Re: [FFmpeg-devel] [PATCH v5] libavcodec/dvbsubenc.c: add a disable_2bpp option to work around some decoders.

[Soft Works <softworkz-at-hotmail.com@ffmpeg.org>]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of Ronan
> Waide
> Sent: Sonntag, 2. März 2025 18:24
> To: ffmpeg-devel@ffmpeg.org
> Cc: Ronan Waide <waider@waider.ie>
> Subject: [FFmpeg-devel] [PATCH v5] libavcodec/dvbsubenc.c: add a
> disable_2bpp option to work around some decoders.
> 
> As noted in the code in several places, some DVB subtitle decoders
> don't handle 2bpp color. This patch adds a disable_2bpp option which
> disables the 2bpp format; subtitles which would use 2bpp will be bumped
> up to 4bpp. Per suggestion from sw, disable_2pp defaults to true.
> 
> Signed-off-by: Ronan Waide <waider@waider.ie>
> ---
>  doc/encoders.texi      | 27 +++++++++++++++++
>  libavcodec/dvbsubenc.c | 69 +++++++++++++++++++++++++++++++-----------
>  2 files changed, 78 insertions(+), 18 deletions(-)
> 
> diff --git a/doc/encoders.texi b/doc/encoders.texi
> index f3fcc1aa60..6ee0065c7d 100644
> --- a/doc/encoders.texi
> +++ b/doc/encoders.texi
> @@ -4484,6 +4484,33 @@ Reduces detail but attempts to preserve color at
> extremely low bitrates.
>  @chapter Subtitles Encoders
>  @c man begin SUBTITLES ENCODERS
> 
> +@section dvbsub
> +
> +This codec encodes the bitmap subtitle format that is used in DVB
> +broadcasts and recordings. The bitmaps are typically embedded in a
> +container such as MPEG-TS as a separate stream.
> +
> +@subsection Options
> +
> +@table @option
> +@item disable_2bpp @var{boolean}
> +Disable the 2 bits-per-pixel encoding format.
> +
> +DVB supports 2, 4, and 8 bits-per-pixel color lookup tables. However,
> +not all players support or properly support 2 bits-per-pixel,
> +resulting in unusable subtitles.
> +@table @option
> +@item 0
> +The 2 bits-per-pixel encoding format will be used for subtitles with 4
> +colors or less.
> +
> +@item 1
> +The 2 bits-per-pixel encoding format will be disabled, and subtitles
> +with 4 colors or less will use a 4 bits-per-pixel format. (default)
> +@end table
> +
> +@end table
> +
>  @section dvdsub
> 
>  This codec encodes the bitmap subtitle format that is used in DVDs.
> diff --git a/libavcodec/dvbsubenc.c b/libavcodec/dvbsubenc.c
> index 822e3a5309..4db2e1ddda 100644
> --- a/libavcodec/dvbsubenc.c
> +++ b/libavcodec/dvbsubenc.c
> @@ -22,9 +22,12 @@
>  #include "bytestream.h"
>  #include "codec_internal.h"
>  #include "libavutil/colorspace.h"
> +#include "libavutil/opt.h"
> 
>  typedef struct DVBSubtitleContext {
> +    AVClass * class;
>      int object_version;
> +    int disable_2bpp;
>  } DVBSubtitleContext;
> 
>  #define PUTBITS2(val)\
> @@ -274,13 +277,15 @@ static int dvbsub_encode(AVCodecContext *avctx,
> uint8_t *outbuf, int buf_size,
>  {
>      DVBSubtitleContext *s = avctx->priv_data;
>      uint8_t *q, *pseg_len;
> -    int page_id, region_id, clut_id, object_id, i, bpp_index,
> page_state;
> +    int page_id, region_id, clut_id, object_id, i, bpp_index,
> page_state, min_colors;
> 
> 
>      q = outbuf;
> 
>      page_id = 1;
> 
> +    min_colors = s->disable_2bpp ? 16 : 0;
> +
>      if (h->num_rects && !h->rects)
>          return AVERROR(EINVAL);
> 
> @@ -326,18 +331,20 @@ static int dvbsub_encode(AVCodecContext *avctx,
> uint8_t *outbuf, int buf_size,
> 
>      if (h->num_rects) {
>          for (clut_id = 0; clut_id < h->num_rects; clut_id++) {
> -            if (buf_size < 6 + h->rects[clut_id]->nb_colors * 6)
> +            int nb_colors = FFMAX(min_colors, h->rects[clut_id]-
> >nb_colors);
> +
> +            if (buf_size < 6 + nb_colors * 6)
>                  return AVERROR_BUFFER_TOO_SMALL;
> 
>              /* CLUT segment */
> 
> -            if (h->rects[clut_id]->nb_colors <= 4) {
> +            if (nb_colors <= 4) {
>                  /* 2 bpp, some decoders do not support it correctly */
>                  bpp_index = 0;
> -            } else if (h->rects[clut_id]->nb_colors <= 16) {
> +            } else if (nb_colors <= 16) {
>                  /* 4 bpp, standard encoding */
>                  bpp_index = 1;
> -            } else if (h->rects[clut_id]->nb_colors <= 256) {
> +            } else if (nb_colors <= 256) {
>                  /* 8 bpp, standard encoding */
>                  bpp_index = 2;
>              } else {
> @@ -354,16 +361,24 @@ static int dvbsub_encode(AVCodecContext *avctx,
> uint8_t *outbuf, int buf_size,
>              *q++ = clut_id;
>              *q++ = (0 << 4) | 0xf; /* version = 0 */
> 
> -            for(i = 0; i < h->rects[clut_id]->nb_colors; i++) {
> +            for(i = 0; i < nb_colors; i++) {
>                  *q++ = i; /* clut_entry_id */
>                  *q++ = (1 << (7 - bpp_index)) | (0xf << 1) | 1; /* 2
> bits/pixel full range */
>                  {
>                      int a, r, g, b;
> -                    uint32_t x= ((uint32_t*)h->rects[clut_id]-
> >data[1])[i];
> -                    a = (x >> 24) & 0xff;
> -                    r = (x >> 16) & 0xff;
> -                    g = (x >>  8) & 0xff;
> -                    b = (x >>  0) & 0xff;
> +                    if (i < h->rects[clut_id]->nb_colors) {
> +                        uint32_t x= ((uint32_t*)h->rects[clut_id]-
> >data[1])[i];
> +                        a = (x >> 24) & 0xff;
> +                        r = (x >> 16) & 0xff;
> +                        g = (x >>  8) & 0xff;
> +                        b = (x >>  0) & 0xff;
> +                    } else {
> +                        /* pad out the CLUT */
> +                        a = 0;
> +                        r = 0;
> +                        g = 0;
> +                        b = 0;
> +                    }
> 
>                      *q++ = RGB_TO_Y_CCIR(r, g, b);
>                      *q++ = RGB_TO_V_CCIR(r, g, b, 0);
> @@ -373,7 +388,7 @@ static int dvbsub_encode(AVCodecContext *avctx,
> uint8_t *outbuf, int buf_size,
>              }
> 
>              bytestream_put_be16(&pseg_len, q - pseg_len - 2);
> -            buf_size -= 6 + h->rects[clut_id]->nb_colors * 6;
> +            buf_size -= 6 + nb_colors * 6;
>          }
> 
>          if (buf_size < h->num_rects * 22)
> @@ -381,14 +396,15 @@ static int dvbsub_encode(AVCodecContext *avctx,
> uint8_t *outbuf, int buf_size,
>          for (region_id = 0; region_id < h->num_rects; region_id++) {
> 
>              /* region composition segment */
> +            int nb_colors = FFMAX(min_colors, h->rects[region_id]-
> >nb_colors);
> 
> -            if (h->rects[region_id]->nb_colors <= 4) {
> +            if (nb_colors <= 4) {
>                  /* 2 bpp, some decoders do not support it correctly */
>                  bpp_index = 0;
> -            } else if (h->rects[region_id]->nb_colors <= 16) {
> +            } else if (nb_colors <= 16) {
>                  /* 4 bpp, standard encoding */
>                  bpp_index = 1;
> -            } else if (h->rects[region_id]->nb_colors <= 256) {
> +            } else if (nb_colors <= 256) {
>                  /* 8 bpp, standard encoding */
>                  bpp_index = 2;
>              } else {
> @@ -424,17 +440,19 @@ static int dvbsub_encode(AVCodecContext *avctx,
> uint8_t *outbuf, int buf_size,
>                                    const uint8_t *bitmap, int linesize,
>                                    int w, int h);
> 
> +            int nb_colors = FFMAX(min_colors, h->rects[object_id]-
> >nb_colors);
> +
>              if (buf_size < 13)
>                  return AVERROR_BUFFER_TOO_SMALL;
> 
>              /* bpp_index maths */
> -            if (h->rects[object_id]->nb_colors <= 4) {
> +            if (nb_colors <= 4) {
>                  /* 2 bpp, some decoders do not support it correctly */
>                  dvb_encode_rle = dvb_encode_rle2;
> -            } else if (h->rects[object_id]->nb_colors <= 16) {
> +            } else if (nb_colors <= 16) {
>                  /* 4 bpp, standard encoding */
>                  dvb_encode_rle = dvb_encode_rle4;
> -            } else if (h->rects[object_id]->nb_colors <= 256) {
> +            } else if (nb_colors <= 256) {
>                  /* 8 bpp, standard encoding */
>                  dvb_encode_rle = dvb_encode_rle8;
>              } else {
> @@ -506,6 +524,20 @@ static int dvbsub_encode(AVCodecContext *avctx,
> uint8_t *outbuf, int buf_size,
>      return q - outbuf;
>  }
> 
> +#define OFFSET(x) offsetof(DVBSubtitleContext, x)
> +#define SE AV_OPT_FLAG_SUBTITLE_PARAM | AV_OPT_FLAG_ENCODING_PARAM
> +static const AVOption options[] = {
> +    {"disable_2bpp", "disable the 2bpp subtitle encoder",
> OFFSET(disable_2bpp), AV_OPT_TYPE_BOOL, {.i64 = 1}, 0, 1, SE},
> +    { NULL },
> +};
> +
> +static const AVClass dvbsubenc_class = {
> +    .class_name = "DVBSUB subtitle encoder",
> +    .item_name  = av_default_item_name,
> +    .option     = options,
> +    .version    = LIBAVUTIL_VERSION_INT,
> +};
> +
>  const FFCodec ff_dvbsub_encoder = {
>      .p.name         = "dvbsub",
>      CODEC_LONG_NAME("DVB subtitles"),
> @@ -513,4 +545,5 @@ const FFCodec ff_dvbsub_encoder = {
>      .p.id           = AV_CODEC_ID_DVB_SUBTITLE,
>      .priv_data_size = sizeof(DVBSubtitleContext),
>      FF_CODEC_ENCODE_SUB_CB(dvbsub_encode),
> +    .p.priv_class   = &dvbsubenc_class,
>  };
> --

Tested & LGTM

Thanks for the patch!

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