webvttenc.c

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/*
 * WebVTT subtitle encoder
 * Copyright (c) 2010  Aurelien Jacobs <aurel@gnuage.org>
 * Copyright (c) 2014  Aman Gupta <ffmpeg@tmm1.net>
 *
 * 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 <stdarg.h>
#include "avcodec.h"
#include "libavutil/bprint.h"
#include "ass_split.h"
#include "ass.h"
#include "codec_internal.h"
 
#define WEBVTT_STACK_SIZE 64
typedef struct {
    AVCodecContext *avctx;
    ASSSplitContext *ass_ctx;
    AVBPrint buffer;
    unsigned timestamp_end;
    int count;
    int64_t cue_start_ms;
    int64_t cue_end_ms;
    int64_t current_time_ms;
    int64_t last_timestamp_ms;
    int64_t pending_duration_ms;
    char stack[WEBVTT_STACK_SIZE];
    int stack_ptr;
} WebVTTContext;
 
static av_printf_format(2, 3) void webvtt_print(WebVTTContext *s, const char *str, ...)
{
    va_list vargs;
    va_start(vargs, str);
    av_vbprintf(&s->buffer, str, vargs);
    va_end(vargs);
}
 
static int webvtt_stack_push(WebVTTContext *s, const char c)
{
    if (s->stack_ptr >= WEBVTT_STACK_SIZE)
        return -1;
    s->stack[s->stack_ptr++] = c;
    return 0;
}
 
static char webvtt_stack_pop(WebVTTContext *s)
{
    if (s->stack_ptr <= 0)
        return 0;
    return s->stack[--s->stack_ptr];
}
 
static int webvtt_stack_find(WebVTTContext *s, const char c)
{
    int i;
    for (i = s->stack_ptr-1; i >= 0; i--)
        if (s->stack[i] == c)
            break;
    return i;
}
 
static void webvtt_close_tag(WebVTTContext *s, char tag)
{
    webvtt_print(s, "</%c>", tag);
}
 
static void webvtt_stack_push_pop(WebVTTContext *s, const char c, int close)
{
    if (close) {
        int i = c ? webvtt_stack_find(s, c) : 0;
        if (i < 0)
            return;
        while (s->stack_ptr != i)
            webvtt_close_tag(s, webvtt_stack_pop(s));
    } else if (webvtt_stack_push(s, c) < 0)
        av_log(s->avctx, AV_LOG_ERROR, "tag stack overflow\n");
}
 
static void webvtt_style_apply(WebVTTContext *s, const char *style)
{
    ASSStyle *st = ff_ass_style_get(s->ass_ctx, style);
    if (st) {
        if (st->bold != ASS_DEFAULT_BOLD) {
            webvtt_print(s, "<b>");
            webvtt_stack_push(s, 'b');
        }
        if (st->italic != ASS_DEFAULT_ITALIC) {
            webvtt_print(s, "<i>");
            webvtt_stack_push(s, 'i');
        }
        if (st->underline != ASS_DEFAULT_UNDERLINE) {
            webvtt_print(s, "<u>");
            webvtt_stack_push(s, 'u');
        }
    }
}
 
static void webvtt_print_timestamp(WebVTTContext *s, int64_t millisec)
{
    int64_t sec = millisec / 1000;
    millisec %= 1000;
    int64_t min = sec / 60;
    sec %= 60;
    int64_t hour = min / 60;
    min %= 60;
    if (hour > 0) {
        webvtt_print(s, "<%02"PRId64":%02"PRId64":%02"PRId64".%03"PRId64">", hour, min, sec, millisec);
    } else {
        webvtt_print(s, "<%02"PRId64":%02"PRId64".%03"PRId64">", min, sec, millisec);
    }
}
 
static void webvtt_flush_karaoke_timestamp(WebVTTContext *s)
{
    if (s->pending_duration_ms <= 0)
        return;
 
    int64_t next_ts = s->current_time_ms <= INT64_MAX - s->pending_duration_ms ?
                      s->current_time_ms + s->pending_duration_ms : INT64_MAX;
    if (next_ts > s->cue_start_ms && next_ts < s->cue_end_ms &&
        next_ts > s->last_timestamp_ms) {
        webvtt_print_timestamp(s, next_ts);
        s->last_timestamp_ms = next_ts;
    }
    s->current_time_ms = next_ts;
    s->pending_duration_ms = 0;
}
 
static void webvtt_text_cb(void *priv, const char *text, int len)
{
    WebVTTContext *s = priv;
    av_bprint_append_data(&s->buffer, text, len);
}
 
static void webvtt_new_line_cb(void *priv, int forced)
{
    WebVTTContext *s = priv;
    webvtt_print(s, "\n");
}
 
static void webvtt_karaoke_cb(void *priv, unsigned int duration)
{
    WebVTTContext *s = priv;
 
    webvtt_flush_karaoke_timestamp(s);
    s->pending_duration_ms = (int64_t)duration * 10;
}
 
static void webvtt_style_cb(void *priv, char style, int close)
{
    if (style == 's') // strikethrough unsupported
        return;
 
    webvtt_stack_push_pop(priv, style, close);
    if (!close)
        webvtt_print(priv, "<%c>", style);
}
 
static void webvtt_cancel_overrides_cb(void *priv, const char *style)
{
    webvtt_stack_push_pop(priv, 0, 1);
    webvtt_style_apply(priv, style);
}
 
static void webvtt_end_cb(void *priv)
{
    WebVTTContext *s = priv;
    webvtt_flush_karaoke_timestamp(s);
    webvtt_stack_push_pop(priv, 0, 1);
}
 
static const ASSCodesCallbacks webvtt_callbacks = {
    .text             = webvtt_text_cb,
    .new_line         = webvtt_new_line_cb,
    .style            = webvtt_style_cb,
    .color            = NULL,
    .font_name        = NULL,
    .font_size        = NULL,
    .alignment        = NULL,
    .cancel_overrides = webvtt_cancel_overrides_cb,
    .move             = NULL,
    .karaoke          = webvtt_karaoke_cb,
    .end              = webvtt_end_cb,
};
 
static int webvtt_encode_frame(AVCodecContext *avctx,
                               unsigned char *buf, int bufsize, const AVSubtitle *sub)
{
    WebVTTContext *s = avctx->priv_data;
    ASSDialog *dialog;
    int i;
 
    av_bprint_init_for_buffer(&s->buffer, buf, bufsize);
 
    /* Multiple rectangles are independent overlays. One monotonic WebVTT
     * cue cannot represent them, so keep the previous plain-text path. */
    if (sub->pts != AV_NOPTS_VALUE && sub->num_rects == 1) {
        s->cue_start_ms = av_rescale_q(sub->pts, AV_TIME_BASE_Q, (AVRational){ 1, 1000 });
        s->cue_end_ms   = s->cue_start_ms + sub->end_display_time;
    } else {
        s->cue_start_ms = s->cue_end_ms = 0;
    }
    s->current_time_ms = s->cue_start_ms;
    s->last_timestamp_ms = -1;
    s->pending_duration_ms = 0;
 
    for (i=0; i<sub->num_rects; i++) {
        const char *ass = sub->rects[i]->ass;
 
        if (sub->rects[i]->type != SUBTITLE_ASS) {
            av_log(avctx, AV_LOG_ERROR, "Only SUBTITLE_ASS type supported.\n");
            return AVERROR(EINVAL);
        }
 
        dialog = ff_ass_split_dialog(s->ass_ctx, ass);
        if (!dialog)
            return AVERROR(ENOMEM);
        webvtt_style_apply(s, dialog->style);
        ff_ass_split_override_codes(&webvtt_callbacks, s, dialog->text);
        ff_ass_free_dialog(&dialog);
    }
 
    if (!s->buffer.len)
        return 0;
 
    if (!av_bprint_is_complete(&s->buffer)) {
        av_log(avctx, AV_LOG_ERROR, "Buffer too small for ASS event.\n");
        return AVERROR_BUFFER_TOO_SMALL;
    }
 
    return s->buffer.len;
}
 
static av_cold int webvtt_encode_close(AVCodecContext *avctx)
{
    WebVTTContext *s = avctx->priv_data;
    ff_ass_split_free(s->ass_ctx);
    return 0;
}
 
static av_cold int webvtt_encode_init(AVCodecContext *avctx)
{
    WebVTTContext *s = avctx->priv_data;
    s->avctx = avctx;
    s->ass_ctx = ff_ass_split(avctx->subtitle_header);
    return s->ass_ctx ? 0 : AVERROR_INVALIDDATA;
}
 
const FFCodec ff_webvtt_encoder = {
    .p.name         = "webvtt",
    CODEC_LONG_NAME("WebVTT subtitle"),
    .p.type         = AVMEDIA_TYPE_SUBTITLE,
    .p.id           = AV_CODEC_ID_WEBVTT,
    .priv_data_size = sizeof(WebVTTContext),
    .init           = webvtt_encode_init,
    FF_CODEC_ENCODE_SUB_CB(webvtt_encode_frame),
    .close          = webvtt_encode_close,
};