Re: [FFmpeg-devel] [PATCH] libavcodec/mpeg12dec.c: rename 0x0502 CC format

[Marton Balint <cus@passwd.hu>]

On Sat, 25 Jan 2025, Scott Theisen wrote:

> The format is used by at least one DVB-S provider, but is not defined in any DVB
> standard, so remove references to DVB.  Since I don't have any standard that
> does define this format, using the magic number as the name seems most
> appropriate.
>
> This is a simple rename, no functional change.
>
> ----
>
> This renaming was discussed briefly on the mailing list on 28 December 2024,
> but I never saw a patch to rename it.
> ---
> libavcodec/mpeg12dec.c | 28 ++++++++++++++--------------
> 1 file changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/libavcodec/mpeg12dec.c b/libavcodec/mpeg12dec.c
> index 9bb995b5be..f28d900cc9 100644
> --- a/libavcodec/mpeg12dec.c
> +++ b/libavcodec/mpeg12dec.c
> @@ -67,7 +67,7 @@ enum Mpeg2ClosedCaptionsFormat {
>     CC_FORMAT_A53_PART4,
>     CC_FORMAT_SCTE20,
>     CC_FORMAT_DVD,
> -    CC_FORMAT_DVB_0502
> +    CC_FORMAT_0x0502
> };
>
> typedef struct Mpeg1Context {
> @@ -2064,39 +2064,39 @@ static int mpeg_decode_a53_cc(AVCodecContext *avctx,
>             mpeg_set_cc_format(avctx, CC_FORMAT_DVD, "DVD");
>         }
>         return 1;
> -    } else if ((!s1->cc_format || s1->cc_format == CC_FORMAT_DVB_0502) &&
> +    } else if ((!s1->cc_format || s1->cc_format == CC_FORMAT_0x0502) &&
>                buf_size >= 12 &&
>                p[0] == 0x05 && p[1] == 0x02) {
> -        /* extract DVB 0502 CC data */
> +        /* extract 0x0502 format CC data */
>         const uint8_t cc_header = 0xf8 | 0x04 /* valid */ | 0x00 /* line 21 field 1 */;
>         uint8_t cc_data[4] = {0};
>         int cc_count = 0;
> -        uint8_t dvb_cc_type = p[7];
> +        uint8_t cc_type = p[7];
>         p += 8;
>         buf_size -= 8;
>
> -        if (dvb_cc_type == 0x05 && buf_size >= 7) {
> -            dvb_cc_type = p[6];
> +        if (cc_type == 0x05 && buf_size >= 7) {
> +            cc_type = p[6];
>             p += 7;
>             buf_size -= 7;
>         }
>
> -        if (dvb_cc_type == 0x02 && buf_size >= 4) { /* 2-byte caption, can be repeated */
> +        if (cc_type == 0x02 && buf_size >= 4) { /* 2-byte caption, can be repeated */
>             cc_count = 1;
>             cc_data[0] = p[1];
>             cc_data[1] = p[2];
> -            dvb_cc_type = p[3];
> +            cc_type = p[3];
>
>             /* Only repeat characters when the next type flag
>              * is 0x04 and the characters are repeatable (i.e., less than
>              * 32 with the parity stripped).
>              */
> -            if (dvb_cc_type == 0x04 && (cc_data[0] & 0x7f) < 32) {
> +            if (cc_type == 0x04 && (cc_data[0] & 0x7f) < 32) {
>                 cc_count = 2;
>                 cc_data[2] = cc_data[0];
>                 cc_data[3] = cc_data[1];
>             }
> -        } else if (dvb_cc_type == 0x04 && buf_size >= 5) { /* 4-byte caption, not repeated */
> +        } else if (cc_type == 0x04 && buf_size >= 5) { /* 4-byte caption, not repeated */
>             cc_count = 2;
>             cc_data[0] = p[1];
>             cc_data[1] = p[2];
> @@ -2124,7 +2124,7 @@ static int mpeg_decode_a53_cc(AVCodecContext *avctx,
>                 }
>             }
>
> -            mpeg_set_cc_format(avctx, CC_FORMAT_DVB_0502, "DVB 0502");
> +            mpeg_set_cc_format(avctx, CC_FORMAT_0x0502, "0x0502");
>         }
>         return 1;
>     }
> @@ -2687,7 +2687,7 @@ const FFCodec ff_mpeg1video_decoder = {
> static const AVOption mpeg2video_options[] = {
>     { "cc_format", "extract a specific Closed Captions format",
>        M2V_OFFSET(cc_format), AV_OPT_TYPE_INT, { .i64 = CC_FORMAT_AUTO },
> -        CC_FORMAT_AUTO, CC_FORMAT_DVB_0502, M2V_PARAM, .unit = "cc_format" },
> +        CC_FORMAT_AUTO, CC_FORMAT_0x0502, M2V_PARAM, .unit = "cc_format" },
>
>        { "auto",   "pick first seen CC substream",  0, AV_OPT_TYPE_CONST,
>         { .i64 =   CC_FORMAT_AUTO },                .flags = M2V_PARAM, .unit = "cc_format" },
> @@ -2697,8 +2697,8 @@ static const AVOption mpeg2video_options[] = {
>         { .i64 =   CC_FORMAT_SCTE20 },              .flags = M2V_PARAM, .unit = "cc_format" },
>        { "dvd",    "pick DVD CC substream",         0, AV_OPT_TYPE_CONST,
>         { .i64 =   CC_FORMAT_DVD },                 .flags = M2V_PARAM, .unit = "cc_format" },
> -       { "dvb_0502", "pick DVB 0502 CC substream",  0, AV_OPT_TYPE_CONST,
> -        { .i64 =   CC_FORMAT_DVB_0502 },            .flags = M2V_PARAM, .unit = "cc_format" },
> +       { "0x0502", "pick 0x0502 format CC substream", 0, AV_OPT_TYPE_CONST,
> +        { .i64 =   CC_FORMAT_0x0502 },              .flags = M2V_PARAM, .unit = "cc_format" },

Named constants which are also numbers are not a good idea.

Maybe the simplest would be to keep the constant name, because constant 
names should not be changed without deprecation anyway.

Regards,
Marton
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