ffmpeg 解码h264数据

转自：http://blog.csdn.net/liushu1231/article/details/9203713

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;
 }