[FFmpeg-devel] [PATCH] movtextdec: MPEG4 Part 17 Timed Text Decoder.

Clément Bœsch ubitux at gmail.com
Thu Jun 28 00:42:27 CEST 2012


On Sat, Jun 23, 2012 at 02:17:49PM -0700, Philip Langdale wrote:
> This change introduces a basic decoder for MPEG4 Part 17 subtitles,
> also known as Timed Text and TX3G.
> 
> This initial change doesn't attempt to parse styling information,
> and just reads the plain text of the subtitles. I intend to add
> support for styles eventually, but it's challenging due to a lack
> of reliable samples and existing players that support them.

Nice. What happens if you use gpac/mp4box to mux
fate-samples/sub/SubRip_capability_tester.srt into a MP4? Wouldn't it
provide an appropriate testing sample?

> ---
>  libavcodec/Makefile     |    1 +
>  libavcodec/allcodecs.c  |    1 +
>  libavcodec/movtextdec.c |  113 +++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 115 insertions(+)
>  create mode 100644 libavcodec/movtextdec.c
> 
> diff --git a/libavcodec/Makefile b/libavcodec/Makefile
> index 95bcb17..ec7ddca 100644
> --- a/libavcodec/Makefile
> +++ b/libavcodec/Makefile
> @@ -265,6 +265,7 @@ OBJS-$(CONFIG_MJPEGB_DECODER)          += mjpegbdec.o mjpegdec.o mjpeg.o
>  OBJS-$(CONFIG_MLP_DECODER)             += mlpdec.o mlpdsp.o
>  OBJS-$(CONFIG_MMVIDEO_DECODER)         += mmvideo.o
>  OBJS-$(CONFIG_MOTIONPIXELS_DECODER)    += motionpixels.o
> +OBJS-$(CONFIG_MOVTEXT_DECODER)         += movtextdec.o ass.o
>  OBJS-$(CONFIG_MP1_DECODER)             += mpegaudiodec.o mpegaudiodecheader.o \
>                                            mpegaudio.o mpegaudiodata.o
>  OBJS-$(CONFIG_MP1FLOAT_DECODER)        += mpegaudiodec_float.o mpegaudiodecheader.o \
> diff --git a/libavcodec/allcodecs.c b/libavcodec/allcodecs.c
> index 8c2bd2b..0674a7c 100644
> --- a/libavcodec/allcodecs.c
> +++ b/libavcodec/allcodecs.c
> @@ -404,6 +404,7 @@ void avcodec_register_all(void)
>      REGISTER_ENCDEC  (DVDSUB, dvdsub);
>      REGISTER_DECODER (JACOSUB, jacosub);
>      REGISTER_DECODER (MICRODVD, microdvd);
> +    REGISTER_DECODER (MOVTEXT, movtext);
>      REGISTER_DECODER (PGSSUB, pgssub);
>      REGISTER_ENCDEC  (SRT, srt);
>      REGISTER_ENCDEC  (XSUB, xsub);
> diff --git a/libavcodec/movtextdec.c b/libavcodec/movtextdec.c
> new file mode 100644
> index 0000000..57f6b71
> --- /dev/null
> +++ b/libavcodec/movtextdec.c
> @@ -0,0 +1,113 @@
> +/*
> + * 3GPP TS 26.245/MPEG4 Part 17 Timed Text decoder
> + * Copyright (c) 2012  Philip Langdale <philipl at 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/bprint.h"
> +#include "libavutil/intreadwrite.h"
> +
> +static int text_to_ass(AVBPrint *buf, const char *text, const char *text_end)
> +{
> +    int line_start = 1;
> +
> +    while (text < text_end) {
> +        switch (*text) {
> +        case '\r':
> +            line_start = 1;
> +            break;
> +        case '\n':
> +            av_bprintf(buf, "\\N");
> +            line_start = 1;
> +            break;
> +        case ' ':
> +            // 1) Don't transfer leading whitespace
> +            // 2) Don't transfer multiple spaces
> +            // 3) Don't transfer trailing whitespace
> +            if (!line_start && *(text + 1) != ' ' &&
> +                *(text + 1) != '\r' && *(text + 1) != '\r')
> +                av_bprint_chars(buf, *text, 1);
> +            break;
> +        default:
> +            av_bprint_chars(buf, *text, 1);
> +            line_start = 0;
> +            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 0;
> +
> +    /*
> +     * 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 + 2 + AV_RB16(ptr);
> +    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});
> +
> +    av_bprint_init(&buf, 0, 2048);
> +    text_to_ass(&buf, ptr, end);
> +    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("MPEG4 Part 17 subtitle"),
> +    .type         = AVMEDIA_TYPE_SUBTITLE,
> +    .id           = CODEC_ID_MOV_TEXT,
> +    .init         = mov_text_init,
> +    .decode       = mov_text_decode_frame,
> +};

The patch LGTM, but I didn't test it (I'd be interested in any sample you
generate/find).

A FATE test for this would be appreciate eventually :)

-- 
Clément B.
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