FFMPEG--裸码流解码AVCodec

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介绍

本例程主要介绍按帧长从裸码流（h264）文件中读取每帧码流，通过avpacket传给解码器进行解码。

创建解码器

//必须先注册编解码器之后， 才能查找解码器等操作    avcodec_register_all();         //查找解码器    enum AVCodecID  emCodecId = AV_CODEC_ID_H264;    AVCodec* ptCodec = avcodec_find_decoder(emCodecId);    ASSERT(ptCodec != NULL);    AVCodecContext* ptCodecCtx = avcodec_alloc_context3(ptCodec);    ASSERT(ptCodecCtx != NULL);    if (avcodec_open2(ptCodecCtx, ptCodec, NULL) < 0) {        printf("Could not open codec\n");        return -1;    }

注意上述代码只是根据解码器类型创建解码器，这对于h264格式是可以的，码流的宽高、pixfmt等信息此时并没有，必须得等到解码器解码第一帧之后才会赋值给AVCodecContext。Mpeg4码流验证也是一样的效果

初始化解码前后帧数据结构

AVFrame–解码后帧结构
AVPacket–解码前帧结构

//初始化解码后ptFrameAVFrame* ptFrame = av_frame_alloc();ASSERT(ptFrame != NULL);//初始化解码前tPacketAVPacket tPacket;av_init_packet(&tPacket);

解码

每帧的数据和长度分别赋非AVPacket，然后传给解码器解码

tPacket.data = pbyInputbuf;     //一帧数据地址tPacket.size = pbyPacketLen[0]; //一帧数据长度

dwGotPicture为非0，表示完全解码一帧
ptFrame是解码后的图像

int dwGotPicture = 0;int dwRet = avcodec_decode_video2(ptCodecCtx, ptFrame, &dwGotPicture, &tPacket);if (dwRet < 0){    printf("decoder error:%d, FrameNum:%d\n", dwRet, dwIdex);    goto NEXT;}if(dwGotPicture){}

例程

#include <stdio.h>extern "C"{#include <libavcodec/avcodec.h>};#define ASSERT(X) if(!(X)){printf("####[%s:%d]assert failed:%s\n", __FUNCTION__, __LINE__, #X);}#define _FILE_NAME_LEN 64       //长度文件名长度#define _READ_BUF_SIZE (10<<20) //输入码流缓存大小#define _FRAME_NUM  20          //码流帧数 #define _VID_WIDTH 1920#define _VID_HEIGHT 1080int main(){    AVCodec* ptCodec = NULL;                //解码器    AVCodecContext* ptCodecCtx = NULL;      //解码器上下文    AVFrame* ptFrame = NULL;                //解码后帧    AVPacket tPacket;                       //待解码帧， 这里的tPacket得保证是完成的一帧    //必须先注册编解码器之后， 才能查找解码器等操作    avcodec_register_all();         //查找解码器    enum AVCodecID  emCodecId = AV_CODEC_ID_H264;    ptCodec = avcodec_find_decoder(emCodecId);    ASSERT(ptCodec != NULL);    ptCodecCtx = avcodec_alloc_context3(ptCodec);    ASSERT(ptCodecCtx != NULL);    if (avcodec_open2(ptCodecCtx, ptCodec, NULL) < 0)     {        printf("Could not open codec\n");        return -1;    }    av_log(NULL, AV_LOG_ERROR,"cur log level:%d\n", av_log_get_level());    av_log(NULL, AV_LOG_ERROR,"codec:%p, %p codectype:%d, pixfmt:%d\n",         ptCodec, ptCodecCtx->codec, ptCodec->type, ptCodecCtx->pix_fmt);    av_log(NULL, AV_LOG_ERROR,"coded_width:%d, coded_height:%d, (%d,%d)\n",         ptCodecCtx->coded_width,        ptCodecCtx->coded_height, ptCodecCtx->width, ptCodecCtx->height);    //初始化解码后ptFrame    ptFrame = av_frame_alloc();    ASSERT(ptFrame != NULL);    //初始化解码前tPacket    av_init_packet(&tPacket);    const char* pszInputFileName = "1080P.h264";//"704x576.mpeg4";//输入为待解码码流文件    const char* pszOutPutFileName = "1080P.yuv";//"704x576.yuv";//解码后图像保存文件    char arbyInputLenFileName[_FILE_NAME_LEN];  //输入码流长度文件，按该文件数据读取固定长度码流为一帧    memset(arbyInputLenFileName, 0, _FILE_NAME_LEN);    sprintf(arbyInputLenFileName, "%s.len", pszInputFileName);    printf("InputLenFileName:%s\n", arbyInputLenFileName);    //打开输入码流文件    FILE* pfInputFile = fopen(pszInputFileName, "rb");    ASSERT(pfInputFile != NULL);    //打开输入码流长度文件    FILE* pfInputLenFile = fopen(arbyInputLenFileName, "rb");    ASSERT(pfInputLenFile != NULL);    //打开输出图像文件    FILE* pfOutputFile = fopen(pszOutPutFileName, "wb");    ASSERT(pfOutputFile != NULL);    uint8_t* pbyInputbuf = new uint8_t[_READ_BUF_SIZE];     //输入码流缓冲    memset(pbyInputbuf, 0, _READ_BUF_SIZE * sizeof(uint8_t));    int* pbyPacketLen = new int[_FRAME_NUM];            //输入码流每帧长度数组    memset(pbyPacketLen, 0, _FRAME_NUM * sizeof(int));    //读取输入码流到buf中    int dwRealBufSize = fread(pbyInputbuf, sizeof(uint8_t), _READ_BUF_SIZE, pfInputFile);    printf("Read Buf Size:%d\n", dwRealBufSize);    fclose(pfInputFile);    pfInputFile = NULL;    //获取实际输入码流帧数即每帧码流长度    int dwRealFrameNum = 0;    for (; dwRealFrameNum < _FRAME_NUM; dwRealFrameNum++)    {        fscanf(pfInputLenFile, "%d", &pbyPacketLen[dwRealFrameNum]);        if (feof(pfInputLenFile))        {            //防止输入码流帧数比 _FRAME_NUM 少的情况            fclose(pfInputLenFile);            pfInputLenFile = NULL;            delete []arbyInputLenFileName;            break;        }    }    tPacket.data = pbyInputbuf;    tPacket.size = pbyPacketLen[0];    for (int dwIdex = 0; dwIdex < dwRealFrameNum; dwIdex++)    {        printf("Idex:%d, FrameLen:%d\n", dwIdex, pbyPacketLen[dwIdex]);        //fwrite(tPacket.data, 1, tPacket.size, pfOutputFile);        av_log(NULL, AV_LOG_ERROR, "pbyInputbuf:%p,%p tPacket.data:%p, %p\n",             pbyInputbuf, pbyInputbuf+dwRealBufSize,            tPacket.data, tPacket.data + pbyPacketLen[0]);        int dwGotPicture = 0;        int dwRet = avcodec_decode_video2(ptCodecCtx, ptFrame, &dwGotPicture, &tPacket);        if (dwRet < 0)        {            printf("decoder error:%d, FrameNum:%d\n", dwRet, dwIdex);            goto NEXT;        }        if (dwGotPicture)        {            printf("num:%d pixfmt:%d, w:%d, h:%d, line:%d,%d,%d\n",             uint8_t* byTempPtr = NULL;            byTempPtr = ptFrame->data[0];            //解码后的图像格式应该是yuv420p，不过很奇怪的是为什么avcodec_open2之后没有设置解码后的pixfmt,而是要等到            //第一帧解码之后，当然码流长度等信息也是第一帧解码之后的            //保存Y            for (int dwHIdex = 0; dwHIdex < ptFrame->height; dwHIdex++)            {                fwrite(byTempPtr, 1, ptFrame->width, pfOutputFile);                //这里要注意每一行的跨度和宽度的区别, 每行的有效数据长度为width                        byTempPtr += ptFrame->linesize[0];            }            //保存U            byTempPtr = ptFrame->data[1];            for (int dwHIdex = 0; dwHIdex < ptFrame->height/2; dwHIdex++)            {                fwrite(byTempPtr, 1, ptFrame->width/2, pfOutputFile);                byTempPtr += ptFrame->linesize[1];            }            //保存V            byTempPtr = ptFrame->data[2];            for (int dwHIdex = 0; dwHIdex < ptFrame->height/2; dwHIdex++)            {                fwrite(byTempPtr, 1, ptFrame->width/2, pfOutputFile);                byTempPtr += ptFrame->linesize[2];            }        }        else        {            printf("decoder failed:%d, FrameNum:%d\n", dwRet, dwIdex);        }NEXT:        tPacket.data += pbyPacketLen[dwIdex];        tPacket.size = pbyPacketLen[dwIdex+1];    }    fclose(pfOutputFile);    av_frame_free(&ptFrame);    avcodec_close(ptCodecCtx);    av_free(ptCodecCtx);    return 0;}

另外需要说明的是解码后图像颜色空间为yuv420p，需要注意的是这里的y/u/v是分别保存在三个空间的。如果需要缩放解码后图像大小和改变其颜色空间，可以使用sws_scale实现

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