1、概述

本程式碼實現了一個muxer並加入到ffmpeg的AVOutputFormat 連結串列中去，使程式碼能直接呼叫。實現的意義一是瞭解ffmpeg程式碼結構，二是可以自己整自己的視訊格式，讓別人播放不了。

2、程式碼

簡單看下程式碼：

/*
*本程式主要實現一個自己的muxer並加入到muxer鏈中去，使其可用,只有視訊
*作者：繆國凱（MK）
*[email protected]
*2015-6-2
*/

#include "stdafx.h"

#ifdef __cplusplus
extern "C"
{
#endif
#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#
#ifdef __cplusplus
};
#endif

#pragma comment(lib, "avcodec.lib")
#pragma comment(lib, "avformat.lib")
#pragma comment(lib, "avutil.lib")
//#pragma comment(lib, "avdevice.lib")
//#pragma comment(lib, "avfilter.lib")
//#pragma comment(lib, "postproc.lib")
//#pragma comment(lib, "swresample.lib")
//#pragma comment(lib, "swscale.lib")

int mk_write_header(struct AVFormatContext *fmt)
{
	//這些地方就可以加入自己的格式定義
	return 0;
}

int mk_write_packet(struct AVFormatContext *fmt, AVPacket *pkt)
{
	avio_write(fmt->pb, pkt->data, pkt->size);//簡單的用file協議寫檔案
	return 0;
}

int mk_write_trailer(struct AVFormatContext *fmt)
{
	//這些地方就可以加入自己的格式定義
	return 0;
}

AVOutputFormat ff_mk_muxer = 
{
	/*.name			= */"mk",
	/*.long_name		= */"mk (MK Video Container)",
	/*.mime_type		= */"mkvideo/x-msvideo",
	/*.extensions		= */"mk",

	/*.audio_codec		= */AV_CODEC_ID_NONE,
	/*.video_codec		= */AV_CODEC_ID_RAWVIDEO,//這裡先用ffmpeg自帶的yuv編碼器，以後改成自己的
	/*.subtitle_codec	= */AV_CODEC_ID_NONE,

	/*.flags		= */AVFMT_NOTIMESTAMPS,
	/*.codec_tag		= */NULL,
	/*.priv_class		= */NULL,
	/*.next				= */NULL,
	/*.priv_data_size	= */0,	

	/*.write_header		= */mk_write_header,
	/*.write_packet		= */mk_write_packet,
	/*.write_trailer	= */mk_write_trailer,
};

void help()
{
	printf("**********************************************\n");
	printf("Usage:\n");
	printf("    MyMuxer [inputfile] [outputfile.mk]\n");
	printf("\n");
	printf("Examples: \n");
	printf("    MyMuxer a.avi a.mk\n");
	printf("**********************************************\n");  
}

int _tmain(int argc, _TCHAR* argv[])
{
	if(argc < 3 || (!strcmp(argv[1],"--help")))
	{
		help();
		return 0;
	}

	AVFormatContext *in_fxt = NULL, *out_fxt = NULL;
	AVStream *out_stream = NULL;
	int video_index = -1;

	av_register_all();
	av_register_output_format(&ff_mk_muxer);//把自己的muxer加入連結串列

	if (avformat_open_input(&in_fxt, argv[1], NULL, NULL) < 0)
	{
		printf("can not open the input file context!\n");
		goto end;
	}
	if (avformat_find_stream_info(in_fxt, NULL) < 0)
	{
		printf("can not find the stream info!\n");
		goto end;
	}

	if(avformat_alloc_output_context2(&out_fxt, NULL, NULL, argv[2]) < 0)
	{
		printf("can not alloc output context!\n");
		goto end;
	}

	for (int i = 0; i < in_fxt->nb_streams; i++)
	{
		if (in_fxt->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO)
		{
			//open decoder
			if(0 > avcodec_open2(in_fxt->streams[i]->codec, avcodec_find_decoder(in_fxt->streams[i]->codec->codec_id), NULL))
			{
				printf("can not find or open decoder!\n");
				goto end;
			}
			video_index = i;
			//new stream
			out_stream = avformat_new_stream(out_fxt, NULL);
			if (!out_stream)
			{
				printf("can not new stream for output!\n");
				goto end;
			}
			//set codec context param
			out_stream->codec->codec = avcodec_find_encoder(out_fxt->oformat->video_codec);
			out_stream->codec->height = in_fxt->streams[i]->codec->height;
			out_stream->codec->width = in_fxt->streams[i]->codec->width;

			out_stream->codec->time_base.num = in_fxt->streams[i]->avg_frame_rate.den;
			out_stream->codec->time_base.den = in_fxt->streams[i]->avg_frame_rate.num;

			out_stream->codec->sample_aspect_ratio = in_fxt->streams[i]->codec->sample_aspect_ratio;			
			out_stream->codec->pix_fmt = in_fxt->streams[i]->codec->pix_fmt;
			if (!out_stream->codec->codec)
			{
				printf("can not find the encoder!\n");
				goto end;
			}
			if ((avcodec_open2(out_stream->codec, out_stream->codec->codec, NULL)) < 0)
			{
				printf("can not open the encoder\n");
				goto end;
			}
			if (out_fxt->oformat->flags & AVFMT_GLOBALHEADER)
				out_stream->codec->flags |= CODEC_FLAG_GLOBAL_HEADER;
			break;
		}
	}

	if (-1 == video_index)
	{
		printf("found no video stream in input file!\n");
		goto end;
	}

	if (!(out_fxt->oformat->flags & AVFMT_NOFILE))
	{
		if(avio_open(&out_fxt->pb, argv[2], AVIO_FLAG_WRITE) < 0)
		{
			printf("can not open output file handle!\n");
			goto end;
		}
	}

	if(avformat_write_header(out_fxt, NULL) < 0)
	{
		printf("can not write the header of the output file!\n");
		goto end;
	}

	AVPacket pkt_in, pkt_out;
	AVFrame *frame;
	frame = av_frame_alloc();
	av_init_packet(&pkt_in);
	av_init_packet(&pkt_out);
	int got_frame, got_picture;
	int i = 0, frame_index = 0;
	while(1)
	{
		got_frame = -1;
		got_picture = -1;
		if (av_read_frame(in_fxt, &pkt_in) < 0)
		{
			break;
		}
		if (avcodec_decode_video2(in_fxt->streams[video_index]->codec, frame, &got_frame, &pkt_in) < 0)
		{
			printf("can not decoder a frame");
			break;
		}
		av_free_packet(&pkt_in);

		if (got_frame)
		{
			frame->pts = i++;
			pkt_out.data = NULL;//主要這裡必須自己初始化，或者必須置為null，不然ff_alloc_packet2函式會報錯
			pkt_out.size = 0;
			if (avcodec_encode_video2(out_stream->codec, &pkt_out, frame, &got_picture) < 0)
			{
				printf("can not encode a frame!\n");
				break;
			}

			if (got_picture)
			{
				printf("Succeed to encode frame: %5d\tsize:%5d\n",frame_index,pkt_out.size);
				pkt_out.stream_index = out_stream->index;
				frame_index++;
				av_write_frame(out_fxt, &pkt_out);
				av_free_packet(&pkt_out);
			}
		}
	}
	av_frame_free(&frame);

	av_write_trailer(out_fxt);

	//clean
	avcodec_close(out_stream->codec);
	avcodec_close(out_fxt->streams[video_index]->codec);
end:
	avformat_close_input(&in_fxt);

	if (out_fxt && !(out_fxt->oformat->flags & AVFMT_NOFILE))
	{
		avio_close(out_fxt->pb);
	}
	avformat_free_context(out_fxt);

	return 0;
}
 

3、解釋

簡單說下基本原理。

首先自己實現一個AVOutputFormat，然後把這個muxer加入到連結串列中去，其他的根本不用自己管了，ffmpeg這套框架做的的確很漂亮，會一層層的掉用到你的muxer來，函式avformat_alloc_output_context2會根據輸入的格式或字尾來在AVOutputFormat鏈中查詢最適合的muxer，並把這個muxer的指標用oformat儲存，下面看看格式匹配的程式碼：

AVOutputFormat *av_guess_format(const char *short_name, const char *filename,
                                const char *mime_type)
{
    AVOutputFormat *fmt = NULL, *fmt_found;
    int score_max, score;

    /* specific test for image sequences */
#if CONFIG_IMAGE2_MUXER
    if (!short_name && filename &&
        av_filename_number_test(filename) &&
        ff_guess_image2_codec(filename) != AV_CODEC_ID_NONE) {
        return av_guess_format("image2", NULL, NULL);
    }
#endif
    /* Find the proper file type. */
    fmt_found = NULL;
    score_max = 0;
    while ((fmt = av_oformat_next(fmt))) {
        score = 0;
        if (fmt->name && short_name && av_match_name(short_name, fmt->name))
            score += 100;
        if (fmt->mime_type && mime_type && !strcmp(fmt->mime_type, mime_type))
            score += 10;
        if (filename && fmt->extensions &&
            av_match_ext(filename, fmt->extensions)) {
            score += 5;
        }
        if (score > score_max) {
            score_max = score;
            fmt_found = fmt;
        }
    }
    return fmt_found;
}
 

再來看看avformat_write_header的程式碼：

int avformat_write_header(AVFormatContext *s, AVDictionary **options)
{
    int ret = 0;

    if (ret = init_muxer(s, options))
        return ret;

    if (s->oformat->write_header) {
        ret = s->oformat->write_header(s);
        if (ret >= 0 && s->pb && s->pb->error < 0)
            ret = s->pb->error;
        if (ret < 0)
            return ret;
        if (s->flush_packets && s->pb && s->pb->error >= 0 && s->flags & AVFMT_FLAG_FLUSH_PACKETS)
            avio_flush(s->pb);
    }

    if ((ret = init_pts(s)) < 0)
        return ret;

    if (s->avoid_negative_ts < 0) {
        av_assert2(s->avoid_negative_ts == AVFMT_AVOID_NEG_TS_AUTO);
        if (s->oformat->flags & (AVFMT_TS_NEGATIVE | AVFMT_NOTIMESTAMPS)) {
            s->avoid_negative_ts = 0;
        } else
            s->avoid_negative_ts = AVFMT_AVOID_NEG_TS_MAKE_NON_NEGATIVE;
    }

    return 0;
}
 

int av_write_frame(AVFormatContext *s, AVPacket *pkt)
{
    int ret;

    ret = check_packet(s, pkt);
    if (ret < 0)
        return ret;

    if (!pkt) {
        if (s->oformat->flags & AVFMT_ALLOW_FLUSH) {
            ret = s->oformat->write_packet(s, NULL);
            if (s->flush_packets && s->pb && s->pb->error >= 0 && s->flags & AVFMT_FLAG_FLUSH_PACKETS)
                avio_flush(s->pb);
            if (ret >= 0 && s->pb && s->pb->error < 0)
                ret = s->pb->error;
            return ret;
        }
        return 1;
    }

    ret = compute_pkt_fields2(s, s->streams[pkt->stream_index], pkt);

    if (ret < 0 && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))
        return ret;

    ret = write_packet(s, pkt);
    if (ret >= 0 && s->pb && s->pb->error < 0)
        ret = s->pb->error;

    if (ret >= 0)
        s->streams[pkt->stream_index]->nb_frames++;
    return ret;
}

最後看看av_write_trailer的程式碼：

int av_write_trailer(AVFormatContext *s)
{
    int ret, i;

    for (;; ) {
        AVPacket pkt;
        ret = interleave_packet(s, &pkt, NULL, 1);
        if (ret < 0)
            goto fail;
        if (!ret)
            break;

        ret = write_packet(s, &pkt);
        if (ret >= 0)
            s->streams[pkt.stream_index]->nb_frames++;

        av_free_packet(&pkt);

        if (ret < 0)
            goto fail;
        if(s->pb && s->pb->error)
            goto fail;
    }

fail:
    if (s->oformat->write_trailer)
        if (ret >= 0) {
        ret = s->oformat->write_trailer(s);
        } else {
            s->oformat->write_trailer(s);
        }

    if (s->pb)
       avio_flush(s->pb);
    if (ret == 0)
       ret = s->pb ? s->pb->error : 0;
    for (i = 0; i < s->nb_streams; i++) {
        av_freep(&s->streams[i]->priv_data);
        av_freep(&s->streams[i]->index_entries);
    }
    if (s->oformat->priv_class)
        av_opt_free(s->priv_data);
    av_freep(&s->priv_data);
    return ret;
}

所以說，只要你按照ffmpeg的格式去定義muxer並加入連結串列，其他的都不用管了。

 最後驗證一下你的結果，執行程式隨便轉一個結果出來，把字尾.mk改為.yuv，用yuv播放器放一下，ok了，大功告成。下一篇介紹如何把這個muxer加入ffmpeg原始碼，使ffmpeg命令列可以用這個muxer。