FFMPEG SDK 快速截取I帧

主要流程 : 

               根据输入的时间，向前seek到最近的I帧，读取I帧，解码，将得到的I帧写为JPEG保存。

主要接口:

          av_seek_frame();

          av_read_frame();

          avcodec_decode_video2();

注意问题:

       1.输入的时间戳单位是秒，需要转换成ffmpeg的时间戳，输入到av_seek_frame(),主要使用av_rescale()实现转换。

       2.ffmpeg解码另开线程，所以decode不是同步返回结果，代码中使用while(){sleep()}的逻辑获取解码结果。

       3.解码得到的frame需要先转换成YUVJ420p,利用sws_scale()实现，转换成YUV420p存在color_range的问题，导致输出的jpeg图像与原图像存在对比度差异,详见: 点击打开链接

////  main.cpp//  test_keyframe////  Created by shiyao.xsy on 16/1/26.//  Copyright © 2016年 rq. All rights reserved.//#include <iostream>#include <stdio.h>extern "C" {#include "libavformat/avformat.h"#include "libswscale/swscale.h"#include "libavcodec/avcodec.h"#include "libavutil/time.h"}int SavetoJPEG(AVFrame *pFrameYUV,AVStream *pVStream,char *filepath,int width, int height){    AVFormatContext* pFormatCtx;    AVOutputFormat* fmt;    AVStream* video_st;    AVCodecContext* pCodecCtx;    AVCodec* pCodec;        uint8_t* picture_buf;    AVFrame* picture;    AVPacket pkt;    int y_size;    int got_picture=0;    int size;        int ret=0;        int in_w=width,in_h=height;    //YUV's width and height    char* out_file = filepath;    //Output file        //Method 1    pFormatCtx = avformat_alloc_context();    //Guess format    fmt = av_guess_format("mjpeg", NULL, NULL);    pFormatCtx->oformat = fmt;    //Output URL    if (avio_open(&pFormatCtx->pb,out_file, AVIO_FLAG_READ_WRITE) < 0){        printf("Couldn't open output file.");        return -1;    }            //Method 2. More simple    //avformat_alloc_output_context2(&pFormatCtx, NULL, NULL, out_file);    //fmt = pFormatCtx->oformat;        video_st = avformat_new_stream(pFormatCtx, 0);    video_st->time_base.num = pVStream->time_base.num;    video_st->time_base.den = pVStream->time_base.den;    if (video_st==NULL){        return -1;    }    pCodecCtx = video_st->codec;    pCodecCtx->codec_id = fmt->video_codec;    pCodecCtx->codec_type = AVMEDIA_TYPE_VIDEO;    pCodecCtx->pix_fmt = PIX_FMT_YUVJ420P;    pCodecCtx->width = in_w;    pCodecCtx->height = in_h;    pCodecCtx->time_base.num = pVStream->codec->time_base.num;    pCodecCtx->time_base.den = pVStream->codec->time_base.den;    //Output some information    av_dump_format(pFormatCtx, 0, out_file, 1);        pCodec = avcodec_find_encoder(pCodecCtx->codec_id);    if (!pCodec){        printf("Codec not found.");        return -1;    }    if (avcodec_open2(pCodecCtx, pCodec,NULL) < 0){        printf("Could not open codec.");        return -1;    }    pCodecCtx->qmin = pCodecCtx->qmax = 3;    pCodecCtx->flags|=CODEC_FLAG_QSCALE;    picture = av_frame_alloc();    size = avpicture_get_size(pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height);    picture_buf = (uint8_t *)av_malloc(size);    if (!picture_buf)    {        return -1;    }    avpicture_fill((AVPicture *)picture, picture_buf, pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height);    //Write Header    avformat_write_header(pFormatCtx,NULL);        y_size = pCodecCtx->width * pCodecCtx->height;    av_new_packet(&pkt,y_size*3);    picture->data[0] = pFrameYUV->data[0];// Y    picture->data[1] = pFrameYUV->data[1];// U    picture->data[2] = pFrameYUV->data[2];// V    picture->width = in_w;    picture->height = in_h;    picture->format = PIX_FMT_YUVJ420P;    picture->pts = 0;    //Encode    ret = avcodec_encode_video2(pCodecCtx, &pkt,picture, &got_picture);    if(ret < 0){        printf("Encode Error.\n");        return -1;    }    if (got_picture==1){        pkt.stream_index = video_st->index;        ret = av_write_frame(pFormatCtx, &pkt);    }        av_packet_unref(&pkt);    //Write Trailer    av_write_trailer(pFormatCtx);        printf("Encode Successful.\n");        if (video_st){        avcodec_close(video_st->codec);        av_free(picture);        av_free(picture_buf);    }    avio_close(pFormatCtx->pb);    avformat_free_context(pFormatCtx);    return 0;}//void SaveYUV(AVFrame *pFrame, int width, int height, int iFrame) {//    FILE *pFile;//    char szFilename[32];//    int  y;////    // Open file//    sprintf(szFilename, "/Users/shiyaoxsy/frame%d.yuv", iFrame);//    pFile=fopen(szFilename, "wb+");//    if(pFile==NULL)//        return;//    // Write data//    int y_size = width * height;//    fwrite(pFrame->data[0], 1, y_size, pFile);//    fwrite(pFrame->data[1], 1, y_size/4, pFile);//    fwrite(pFrame->data[2], 1, y_size/4, pFile);////    // Close file//    fclose(pFile);//}int main(int argc, char * argv[]) {        if(argc != 4){        fprintf(stderr, "usage: %s timestamp input_file video_output_file \n",argv[0]);        return -1;    }    AVFormatContext *pFormatCtx;    int             i, videoindex,PictureSize;    AVCodecContext  *pCodecCtx;    AVCodec         *pCodec;    AVFrame *pFrame,*pFrameRGB;    AVPacket packet;    int ret, got_picture;    char *filepath = argv[2];    int64_t time_,time_total;        struct SwsContext *pSwsCtx;    uint8_t *outBuff;    time_total = av_gettime_relative();    av_register_all();    avformat_network_init();        pFormatCtx = avformat_alloc_context();    time_ = av_gettime_relative();        if(avformat_open_input(&pFormatCtx,filepath,NULL,NULL)!=0){        printf("Couldn't open input stream.\n");        return -1;    }        if(avformat_find_stream_info(pFormatCtx,NULL)<0){        printf("Couldn't find stream information.\n");        return -1;    }        videoindex=-1;        for(i=0; i<pFormatCtx->nb_streams; i++){        if(pFormatCtx->streams[i]->codec->codec_type==AVMEDIA_TYPE_VIDEO){            videoindex=i;            break;        }    }        if(videoindex == -1){        printf("Didn't find a video stream.\n");        return -1;    }        pCodecCtx=pFormatCtx->streams[videoindex]->codec;    pCodec=avcodec_find_decoder(pCodecCtx->codec_id);    if(pCodec==NULL){        printf("Codec not found.\n");        return -1;    }    if(avcodec_open2(pCodecCtx, pCodec,NULL)<0){        printf("Could not open codec.\n");        return -1;    }        pFrame=av_frame_alloc();    pFrameRGB=av_frame_alloc();    if( pFrame == NULL || pFrameRGB == NULL)    {        printf("avframe malloc failed!\n");        return -1;    }    PictureSize = avpicture_get_size(PIX_FMT_YUVJ420P, pCodecCtx->width, pCodecCtx->height);    outBuff = (uint8_t*)av_malloc(PictureSize*sizeof(uint8_t));        if(outBuff == NULL){        printf("av malloc failed!\n");        return -1;    }    avpicture_fill((AVPicture *)pFrameRGB, outBuff, PIX_FMT_YUVJ420P, pCodecCtx->width, pCodecCtx->height);    pSwsCtx = sws_getContext(pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height, PIX_FMT_YUVJ420P, SWS_BICUBIC, NULL, NULL, NULL);    time_ = av_gettime_relative() - time_;    printf("open file and find info, cost time: %0.3fs\n",time_/1000000.0);        time_ = av_gettime_relative();    int64_t timestamp = atoi(argv[1]);    timestamp = av_rescale(timestamp, pFormatCtx->streams[videoindex]->time_base.den, (int64_t)pFormatCtx->streams[videoindex]->time_base.num);        av_seek_frame(pFormatCtx, videoindex, timestamp, AVSEEK_FLAG_BACKWARD);    avcodec_flush_buffers(pFormatCtx->streams[videoindex]->codec);    time_ = av_gettime_relative()-time_;    printf("seek frame, costs time: %0.3fs\n",time_ / 1000000.0);    AVPacket avpkt;    av_init_packet(&avpkt);    avpkt.data = NULL;    avpkt.size = 0;        time_ = av_gettime_relative();    while (av_read_frame(pFormatCtx, &packet) >=0 ) {        if(packet.stream_index == videoindex ){            if(packet.flags){                if((ret = avcodec_decode_video2(pCodecCtx, pFrame, &got_picture, &packet))<0){                    printf("Decode Error!\n");                    return -1;                }                while(got_picture==0){                    sleep(0.01);                    if((ret = avcodec_decode_video2(pCodecCtx, pFrame, &got_picture, &avpkt))<0){                        printf("Decode Error!\n");                        return -1;                    }                }                if(got_picture){                    time_ = av_gettime_relative()-time_;                    printf("read and decode frame, costs time: %0.3fs\n",time_/1000000.0);                                        time_ = av_gettime_relative();                    sws_scale(pSwsCtx, (uint8_t const * const *)pFrame->data, pFrame->linesize, 0, pCodecCtx->height, pFrameRGB->data, pFrameRGB->linesize);                    if (SavetoJPEG(pFrameRGB,pFormatCtx->streams[videoindex],argv[3],pCodecCtx->width,pCodecCtx->height)<0) {                        printf("Write Image Error!\n");                        return -1;                    }                    time_ = av_gettime_relative()-time_;                    printf("write frame, costs time: %0.3fs\n",time_/1000000.0);                    av_packet_unref(&packet);                    break;                }                            }                    }        av_packet_unref(&packet);    }    sws_freeContext(pSwsCtx);    av_packet_unref(&avpkt);    av_free(outBuff);    av_free(pFrameRGB);    av_free(pFrame);    avcodec_close(pCodecCtx);    avformat_close_input(&pFormatCtx);    time_total = av_gettime_relative()-time_total;    printf("all done, costs time: %0.3fs\n",time_total/1000000.0);    return 0;}