[FFmpeg-trac] #2909(avcodec:new): Make dvbsubs more compliant among stb's

[FFmpeg]

#2909: Make dvbsubs more compliant among stb's
-------------------------------------+-----------------------------------
             Reporter:  tommy2d      |                    Owner:
                 Type:  enhancement  |                   Status:  new
             Priority:  normal       |                Component:  avcodec
              Version:  1.2.2        |               Resolution:
             Keywords:  dvbsub       |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-----------------------------------

Old description:

> For a project I'm using avcodec to encode various dvbsubtitles and mux
> them in an mpeg transport stream using a self-made multiplexer.
>
> The dvdsubtitles that are produced by avcodec are not working on some
> stubborn set-top-boxes (STBs). About 80% of the boxes i used to test are
> working. 20% is not.
>
> I was able to create my own encoder, but with some minor changes in the
> dvbsubenc.c code, i was able to use ffmpeg to produce subtitles that work
> on 100% of the STBs we used to test. These changes are:
>
> 1. Make sure all reserved bits, especially in the region composition
> segment, are set to 0 rather than 1:
>
> -        *q++ = ((1 + bpp_index) << 5) | ((1 + bpp_index) << 2) | 0x03;
> +        *q++ = ((1 + bpp_index) << 5) | ((1 + bpp_index) << 2) | 0x00;
>
> -        *q++ = 0xf0 | ((y >> 8) & 0xf);
> +        *q++ = 0x00 | ((y >> 8) & 0xf);
>
> 2. Make sure all dvb-subtitles are ordered as described in the spec:
>
> 0x10 page composition segment
> 0x11 region composition segment
> 0x12 CLUT
> 0x13 Object data segment
> 0x14 DDS
> 0x80 end of display
>
> Failing to do so will cause some STBs to fail selecting the proper
> colours from the CLUT.
>
> 3. Define / calculate fixed regions that can be used again and again.
> Defining and/or using new regions for each subtitle, like avcodec does by
> default, causes some serious graphical distortion,  most notably in high-
> end professional tandberg decoders.
>
> 4. Implement subtitle clear packets that only contain a page composition
> segment (no end of display segment) and use page_state = 0x01;.
>
> 5. Make sure actual subtitle payload packets use page_state = 0x02.
>
> 6. add display defintion segments for low res subs that are muxed with HD
> video:
>
>         *p++ = 0x0F; //sync_byte
>         *p++ = 0x14; //segment_type
>         *p++ = 0x00; //page_id 1
>         *p++ = 0x01; //page_id 2
>         *p++ = 0x00; //segment_length 1
>         *p++ = 0x05; //segment_length 2
>
>         *p++ = 0x00;//dds_version_number + display_window_flag + reserved
>         *p++ = (ddsWidth >> 8) & 0xFF;
>         *p++ = ddsWidth & 0xFF;
>         *p++ = (ddsHeight >> 8) & 0xFF;
>         *p++ = ddsHeight & 0xFF;
>
> That's about it. I know this feature is not used a lot, especially since
> the mpeg-ts muxer provided by ffmpeg is not really suitable for
> broadcasting purposes, but these minor changes really contribute to the
> compatibility of the dvbsubs that avcodec encodes.
>
> F.y.i: this information is based on reverse-engineering highly-compatbile
> broadcast TS files that contain dvbsubtitles. If necessary i can deliver
> some raw dvbsub payloads that are tested on multiple STBs and that can
> used in further research and/or development.

New description:

 For a project I'm using avcodec to encode various dvbsubtitles and mux
 them in an mpeg transport stream using a self-made multiplexer.

 The dvdsubtitles that are produced by avcodec are not working on some
 stubborn set-top-boxes (STBs). About 80% of the boxes i used to test are
 working. 20% is not.

 I was able to create my own encoder, but with some minor changes in the
 dvbsubenc.c code, i was able to use ffmpeg to produce subtitles that work
 on 100% of the STBs we used to test. These changes are:

 1. Make sure all reserved bits, especially in the region composition
 segment, are set to 0 rather than 1:

 {{{
 -        *q++ = ((1 + bpp_index) << 5) | ((1 + bpp_index) << 2) | 0x03;
 +        *q++ = ((1 + bpp_index) << 5) | ((1 + bpp_index) << 2) | 0x00;

 -        *q++ = 0xf0 | ((y >> 8) & 0xf);
 +        *q++ = 0x00 | ((y >> 8) & 0xf);
 }}}

 2. Make sure all dvb-subtitles are ordered as described in the spec:

 {{{
 0x10 page composition segment
 0x11 region composition segment
 0x12 CLUT
 0x13 Object data segment
 0x14 DDS
 0x80 end of display
 }}}

 Failing to do so will cause some STBs to fail selecting the proper colours
 from the CLUT.

 3. Define / calculate fixed regions that can be used again and again.
 Defining and/or using new regions for each subtitle, like avcodec does by
 default, causes some serious graphical distortion,  most notably in high-
 end professional tandberg decoders.

 4. Implement subtitle clear packets that only contain a page composition
 segment (no end of display segment) and use page_state = 0x01;.

 5. Make sure actual subtitle payload packets use page_state = 0x02.

 6. add display defintion segments for low res subs that are muxed with HD
 video:

 {{{
         *p++ = 0x0F; //sync_byte
         *p++ = 0x14; //segment_type
         *p++ = 0x00; //page_id 1
         *p++ = 0x01; //page_id 2
         *p++ = 0x00; //segment_length 1
         *p++ = 0x05; //segment_length 2

         *p++ = 0x00;//dds_version_number + display_window_flag + reserved
         *p++ = (ddsWidth >> 8) & 0xFF;
         *p++ = ddsWidth & 0xFF;
         *p++ = (ddsHeight >> 8) & 0xFF;
         *p++ = ddsHeight & 0xFF;
 }}}

 That's about it. I know this feature is not used a lot, especially since
 the mpeg-ts muxer provided by ffmpeg is not really suitable for
 broadcasting purposes, but these minor changes really contribute to the
 compatibility of the dvbsubs that avcodec encodes.

 F.y.i: this information is based on reverse-engineering highly-compatbile
 broadcast TS files that contain dvbsubtitles. If necessary i can deliver
 some raw dvbsub payloads that are tested on multiple STBs and that can
 used in further research and/or development.

--

Comment (by ubitux):

 Added some code quotation in the description to make it readable.

-- 
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2909#comment:3>
FFmpeg <http://ffmpeg.org>
FFmpeg issue tracker