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FFmpeg/libavcodec/dvbsubenc.c
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Jun Zhao 4811625b5f avcodec/dvbsubenc: provide the page ids in extradata 
The DVB subtitles written by this encoder always use page 1 as
composition and ancillary page id, but the encoder never told the muxers
about it: it never set any extradata.

Muxers need it. The Matroska CodecPrivate of an S_DVBSUB track is
mandatory and holds the composition page id, the ancillary page id and
the subtitling type, five bytes in that order; MPEG-TS reorders these
fields into its subtitling descriptor, which carries the language and
the type before the page ids, and the decoder only reads the page ids.
A file without them is invalid, and players that require them (VLC
refuses to create such a track and then filters the segments by page id)
show no subtitles at all, while players that do not, like mpv, are
unaffected.

Set the five bytes in the encoder so that every container gets them,
including MPEG-TS, which previously filled in the page ids with the same
hardcoded default. The encoded packets are unchanged; only the
fate-sub-dvb reference gains the extradata line of the output stream.

Fixes: https://code.ffmpeg.org/FFmpeg/FFmpeg/issues/25610
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>
2026-10-31 11:00:25 +00:00

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/*
* DVB subtitle encoding
* Copyright (c) 2005 Fabrice Bellard
*
* This file is part of FFmpeg.
*
* FFmpeg is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* FFmpeg is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with FFmpeg; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
#include "avcodec.h"
#include "bytestream.h"
#include "codec_internal.h"
#include "libavutil/avassert.h"
#include "libavutil/colorspace.h"
#include "libavutil/intreadwrite.h"
#include "libavutil/mem.h"
#include "libavutil/opt.h"
/* Every display set produced by this encoder uses this single page, i.e.
* the composition page id and the ancillary page id are both 1. */
#define DVB_SUB_PAGE_ID 1
typedef struct DVBSubtitleContext {
AVClass * class;
int object_version;
int min_bpp;
} DVBSubtitleContext;
#define PUTBITS2(val)\
{\
bitbuf |= (val) << bitcnt;\
bitcnt -= 2;\
if (bitcnt < 0) {\
bitcnt = 6;\
*q++ = bitbuf;\
bitbuf = 0;\
}\
}
static int dvb_encode_rle2(uint8_t **pq, int buf_size,
const uint8_t *bitmap, int linesize,
int w, int h)
{
uint8_t *q, *line_begin;
unsigned int bitbuf;
int bitcnt;
int x, y, len, x1, v, color;
q = *pq;
for(y = 0; y < h; y++) {
// Worst case line is 3 bits per value + 4 bytes overhead
if (buf_size * 8LL < w * 3 + 32)
return AVERROR_BUFFER_TOO_SMALL;
line_begin = q;
*q++ = 0x10;
bitbuf = 0;
bitcnt = 6;
x = 0;
while (x < w) {
x1 = x;
color = bitmap[x1++];
while (x1 < w && bitmap[x1] == color)
x1++;
len = x1 - x;
if (color == 0 && len == 2) {
PUTBITS2(0);
PUTBITS2(0);
PUTBITS2(1);
} else if (len >= 3 && len <= 10) {
v = len - 3;
PUTBITS2(0);
PUTBITS2((v >> 2) | 2);
PUTBITS2(v & 3);
PUTBITS2(color);
} else if (len >= 12 && len <= 27) {
v = len - 12;
PUTBITS2(0);
PUTBITS2(0);
PUTBITS2(2);
PUTBITS2(v >> 2);
PUTBITS2(v & 3);
PUTBITS2(color);
} else if (len >= 29) {
/* length = 29 ... 284 */
if (len > 284)
len = 284;
v = len - 29;
PUTBITS2(0);
PUTBITS2(0);
PUTBITS2(3);
PUTBITS2((v >> 6));
PUTBITS2((v >> 4) & 3);
PUTBITS2((v >> 2) & 3);
PUTBITS2(v & 3);
PUTBITS2(color);
} else {
PUTBITS2(color);
if (color == 0) {
PUTBITS2(1);
}
len = 1;
}
x += len;
}
/* end of line */
PUTBITS2(0);
PUTBITS2(0);
PUTBITS2(0);
if (bitcnt != 6) {
*q++ = bitbuf;
}
*q++ = 0xf0;
bitmap += linesize;
buf_size -= q - line_begin;
}
len = q - *pq;
*pq = q;
return len;
}
#define PUTBITS4(val)\
{\
bitbuf |= (val) << bitcnt;\
bitcnt -= 4;\
if (bitcnt < 0) {\
bitcnt = 4;\
*q++ = bitbuf;\
bitbuf = 0;\
}\
}
/* some DVB decoders only implement 4 bits/pixel */
static int dvb_encode_rle4(uint8_t **pq, int buf_size,
const uint8_t *bitmap, int linesize,
int w, int h)
{
uint8_t *q, *line_begin;
unsigned int bitbuf;
int bitcnt;
int x, y, len, x1, v, color;
q = *pq;
for(y = 0; y < h; y++) {
// Worst case line is 6 bits per value, + 4 bytes overhead
if (buf_size * 8LL < w * 6 + 32)
return AVERROR_BUFFER_TOO_SMALL;
line_begin = q;
*q++ = 0x11;
bitbuf = 0;
bitcnt = 4;
x = 0;
while (x < w) {
x1 = x;
color = bitmap[x1++];
while (x1 < w && bitmap[x1] == color)
x1++;
len = x1 - x;
if (color == 0 && len == 2) {
PUTBITS4(0);
PUTBITS4(0xd);
} else if (color == 0 && (len >= 3 && len <= 9)) {
PUTBITS4(0);
PUTBITS4(len - 2);
} else if (len >= 4 && len <= 7) {
PUTBITS4(0);
PUTBITS4(8 + len - 4);
PUTBITS4(color);
} else if (len >= 9 && len <= 24) {
PUTBITS4(0);
PUTBITS4(0xe);
PUTBITS4(len - 9);
PUTBITS4(color);
} else if (len >= 25) {
if (len > 280)
len = 280;
v = len - 25;
PUTBITS4(0);
PUTBITS4(0xf);
PUTBITS4(v >> 4);
PUTBITS4(v & 0xf);
PUTBITS4(color);
} else {
PUTBITS4(color);
if (color == 0) {
PUTBITS4(0xc);
}
len = 1;
}
x += len;
}
/* end of line */
PUTBITS4(0);
PUTBITS4(0);
if (bitcnt != 4) {
*q++ = bitbuf;
}
*q++ = 0xf0;
bitmap += linesize;
buf_size -= q - line_begin;
}
len = q - *pq;
*pq = q;
return len;
}
static int dvb_encode_rle8(uint8_t **pq, int buf_size,
const uint8_t *bitmap, int linesize,
int w, int h)
{
uint8_t *q, *line_begin;
int x, y, len, x1, color;
q = *pq;
for (y = 0; y < h; y++) {
// Worst case line is 12 bits per value, + 4 bytes overhead
if (buf_size * 8LL < w * 12 + 32)
return AVERROR_BUFFER_TOO_SMALL;
line_begin = q;
*q++ = 0x12;
x = 0;
while (x < w) {
x1 = x;
color = bitmap[x1++];
while (x1 < w && bitmap[x1] == color)
x1++;
len = x1 - x;
if (len == 1 && color) {
// 00000001 to 11111111 1 pixel in colour x
*q++ = color;
} else {
if (color == 0x00) {
// 00000000 0LLLLLLL L pixels (1-127) in colour 0 (L > 0)
len = FFMIN(len, 127);
*q++ = 0x00;
*q++ = len;
} else if (len > 2) {
// 00000000 1LLLLLLL CCCCCCCC L pixels (3-127) in colour C (L > 2)
len = FFMIN(len, 127);
*q++ = 0x00;
*q++ = 0x80+len;
*q++ = color;
}
else if (len == 2) {
*q++ = color;
*q++ = color;
} else {
*q++ = color;
len = 1;
}
}
x += len;
}
/* end of line */
// 00000000 00000000 end of 8-bit/pixel_code_string
*q++ = 0x00;
*q++ = 0x00;
*q++ = 0xf0;
bitmap += linesize;
buf_size -= q - line_begin;
}
len = q - *pq;
*pq = q;
return len;
}
static av_cold int dvbsub_encode_init(AVCodecContext *avctx)
{
/* Muxers use this FFmpeg/Matroska extradata layout: composition page
* id, ancillary page id, then subtitling type. MPEG-TS writes the
* type before the page ids in its subtitling descriptor. The decoder
* reads the page ids. Type 0x10 denotes normal subtitles with no
* monitor aspect ratio criticality. */
uint8_t *extradata = av_mallocz(5 + AV_INPUT_BUFFER_PADDING_SIZE);
if (!extradata)
return AVERROR(ENOMEM);
AV_WB16(extradata, DVB_SUB_PAGE_ID);
AV_WB16(extradata + 2, DVB_SUB_PAGE_ID);
extradata[4] = 0x10;
/* Extradata of the input stream has been copied into the context; it is
* of no use here, the page ids are always the ones above. */
av_freep(&avctx->extradata);
avctx->extradata = extradata;
avctx->extradata_size = 5;
return 0;
}
static int dvbsub_encode(AVCodecContext *avctx, uint8_t *outbuf, int buf_size,
const AVSubtitle *h)
{
DVBSubtitleContext *s = avctx->priv_data;
uint8_t *q, *pseg_len;
int page_id, region_id, clut_id, object_id, i, bpp_index, page_state, min_colors;
q = outbuf;
page_id = DVB_SUB_PAGE_ID;
switch(s->min_bpp) {
case 2:
case 4:
case 8:
min_colors = 1 << s->min_bpp;
break;
default:
av_log(avctx, AV_LOG_ERROR, "Invalid min_bpp value %d.\n", s->min_bpp);
return AVERROR(EINVAL);
}
if (h->num_rects && !h->rects)
return AVERROR(EINVAL);
if (h->num_rects >= 256)
return AVERROR(EINVAL);
for (i = 0; i < h->num_rects; i++) {
av_assert0(h->rects[i]->w >= 0 && h->rects[i]->h >= 0 &&
h->rects[i]->nb_colors >= 0 && h->rects[i]->nb_colors <= 256);
if (h->rects[i]->w > 0xFFFF || h->rects[i]->h > 0xFFFF)
return AVERROR(EINVAL);
}
if (avctx->width > 0 && avctx->height > 0) {
if (buf_size < 11)
return AVERROR_BUFFER_TOO_SMALL;
/* display definition segment */
*q++ = 0x0f; /* sync_byte */
*q++ = 0x14; /* segment_type */
bytestream_put_be16(&q, page_id);
pseg_len = q;
q += 2; /* segment length */
*q++ = 0x00; /* dds version number & display window flag */
bytestream_put_be16(&q, avctx->width - 1); /* display width */
bytestream_put_be16(&q, avctx->height - 1); /* display height */
bytestream_put_be16(&pseg_len, q - pseg_len - 2);
buf_size -= 11;
}
/* page composition segment */
if (buf_size < 8 + h->num_rects * 6)
return AVERROR_BUFFER_TOO_SMALL;
*q++ = 0x0f; /* sync_byte */
*q++ = 0x10; /* segment_type */
bytestream_put_be16(&q, page_id);
pseg_len = q;
q += 2; /* segment length */
*q++ = 30; /* page_timeout (seconds) */
page_state = 2; /* mode change */
/* page_version = 0 + page_state */
*q++ = (s->object_version << 4) | (page_state << 2) | 3;
for (region_id = 0; region_id < h->num_rects; region_id++) {
*q++ = region_id;
*q++ = 0xff; /* reserved */
bytestream_put_be16(&q, h->rects[region_id]->x); /* left pos */
bytestream_put_be16(&q, h->rects[region_id]->y); /* top pos */
}
bytestream_put_be16(&pseg_len, q - pseg_len - 2);
buf_size -= 8 + h->num_rects * 6;
if (h->num_rects) {
for (clut_id = 0; clut_id < h->num_rects; clut_id++) {
/* CLUT segment */
int nb_colors = FFMAX(min_colors, h->rects[clut_id]->nb_colors);
if (nb_colors <= 4U) {
/* 2 bpp, some decoders do not support it correctly */
bpp_index = 0;
} else if (nb_colors <= 16U) {
/* 4 bpp, standard encoding */
bpp_index = 1;
} else if (nb_colors <= 256U) {
/* 8 bpp, standard encoding */
bpp_index = 2;
} else {
return AVERROR(EINVAL);
}
if (buf_size < 8 + h->rects[clut_id]->nb_colors * 6)
return AVERROR_BUFFER_TOO_SMALL;
/* CLUT segment */
*q++ = 0x0f; /* sync byte */
*q++ = 0x12; /* CLUT definition segment */
bytestream_put_be16(&q, page_id);
pseg_len = q;
q += 2; /* segment length */
*q++ = clut_id;
*q++ = (0 << 4) | 0xf; /* version = 0 */
for(i = 0; i < h->rects[clut_id]->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;
*q++ = RGB_TO_Y_CCIR(r, g, b);
*q++ = RGB_TO_V_CCIR(r, g, b, 0);
*q++ = RGB_TO_U_CCIR(r, g, b, 0);
*q++ = 255 - a;
}
}
bytestream_put_be16(&pseg_len, q - pseg_len - 2);
buf_size -= 8 + h->rects[clut_id]->nb_colors * 6;
}
if (buf_size < h->num_rects * 22)
return AVERROR_BUFFER_TOO_SMALL;
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 (nb_colors <= 4) {
/* 2 bpp, some decoders do not support it correctly */
bpp_index = 0;
} else if (nb_colors <= 16) {
/* 4 bpp, standard encoding */
bpp_index = 1;
} else if (nb_colors <= 256) {
/* 8 bpp, standard encoding */
bpp_index = 2;
} else {
return AVERROR(EINVAL);
}
*q++ = 0x0f; /* sync_byte */
*q++ = 0x11; /* segment_type */
bytestream_put_be16(&q, page_id);
pseg_len = q;
q += 2; /* segment length */
*q++ = region_id;
*q++ = (s->object_version << 4) | (0 << 3) | 0x07; /* version , no fill */
bytestream_put_be16(&q, h->rects[region_id]->w); /* region width */
bytestream_put_be16(&q, h->rects[region_id]->h); /* region height */
*q++ = ((1 + bpp_index) << 5) | ((1 + bpp_index) << 2) | 0x03;
*q++ = region_id; /* clut_id == region_id */
*q++ = 0; /* 8 bit fill colors */
*q++ = 0x03; /* 4 bit and 2 bit fill colors */
bytestream_put_be16(&q, region_id); /* object_id == region_id */
*q++ = (0 << 6) | (0 << 4);
*q++ = 0;
*q++ = 0xf0;
*q++ = 0;
bytestream_put_be16(&pseg_len, q - pseg_len - 2);
}
buf_size -= h->num_rects * 22;
for (object_id = 0; object_id < h->num_rects; object_id++) {
int (*dvb_encode_rle)(uint8_t **pq, 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 (nb_colors <= 4) {
/* 2 bpp, some decoders do not support it correctly */
dvb_encode_rle = dvb_encode_rle2;
} else if (nb_colors <= 16) {
/* 4 bpp, standard encoding */
dvb_encode_rle = dvb_encode_rle4;
} else if (nb_colors <= 256) {
/* 8 bpp, standard encoding */
dvb_encode_rle = dvb_encode_rle8;
} else {
return AVERROR(EINVAL);
}
/* Object Data segment */
*q++ = 0x0f; /* sync byte */
*q++ = 0x13;
bytestream_put_be16(&q, page_id);
pseg_len = q;
q += 2; /* segment length */
bytestream_put_be16(&q, object_id);
*q++ = (s->object_version << 4) | (0 << 2) | (0 << 1) | 1; /* version = 0,
object_coding_method,
non_modifying_color_flag */
{
uint8_t *ptop_field_len, *pbottom_field_len, *top_ptr, *bottom_ptr;
int ret;
ptop_field_len = q;
q += 2;
pbottom_field_len = q;
q += 2;
buf_size -= 13;
top_ptr = q;
ret = dvb_encode_rle(&q, buf_size,
h->rects[object_id]->data[0],
h->rects[object_id]->w * 2,
h->rects[object_id]->w,
h->rects[object_id]->h >> 1);
if (ret < 0)
return ret;
buf_size -= ret;
bottom_ptr = q;
ret = dvb_encode_rle(&q, buf_size,
h->rects[object_id]->data[0] + h->rects[object_id]->w,
h->rects[object_id]->w * 2,
h->rects[object_id]->w,
h->rects[object_id]->h >> 1);
if (ret < 0)
return ret;
buf_size -= ret;
bytestream_put_be16(&ptop_field_len, bottom_ptr - top_ptr);
bytestream_put_be16(&pbottom_field_len, q - bottom_ptr);
}
bytestream_put_be16(&pseg_len, q - pseg_len - 2);
}
}
/* end of display set segment */
if (buf_size < 6)
return AVERROR_BUFFER_TOO_SMALL;
*q++ = 0x0f; /* sync_byte */
*q++ = 0x80; /* segment_type */
bytestream_put_be16(&q, page_id);
pseg_len = q;
q += 2; /* segment length */
bytestream_put_be16(&pseg_len, q - pseg_len - 2);
buf_size -= 6;
s->object_version = (s->object_version + 1) & 0xf;
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[] = {
{"min_bpp", "minimum bits-per-pixel for subtitle colors (2, 4 or 8)", OFFSET(min_bpp), AV_OPT_TYPE_INT, {.i64 = 4}, 2, 8, 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"),
.p.type = AVMEDIA_TYPE_SUBTITLE,
.p.id = AV_CODEC_ID_DVB_SUBTITLE,
.priv_data_size = sizeof(DVBSubtitleContext),
.init = dvbsub_encode_init,
FF_CODEC_ENCODE_SUB_CB(dvbsub_encode),
.p.priv_class = &dvbsubenc_class,
};
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