经常遇到ivf格式，下面看看它的头：

typedef struct ivf_header {
    
    
    //     0-3	固定的'DKIF'字符串
    // 4-5	version，应该为0
    // 6-7	header的字节长度
    // 8-11	编码器的FourCC (e.g., 'VP80')
    // 12-13	width in pixels
    // 14-15	height in pixels
    // 16-19	framerate，单位为(1/timescale)
    // 20-23	timescale
    // 24-27	帧的个数
    // 28-31	unused

    uint32_t signature;
    uint16_t version;
    uint16_t lenght;
    uint32_t fourcc;
    uint16_t width;
    uint16_t height;
    uint32_t framerate;
    uint32_t timescale;
    uint32_t frame_count;
    uint32_t unused;

} ivf_header;

所以整个头文件大小是：32个字节。
另外每一帧还有4+8=12字节的头。
关于上述解析，ffmpeg-ivfdec.c中有：

/*
 * Copyright (c) 2010 David Conrad
 *
 * 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 "avformat.h"
#include "internal.h"
#include "riff.h"
#include "libavutil/intreadwrite.h"

static int probe(const AVProbeData *p)
{
    
    
    if (AV_RL32(p->buf) == MKTAG('D','K','I','F')
        && !AV_RL16(p->buf+4) && AV_RL16(p->buf+6) == 32)
        return AVPROBE_SCORE_MAX-2;

    return 0;
}

static int read_header(AVFormatContext *s)
{
    
    
    AVStream *st;
    AVRational time_base;

    avio_rl32(s->pb); // DKIF
    avio_rl16(s->pb); // version
    avio_rl16(s->pb); // header size

    st = avformat_new_stream(s, NULL);
    if (!st)
        return AVERROR(ENOMEM);


    st->codecpar->codec_type = AVMEDIA_TYPE_VIDEO;
    st->codecpar->codec_tag  = avio_rl32(s->pb);
    st->codecpar->codec_id   = ff_codec_get_id(ff_codec_bmp_tags, st->codecpar->codec_tag);
    st->codecpar->width      = avio_rl16(s->pb);
    st->codecpar->height     = avio_rl16(s->pb);
    time_base.den         = avio_rl32(s->pb);
    time_base.num         = avio_rl32(s->pb);
    st->duration          = avio_rl32(s->pb);
    avio_skip(s->pb, 4); // unused

    st->need_parsing      = AVSTREAM_PARSE_HEADERS;
//如果这两个有问题，一般会失败，所以ivf必须要填写
    if (!time_base.den || !time_base.num) {
    
    
        av_log(s, AV_LOG_ERROR, "Invalid frame rate\n");
        return AVERROR_INVALIDDATA;
    }

    avpriv_set_pts_info(st, 64, time_base.num, time_base.den);

    return 0;
}

static int read_packet(AVFormatContext *s, AVPacket *pkt)
{
    
    
    int ret, size = avio_rl32(s->pb);
    int64_t   pts = avio_rl64(s->pb);

    ret = av_get_packet(s->pb, pkt, size);
    pkt->stream_index = 0;
    pkt->pts          = pts;
    pkt->pos         -= 12;

    return ret;
}

AVInputFormat ff_ivf_demuxer = {
    
    
    .name           = "ivf",
    .long_name      = NULL_IF_CONFIG_SMALL("On2 IVF"),
    .read_probe     = probe,
    .read_header    = read_header,
    .read_packet    = read_packet,
    .flags          = AVFMT_GENERIC_INDEX,
    .codec_tag      = (const AVCodecTag* const []){
    
     ff_codec_bmp_tags, 0 },
};