FFmpeg/libavformat/gif.c

/*
 * Animated GIF muxer
 * Copyright (c) 2000 Fabrice Bellard
 *
 * first version by Francois Revol <revol@free.fr>
 *
 * 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
 */

/*
 * Features and limitations:
 * - currently no compression is performed,
 *   in fact the size of the data is 9/8 the size of the image in 8bpp
 * - uses only a global standard palette
 * - tested with IE 5.0, Opera for BeOS, NetPositive (BeOS), and Mozilla (BeOS).
 *
 * Reference documents:
 * http://www.goice.co.jp/member/mo/formats/gif.html
 * http://astronomy.swin.edu.au/pbourke/dataformats/gif/
 * http://www.dcs.ed.ac.uk/home/mxr/gfx/2d/GIF89a.txt
 *
 * this url claims to have an LZW algorithm not covered by Unisys patent:
 * http://www.msg.net/utility/whirlgif/gifencod.html
 * could help reduce the size of the files _a lot_...
 * some sites mentions an RLE type compression also.
 */

#include "avformat.h"
#include "libavutil/avassert.h"
#include "libavutil/imgutils.h"
#include "libavutil/log.h"
#include "libavutil/opt.h"

/* slows down the decoding (and some browsers don't like it) */
/* update on the 'some browsers don't like it issue from above:
 * this was probably due to missing 'Data Sub-block Terminator'
 * (byte 19) in the app_header */
#define GIF_ADD_APP_HEADER // required to enable looping of animated gif

static int gif_image_write_header(AVIOContext *pb, int width, int height,
                                  int loop_count, uint32_t *palette)
{
    int i;

    avio_write(pb, "GIF", 3);
    avio_write(pb, "89a", 3);
    avio_wl16(pb, width);
    avio_wl16(pb, height);

    if (palette) {
        avio_w8(pb, 0xf7); /* flags: global clut, 256 entries */
        avio_w8(pb, 0x1f); /* background color index */
        avio_w8(pb, 0);    /* aspect ratio */
        for (i = 0; i < 256; i++) {
            const uint32_t v = palette[i];
            avio_wb24(pb, v);
        }
    } else {
        avio_w8(pb, 0); /* flags */
        avio_w8(pb, 0); /* background color index */
        avio_w8(pb, 0); /* aspect ratio */
    }

    /* update: this is the 'NETSCAPE EXTENSION' that allows for looped animated
     * GIF, see http://members.aol.com/royalef/gifabout.htm#net-extension
     *
     * byte   1       : 33 (hex 0x21) GIF Extension code
     * byte   2       : 255 (hex 0xFF) Application Extension Label
     * byte   3       : 11 (hex (0x0B) Length of Application Block
     *                          (eleven bytes of data to follow)
     * bytes  4 to 11 : "NETSCAPE"
     * bytes 12 to 14 : "2.0"
     * byte  15       : 3 (hex 0x03) Length of Data Sub-Block
     *                          (three bytes of data to follow)
     * byte  16       : 1 (hex 0x01)
     * bytes 17 to 18 : 0 to 65535, an unsigned integer in
     *                          lo-hi byte format. This indicate the
     *                          number of iterations the loop should
     *                          be executed.
     * bytes 19       : 0 (hex 0x00) a Data Sub-block Terminator
     */

    /* application extension header */
#ifdef GIF_ADD_APP_HEADER
    if (loop_count >= 0 && loop_count <= 65535) {
        avio_w8(pb, 0x21);
        avio_w8(pb, 0xff);
        avio_w8(pb, 0x0b);
        // bytes 4 to 14
        avio_write(pb, "NETSCAPE2.0", sizeof("NETSCAPE2.0") - 1);
        avio_w8(pb, 0x03); // byte 15
        avio_w8(pb, 0x01); // byte 16
        avio_wl16(pb, (uint16_t)loop_count);
        avio_w8(pb, 0x00); // byte 19
    }
#endif
    return 0;
}

typedef struct {
    AVClass *class;         /** Class for private options. */
    int64_t time, file_time;
    uint8_t buffer[100]; /* data chunks */
    int loop;
} GIFContext;

static int gif_write_header(AVFormatContext *s)
{
    GIFContext *gif = s->priv_data;
    AVIOContext *pb = s->pb;
    AVCodecContext *enc, *video_enc;
    int i, width, height /*, rate*/;
    uint32_t palette[AVPALETTE_COUNT];

/* XXX: do we reject audio streams or just ignore them ?
 *  if (s->nb_streams > 1)
 *      return -1;
 */
    gif->time      = 0;
    gif->file_time = 0;

    video_enc = NULL;
    for (i = 0; i < s->nb_streams; i++) {
        enc = s->streams[i]->codec;
        if (enc->codec_type != AVMEDIA_TYPE_AUDIO)
            video_enc = enc;
    }

    if (!video_enc) {
        av_free(gif);
        return -1;
    } else {
        width  = video_enc->width;
        height = video_enc->height;
//        rate = video_enc->time_base.den;
    }

    if (avpriv_set_systematic_pal2(palette, video_enc->pix_fmt) < 0) {
        av_assert0(video_enc->pix_fmt == AV_PIX_FMT_PAL8);
        gif_image_write_header(pb, width, height, gif->loop, NULL);
    } else {
        gif_image_write_header(pb, width, height, gif->loop, palette);
    }

    avio_flush(s->pb);
    return 0;
}

static int gif_write_video(AVFormatContext *s, AVCodecContext *enc,
                           const uint8_t *buf, int size)
{
    AVIOContext *pb = s->pb;
    int jiffies;

    /* graphic control extension block */
    avio_w8(pb, 0x21);
    avio_w8(pb, 0xf9);
    avio_w8(pb, 0x04); /* block size */
    avio_w8(pb, 0x04); /* flags */

    /* 1 jiffy is 1/70 s */
    /* the delay_time field indicates the number of jiffies - 1 */
    /* XXX: should use delay, in order to be more accurate */
    /* instead of using the same rounded value each time */
    /* XXX: don't even remember if I really use it for now */
    jiffies = (70 * enc->time_base.num / enc->time_base.den) - 1;

    avio_wl16(pb, jiffies);

    avio_w8(pb, 0x1f); /* transparent color index */
    avio_w8(pb, 0x00);

    avio_write(pb, buf, size);

    return 0;
}

static int gif_write_packet(AVFormatContext *s, AVPacket *pkt)
{
    AVCodecContext *codec = s->streams[pkt->stream_index]->codec;
    if (codec->codec_type == AVMEDIA_TYPE_AUDIO)
        return 0; /* just ignore audio */
    else
        return gif_write_video(s, codec, pkt->data, pkt->size);
}

static int gif_write_trailer(AVFormatContext *s)
{
    AVIOContext *pb = s->pb;

    avio_w8(pb, 0x3b);

    return 0;
}

#define OFFSET(x) offsetof(GIFContext, x)
#define ENC AV_OPT_FLAG_ENCODING_PARAM
static const AVOption options[] = {
    { "loop", "Number of times to loop the output.", OFFSET(loop),
      AV_OPT_TYPE_INT, { .i64 = 0 }, 0, 65535, ENC },
    { NULL },
};

static const AVClass gif_muxer_class = {
    .class_name = "GIF muxer",
    .item_name  = av_default_item_name,
    .version    = LIBAVUTIL_VERSION_INT,
    .option     = options,
};

AVOutputFormat ff_gif_muxer = {
    .name           = "gif",
    .long_name      = NULL_IF_CONFIG_SMALL("GIF Animation"),
    .mime_type      = "image/gif",
    .extensions     = "gif",
    .priv_data_size = sizeof(GIFContext),
    .audio_codec    = AV_CODEC_ID_NONE,
    .video_codec    = AV_CODEC_ID_GIF,
    .write_header   = gif_write_header,
    .write_packet   = gif_write_packet,
    .write_trailer  = gif_write_trailer,
    .priv_class     = &gif_muxer_class,
};