最近做的一個專案中需要對h264編碼的視訊檔案進行解碼並轉換為openc可以處理的格式進行顯示和相關的影象處理操作。從網上找了借鑑了很多資料，但做的相對來說比較簡單。因此，在網上現有的程式碼基礎上進行了整理和優化。目前在專案中整合，效果良好。特分享給有共同需求的朋友，如有可繼續優化的空間，還望各位朋友能指出，我們共同學習與提高。
下面，貼出相關程式碼。
decode_video.h

#ifndef _DECODE_VIDEO_H
#define _DECODE_VIDEO_H

#include "opencv2/opencv.hpp"
extern "C"
{
    #include "libavcodec/avcodec.h"
 

    #include "libavformat/avformat.h"
    #include "libswscale/swscale.h"
};

typedef struct video_info
{
    AVPacket *packet;
    AVFrame *pAvFrame;
    AVCodec         *pCodec;
    AVFormatContext *pFormatCtx;
    AVCodecContext  *pCodecCtx;
    SwsContext *img_convert_ctx;
    int videoindex;
}video_t;

#ifndef G_DECODE_VIDEO_H
 

#define G_DECODE_VIDEO_H extern
#endif
G_DECODE_VIDEO_H video_t* video_init(const char* video_filename,int*ret);
G_DECODE_VIDEO_H int video_get_frame(video_t* handel,cv::Mat* pCvMat);
G_DECODE_VIDEO_H int video_get_alltime(video_t* handel);
G_DECODE_VIDEO_H int video_seek_frame(video_t* handel,long
 
 time_start);
G_DECODE_VIDEO_H int video_uninit(video_t* handel);

#endif

decode_video.cpp

#include "decode_video.h"

G_DECODE_VIDEO_H video_t* video_init(const char* video_filename,int *ret)
{
    video_t* video_info = (video_t*)malloc(sizeof(video_t));
    video_info->packet = NULL;
    video_info->pAvFrame = NULL;
    video_info->pCodec = NULL;
    video_info->pFormatCtx = NULL;
    video_info->pCodecCtx = NULL;
    video_info->img_convert_ctx = NULL;
    video_info->videoindex = -1;
    av_register_all();
    if (avformat_open_input(&(video_info->pFormatCtx),video_filename, NULL, NULL) != 0)
    {
        //無法開啟檔案
        (*ret) = -1;
        return NULL;
    }
    if (avformat_find_stream_info(video_info->pFormatCtx,NULL) < 0)
    {
        //無法查詢到流資訊
        (*ret) = -2;
        return NULL;
    }
    for(int i = 0;i < video_info->pFormatCtx->nb_streams;i++)
    {

        if (video_info->pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO)
        {
            video_info->videoindex = i;
            break;
        }
    }
    if(video_info->videoindex == -1)
    {
       //無法找到視訊流
        (*ret) = -3;
        return NULL;
    }
    video_info->pCodecCtx = video_info->pFormatCtx->streams[video_info->videoindex]->codec;

    video_info->pCodec = avcodec_find_decoder(video_info->pCodecCtx->codec_id);
    if (video_info->pCodec == NULL)
    {
        (*ret) = -4;
        return NULL;
    }

    if (avcodec_open2(video_info->pCodecCtx,video_info->pCodec, NULL) < 0)
    {
        //無法開啟解碼器
        (*ret) = -5;
        return NULL;
    }
    video_info->pAvFrame = av_frame_alloc();
    int y_size = video_info->pCodecCtx->width * video_info->pCodecCtx->height;
    video_info->packet = (AVPacket *)av_malloc(sizeof(AVPacket));
    av_new_packet(video_info->packet, y_size);
    (*ret) = 0;
    return video_info;

}

void video_getimg(AVCodecContext * pCodecCtx, SwsContext * img_convert_ctx, AVFrame * pFrame,cv::Mat* pCvMat)
{
    if (pCvMat->empty())
    {
        pCvMat->create(cv::Size(pCodecCtx->width, pCodecCtx->height), CV_8UC3);
    }

    AVFrame *pFrameRGB = NULL;
    uint8_t  *out_bufferRGB = NULL;
    pFrameRGB = av_frame_alloc();

    //給pFrameRGB幀加上分配的記憶體;
    int size = avpicture_get_size(AV_PIX_FMT_BGR24, pCodecCtx->width, pCodecCtx->height);
    out_bufferRGB = new uint8_t[size];
    avpicture_fill((AVPicture *)pFrameRGB, out_bufferRGB, AV_PIX_FMT_BGR24, pCodecCtx->width, pCodecCtx->height);

    //YUV to RGB
    sws_scale(img_convert_ctx, pFrame->data, pFrame->linesize, 0, pCodecCtx->height, pFrameRGB->data, pFrameRGB->linesize);

    memcpy(pCvMat->data, out_bufferRGB, size);

    delete[] out_bufferRGB;
    av_free(pFrameRGB);
}

G_DECODE_VIDEO_H int video_get_frame(video_t* handel,cv::Mat* pCvMat)
{
    int result = 0;
    int pic_got = -1;
    result = av_read_frame(handel->pFormatCtx,handel->packet);
    if(result < 0)
    {
        //視訊播放完成
        pCvMat = NULL;
        return -6;
    }
    //此處需注意，視訊播放完成後，並不會輸出-6，而是會再進行解碼導致解碼錯誤輸出-7
    if (handel->packet->stream_index == handel->videoindex)
    {
        int state = avcodec_decode_video2(handel->pCodecCtx, handel->pAvFrame, &pic_got, handel->packet);
        if (state < 0)
        {
            //解碼錯誤
            pCvMat = NULL;
            return -7;
        }
        if (pic_got)
        {
            if (handel->img_convert_ctx == NULL)
            {
                handel->img_convert_ctx = sws_getContext(handel->pCodecCtx->width, handel->pCodecCtx->height,handel->pCodecCtx->pix_fmt, handel->pCodecCtx->width, handel->pCodecCtx->height,AV_PIX_FMT_BGR24, SWS_BICUBIC, NULL, NULL, NULL);
            }
            if (pCvMat->empty())
            {
                pCvMat->create(cv::Size(handel->pCodecCtx->width, handel->pCodecCtx->height), CV_8UC3);
            }

            if (handel->img_convert_ctx != NULL)
            {
                video_getimg(handel->pCodecCtx, handel->img_convert_ctx, handel->pAvFrame,pCvMat);
            }
        }
    }
    av_free_packet(handel->packet);
    return 0;
}

G_DECODE_VIDEO_H int video_get_alltime(video_t* handel)
{
    int hours, mins, secs, us;
    if (handel->pFormatCtx->duration != AV_NOPTS_VALUE)
    {
        int64_t duration = handel->pFormatCtx->duration + 5000;
        secs = duration / AV_TIME_BASE;
        us = duration % AV_TIME_BASE;
        mins = secs / 60;
        secs %= 60;
        hours = mins / 60;
        mins %= 60;
        return (hours * 3600 + mins * 60 + secs);
    }
    else
    {
        return 0;
    }
}

G_DECODE_VIDEO_H int video_seek_frame(video_t* handel,long time_start)
{
    int64_t seek_pos = 0;
    if (time_start < 0)
    { 
        return -1;
    }
    seek_pos = time_start * AV_TIME_BASE;
    if (handel->pFormatCtx->start_time != AV_NOPTS_VALUE)
    {
        seek_pos += handel->pFormatCtx->start_time;
    }
    if (av_seek_frame(handel->pFormatCtx, -1, seek_pos, AVSEEK_FLAG_ANY) < 0)
    {
        return -2;
    }
    return 0;
}

G_DECODE_VIDEO_H int video_uninit(video_t* handel)
{
    if(handel != NULL)
    {
        av_free_packet(handel->packet);
        avcodec_close(handel->pCodecCtx);
        avformat_close_input(&(handel->pFormatCtx));
        return 0;
    }
    else
    {
        return -1;
    }
}