ffmpeg 解码h264数据

本文内容均为原创，转载请注明出处。

使用ffmpeg解码h264数据其实相对使用x264进行视频编码是简单了许多的，因为ffmpeg提供了一个decoding_encoding.c的文件，这个文件里面有简单的使用ffmpeg进行视频、音频编解码的例子，不过可能有的人也会找不到这个示例，我就讲我改造过的这个示例放在这里，同时加一些解释。

其中需要注意的的一点我需要在此说明，就是ffmpeg在进行解码的时候是会考虑要解码的数据包是否有0x00 00 001这样的头的，如果没有的话，ffmpeg会认为是错误的数据包。下面是使用opencv对解码后的图像进行显示，所以还要配置opencv的环境，如果没有的话，可以注释掉ShowImage这个函数，然后使用pgm_save这个函数将解码后的图像保存。

下面将我的代码放在下面，同样，过程参见代码注释，相对来说比较简单，不在此过多叙述：

static void pgm_save(unsigned char *buf, int wrap, int xsize, int ysize,char*filename)
{
    FILE *f;
    int i;
 
    f=fopen(filename,"wb");
    fprintf(f,"P5\n%d%d\n%d\n",xsize,ysize,255);
    for(i=0;i<ysize;i++)
        fwrite(buf + i * wrap,1,xsize,f);
    fclose(f);
}
 
 //通过查找0x000001或者0x00000001找到下一个数据包的头部
static int _find_head(unsigned char*buffer, int len)
{
    int i;
 
    for(i=512;i<len;i++)
    {
        if(buffer[i] == 0 && buffer[i+1] == 0 && buffer[i+2] == 0&& buffer[i+3] == 1)
            break;
        if(buffer[i]== 0 && buffer[i+1] == 0 && buffer[i+2] == 1)
            break;
    }
    if (i ==len)
        return0;
    if (i ==512)
        return0;
    return i;
}
 
 
 //将文件中的一个数据包转换成AVPacket类型以便ffmpeg进行解码
#define FILE_READING_BUFFER (1*1024*1024)
static void build_avpkt(AVPacket *avpkt, FILE *fp)
{
    static unsigned charbuffer[1*1024*1024];
    static int readptr = 0;
    static int writeptr = 0;
    intlen,toread;
 
    intnexthead;
 
    if (writeptr- readptr < 200 * 1024)
    {
        memmove(buffer, &buffer[readptr],writeptr - readptr);
        writeptr -= readptr;
        readptr = 0;
        toread = FILE_READING_BUFFER - writeptr;
        len = fread(&buffer[writeptr], 1,toread, fp);
        writeptr += len;
    }
 
    nexthead = _find_head(&buffer[readptr], writeptr-readptr);
    if (nexthead== 0)
    {
        printf("failedfind next head...\n");
        nexthead = writeptr - readptr;
    }
 
    avpkt->size = nexthead;
    avpkt->data = &buffer[readptr];
    readptr += nexthead;
 
}
 
static voidvideo_decode_example(const char *outfilename, constchar *filename)
{
    AVCodec *codec;
    AVCodecContext *c= NULL;
    int frame,got_picture, len;
    FILE *f, *fout;
    AVFrame *picture;
    uint8_t inbuf[INBUF_SIZE +FF_INPUT_BUFFER_PADDING_SIZE];
    charbuf[1024];
    AVPacket avpkt;
 
    av_init_packet(&avpkt);
 
    /* set end ofbuffer to 0 (this ensures that no overreading happens for damaged mpeg streams)*/
    memset(inbuf + INBUF_SIZE, 0,FF_INPUT_BUFFER_PADDING_SIZE);
 
    printf("Videodecoding\n");
    opts = NULL;
    //av_dict_set(&opts,"b", "2.5M", 0);
    /* find the h264video decoder */
    codec = avcodec_find_decoder(CODEC_ID_H264);
    if (!codec){
        fprintf(stderr, "codecnot found\n");
        return ;
    }
 
    c = avcodec_alloc_context3(codec);
    picture= avcodec_alloc_frame();
 
    if(codec->capabilities&CODEC_CAP_TRUNCATED)
    c->flags|= CODEC_FLAG_TRUNCATED; /* we do not send complete frames */
 
    /* For somecodecs, such as msmpeg4 and mpeg4, width and height
    MUST be initialized there because thisinformation is not
    available in the bitstream. */
 
    /* open it */
    if(avcodec_open2(c, codec, NULL) < 0) {
        fprintf(stderr, "couldnot open codec\n");
        exit(1);
    }
 
 
//  fout=fopen(outfilename,"wb");
    /* the codec givesus the frame size, in samples */
 
    f = fopen(filename, "rb");
    if (!f) {
        fprintf(stderr, "couldnot open %s\n", filename);
        exit(1);
    }
     //解码与显示需要的辅助的数据结构，需要注意的是，AVFrame必须经过alloc才能使用，不然其内存的缓存空间指针是空的，程序会崩溃
    AVFrame frameRGB;
    IplImage *showImage =cvCreateImage(cvSize(352,288),8,3);
    avpicture_alloc((AVPicture*)&frameRGB,PIX_FMT_RGB24,352,288);
    cvNamedWindow("decode");
 
    frame = 0;
    for(;;) {
 
        build_avpkt(&avpkt, f);
 
        if(avpkt.size == 0)
            break;
 
        while(avpkt.size > 0) {
            len = avcodec_decode_video2(c,picture, &got_picture, &avpkt);//解码每一帧
            if(len < 0) {
                fprintf(stderr, "Error while decoding frame %d\n",frame);
                break;
            }
            if(got_picture) {
                printf("savingframe %3d\n", frame);
                fflush(stdout);
 
                /* thepicture is allocated by the decoder. no need to free it */
               //将YUV420格式的图像转换成RGB格式所需要的转换上下文
                SwsContext* scxt =sws_getContext(picture->width,picture->height,PIX_FMT_YUV420P,
                                                  picture->width,picture->height,PIX_FMT_RGB24,
                                                  2,NULL,NULL,NULL);
                if(scxt != NULL)
                {
                    sws_scale(scxt,picture->data,picture->linesize,0,c->height,frameRGB.data,frameRGB.linesize);//图像格式转换
                    showImage->imageSize =frameRGB.linesize[0];//指针赋值给要显示的图像
                    showImage->imageData = (char *)frameRGB.data[0];
                    cvShowImage("decode",showImage);//显示
                    cvWaitKey(0.5);//设置0.5s显示一帧，如果不设置由于这是个循环，会导致看不到显示出来的图像
                }
 
                //sprintf(buf,outfilename,frame);
 
                //pgm_save(picture->data[0],picture->linesize[0],
                //c->width,c->height, buf);
                //pgm_save(picture->data[1],picture->linesize[1],
                //c->width/2,c->height/2, fout);
                //pgm_save(picture->data[2],picture->linesize[2],
                //c->width/2,c->height/2, fout);
                frame++;
            }
            avpkt.size -= len;
            avpkt.data += len;
        }
    }
 
    /* some codecs,such as MPEG, transmit the I and P frame with a
    latency of one frame. You must do thefollowing to have a
    chance to get the last frame of the video */
    avpkt.data = NULL;
    avpkt.size = 0;
    len = avcodec_decode_video2(c, picture,&got_picture, &avpkt);
    if(got_picture) {
        printf("savinglast frame %3d\n", frame);
        fflush(stdout);
 
        /* the pictureis allocated by the decoder. no need to
        free it */
        sprintf(buf, outfilename, frame);
        //pgm_save(picture->data[0],picture->linesize[0],
        //       c->width, c->height, fout);
        pgm_save(picture->data[0],picture->linesize[0],c->width, c->height, fout);
        pgm_save(picture->data[1],picture->linesize[1],c->width/2, c->height/2, fout);
        pgm_save(picture->data[2],picture->linesize[2],c->width/2, c->height/2, fout);
 
        frame++;
    }
 
    fclose(f);
//  fclose(fout);
 
    avcodec_close(c);
    av_free(c);
    av_free(picture);
    printf("\n");
}
 
 
int main(int argc, char* argv[])
{
    avcodec_register_all();//注册所有的编解码器，一定要注意，如果没有这行代码则会出错，提示没有找不到编解码器
    video_decode_example("%3d.pgm","test.264");//可以使用x264编码出来的264文件
    system("pause");
    return 0;
}