ffmpeg 解碼h264資料

本文內容均為原創，轉載請註明出處。

使用ffmpeg解碼h264資料其實相對使用x264進行視頻編碼是簡單了許多的，因為ffmpeg提供了一個decoding_encoding.c的檔案，這個檔案裡面有簡單的使用ffmpeg進行視頻、音頻編解碼的例子，不過可能有的人也會找不到這個樣本，我就講我改造過的這個樣本放在這裡，同時加一些解釋。

其中需要注意的的一點我需要在此說明，就是ffmpeg在進行解碼的時候是會考慮要解碼的資料包是否有0x00 00 001這樣的頭的，如果沒有的話，ffmpeg會認為是錯誤的資料包。下面是使用opencv對解碼後的映像進行顯示，所以還要配置opencv的環境，如果沒有的話，可以注釋掉ShowImage這個函數，然後使用pgm_save這個函數將解碼後的映像儲存。

下面將My Code放在下面，同樣，過程參見代碼注釋，相對來說比較簡單，不在此過多敘述：

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);}  //通過尋找0x000001或者0x00000001找到下一個資料包的頭部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;}   //將檔案中的一個資料包轉換成AVPacket類型以便ffmpeg進行解碼#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);    }     //解碼與顯示需要的輔助的資料結構，需要注意的是，AVFrame必須經過alloc才能使用，不然其記憶體的緩衝空間指標是空的，程式會崩潰    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);//解碼每一幀            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 */               //將YUV420格式的映像轉換成RGB格式所需要的轉換上下文                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);//映像格式轉換                    showImage->imageSize =frameRGB.linesize[0];//指標賦值給要顯示的映像                    showImage->imageData = (char *)frameRGB.data[0];                    cvShowImage("decode",showImage);//顯示                    cvWaitKey(0.5);//設定0.5s顯示一幀，如果不設定由於這是個迴圈，會導致看不到顯示出來的映像                }                 //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();//註冊所有的轉碼器，一定要注意，如果沒有這行代碼則會出錯，提示沒有找不到轉碼器    video_decode_example("%3d.pgm","test.264");//可以使用x264編碼出來的264檔案    system("pause");    return 0;}