movtextdec.c

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/*
 * 3GPP TS 26.245 Timed Text decoder
 * Copyright (c) 2012  Philip Langdale <philipl@overt.org>
 *
 * 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 "ass.h"
#include "libavutil/avstring.h"
#include "libavutil/common.h"
#include "libavutil/bprint.h"
#include "libavutil/intreadwrite.h"

static int text_to_ass(AVBPrint *buf, const char *text, const char *text_end)
{
    while (text < text_end) {
        switch (*text) {
        case '\r':
            break;
        case '\n':
            av_bprintf(buf, "\\N");
            break;
        default:
            av_bprint_chars(buf, *text, 1);
            break;
        }
        text++;
    }

    av_bprintf(buf, "\r\n");
    return 0;
}

static int mov_text_init(AVCodecContext *avctx) {
    /*
     * TODO: Handle the default text style.
     * NB: Most players ignore styles completely, with the result that
     * it's very common to find files where the default style is broken
     * and respecting it results in a worse experience than ignoring it.
     */
    return ff_ass_subtitle_header_default(avctx);
}

static int mov_text_decode_frame(AVCodecContext *avctx,
                            void *data, int *got_sub_ptr, AVPacket *avpkt)
{
    AVSubtitle *sub = data;
    int ts_start, ts_end;
    AVBPrint buf;
    const char *ptr = avpkt->data;
    const char *end;

    if (!ptr || avpkt->size < 2)
        return AVERROR_INVALIDDATA;

    /*
     * A packet of size two with value zero is an empty subtitle
     * used to mark the end of the previous non-empty subtitle.
     * We can just drop them here as we have duration information
     * already. If the value is non-zero, then it's technically a
     * bad packet.
     */
    if (avpkt->size == 2)
        return AV_RB16(ptr) == 0 ? 0 : AVERROR_INVALIDDATA;

    /*
     * The first two bytes of the packet are the length of the text string
     * In complex cases, there are style descriptors appended to the string
     * so we can't just assume the packet size is the string size.
     */
    end = ptr + FFMIN(2 + AV_RB16(ptr), avpkt->size);
    ptr += 2;

    ts_start = av_rescale_q(avpkt->pts,
                            avctx->time_base,
                            (AVRational){1,100});
    ts_end   = av_rescale_q(avpkt->pts + avpkt->duration,
                            avctx->time_base,
                            (AVRational){1,100});

    // Note that the spec recommends lines be no longer than 2048 characters.
    av_bprint_init(&buf, 0, AV_BPRINT_SIZE_UNLIMITED);
    text_to_ass(&buf, ptr, end);

    if (!av_bprint_is_complete(&buf))
        return AVERROR(ENOMEM);

    ff_ass_add_rect(sub, buf.str, ts_start, ts_end-ts_start, 0);
    *got_sub_ptr = sub->num_rects > 0;
    av_bprint_finalize(&buf, NULL);
    return avpkt->size;
}

AVCodec ff_movtext_decoder = {
    .name         = "mov_text",
    .long_name    = NULL_IF_CONFIG_SMALL("3GPP Timed Text subtitle"),
    .type         = AVMEDIA_TYPE_SUBTITLE,
    .id           = AV_CODEC_ID_MOV_TEXT,
    .init         = mov_text_init,
    .decode       = mov_text_decode_frame,
};