.texi
> index ed5341be39..c9bafeec19 100644
> --- a/doc/muxers.texi
> +++ b/doc/muxers.texi
> @@ -509,6 +509,8 @@ ffmpeg -re -i ... -c:v libx264 -c:a aac -f fifo -fifo_format flv -map 0:v -map 0
>   @section flv
>   
>   Adobe Flash Video Format muxer.
> +This muxer will store user metadata whose keys start with @code{num_} and whose value is identified as a
> +pure value of type double as an AMF Number. All other user metadata is stored as a String.
>   
>   This muxer accepts the following options:
>   
> diff --git a/libavformat/flvenc.c b/libavformat/flvenc.c
> index 81d9b6100d..25d09ef304 100644
> --- a/libavformat/flvenc.c
> +++ b/libavformat/flvenc.c
> @@ -23,6 +23,7 @@
>   #include "libavutil/dict.h"
>   #include "libavutil/intfloat.h"
>   #include "libavutil/avassert.h"
> +#include "libavutil/eval.h"
>   #include "libavutil/mathematics.h"
>   #include "libavcodec/codec_desc.h"
>   #include "libavcodec/mpeg4audio.h"
> @@ -275,8 +276,10 @@ static void write_metadata(AVFormatContext *s, unsigned int ts)
>       AVIOContext *pb = s->pb;
>       FLVContext *flv = s->priv_data;
>       int write_duration_filesize = !(flv->flags & FLV_NO_DURATION_FILESIZE);
> -    int metadata_count = 0;
> +    int metadata_count = 0, is_num;
>       int64_t metadata_count_pos;
> +    double numvalue;
> +    char *key, *tail;
>       const AVDictionaryEntry *tag = NULL;
>   
>       /* write meta_tag */
> @@ -378,10 +381,30 @@ static void write_metadata(AVFormatContext *s, unsigned int ts)
>               av_log(s, AV_LOG_DEBUG, "Ignoring metadata for %s\n", tag->key);
>               continue;
>           }
> -        put_amf_string(pb, tag->key);
> -        avio_w8(pb, AMF_DATA_TYPE_STRING);
> -        put_amf_string(pb, tag->value);
> -        metadata_count++;
> +
> +        is_num = !strncmp(tag->key, "num_", 4) && *(tag->key + 4);
> +        key = is_num ? (tag->key + 4) : tag->key;
> +
> +        if (is_num) {
> +            numvalue = av_strtod(tag->value, &tail);
> +            if (*tail || numvalue == HUGE_VAL) {
> +                av_log(s, AV_LOG_ERROR, "Value %s for key %s cannot be stored as a double-typed number. Will be written as a string\n", tag->value, key);
> +                key = tag->key;
> +                is_num = 0;
> +            }
> +        }
> +
> +        av_log(s, AV_LOG_DEBUG, "Writing tag %s with value %s count: %d\n", key, tag->value, metadata_count);
> +
> +        put_amf_string(pb, key);
> +        if (is_num) {
> +            put_amf_double(pb, numvalue);
> +        } else {
> +            avio_w8(pb, AMF_DATA_TYPE_STRING);
> +            put_amf_string(pb, tag->value);
> +        }
> +
> +            metadata_count++;
>       }
>   
>       if (write_duration_filesize) {

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