1.初始化格式上下文。

    AVFormatContext *formatCtx = avformat_alloc_context();
    if (avformat_open_input(&formatCtx, [videoPath UTF8String], NULL, NULL) < 0) {
        NSLog(@"不能打开输入文件");
        return nil;
    }
    
    if (avformat_find_stream_info(formatCtx, NULL) < 0) {
        NSLog(@"没有流信息");
        return nil;
    }

2.获取视频流。

    int stream_index = av_find_best_stream(formatCtx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
    if (stream_index < 0) {
        NSLog(@"没有发现视频流");
        return nil;
    }
    AVStream *videoStream = formatCtx->streams[stream_index];

3.获取解码器、解码上下文。

    AVCodecContext *codeCtx = avcodec_alloc_context3(NULL);
    if (!codeCtx) {
        NSLog(@"没有发现解码上下文");
        return nil;
    }

    if ((avcodec_parameters_to_context(codeCtx, videoStream->codecpar)) < 0) {
        NSLog(@"复制解码参数失败");
        return nil;
    }
    avcodec_open2(codeCtx, codec, NULL);

4.解码出视频帧。

    AVPacket *packet = av_packet_alloc();
    AVFrame *frame = av_frame_alloc();
    int frameCount = 0;
    NSMutableArray *imgArray = [NSMutableArray array];
    while (av_read_frame(formatCtx, packet) >= 0) {
        if (packet->stream_index == stream_index) {
            int send_result = avcodec_send_packet(codeCtx, packet);
            int receive_result = 0;
            if (send_result == AVERROR(EAGAIN)) {
                while (receive_result != AVERROR(EAGAIN)) {
                    receive_result = avcodec_receive_frame(codeCtx, frame);
                    if (receive_result == 0) {
                        if (onlyIFrame && frame->pict_type != AV_PICTURE_TYPE_I) {
                            continue;
                        }

                        NSString *imagePath = [DYMediaTools creatFile:[NSString stringWithFormat:@"img%@",@(frameCount++)] ofType:@"jpg"];
                        int r = frameToJPG(frame, [imagePath UTF8String]);
                        if (r == 0){
                            [imgArray addObject:[UIImage imageWithContentsOfFile:imagePath]];
                        }
                    }
                }
            } else {
                receive_result = avcodec_receive_frame(codeCtx, frame);
                if (receive_result == AVERROR(EAGAIN)) {
                    continue;
                } else if (receive_result == 0) {
                    if (onlyIFrame && frame->pict_type != AV_PICTURE_TYPE_I) {
                        continue;
                    }
        
                    NSString *imagePath = [DYMediaTools creatFile:[NSString stringWithFormat:@"img%@",@(frameCount++)] ofType:@"jpg"];
                    int r = frameToJPG(frame, [imagePath UTF8String]);
                    if (r == 0){
                        [imgArray addObject:[UIImage imageWithContentsOfFile:imagePath]];
                    }
                }
            }
        }
        av_packet_unref(packet);
    }

5.将视频帧编码为jpg。

int frameToJPG(AVFrame *videoFrame,const char *imageName) {
    
    AVFormatContext *formatCtx = avformat_alloc_context();
    avformat_alloc_output_context2(&formatCtx, NULL, NULL, imageName);

    if (avio_open(&formatCtx->pb, imageName, AVIO_FLAG_READ_WRITE) < 0) {
        NSLog(@"打开输出文件失败");
        return -1;
    }

    AVStream *pAVStream = avformat_new_stream(formatCtx, 0);
    if (pAVStream == NULL) {
        return -1;
    }

    AVCodecParameters *parameters = pAVStream->codecpar;
    parameters->codec_id = formatCtx->oformat->video_codec;
    parameters->codec_type = AVMEDIA_TYPE_VIDEO;
    parameters->format = AV_PIX_FMT_YUVJ420P;
    parameters->width = videoFrame->width;
    parameters->height = videoFrame->height;

    AVCodec *codec = avcodec_find_encoder(pAVStream->codecpar->codec_id);

    if (!codec) {
        NSLog(@"没有发现编码器");
        return -1;
    }

    AVCodecContext *codectx = avcodec_alloc_context3(codec);
    if (!codectx) {
        NSLog(@"没有发现编码上下文");
        return -1;
    }

    if ((avcodec_parameters_to_context(codectx, pAVStream->codecpar)) < 0) {
        NSLog(@"复制编码参数失败");
        return -1;
    }

    codectx->time_base = (AVRational) {1, 25};

    if (avcodec_open2(codectx, codec, NULL) < 0) {
        NSLog(@"打开编码器失败");
        return -1;
    }

    int ret = avformat_write_header(formatCtx, NULL);
    if (ret < 0) {
        NSLog(@"头文件写入失败");
        return -1;
    }

    AVPacket *packet = av_packet_alloc();
    //分配足够的空间保证能一次接收完
    av_new_packet(packet, videoFrame->width * videoFrame->height * 3);
    ret = avcodec_send_frame(codectx, videoFrame);
    if (ret < 0) {
        NSLog(@"发送视频帧失败");
        return -1;
    }

    ret = avcodec_receive_packet(codectx, packet);
    if (ret < 0) {
        NSLog(@"接收视频帧失败");
        return -1;
    }

    ret = av_write_frame(formatCtx, packet);

    if (ret < 0) {
        NSLog(@"写入帧失败");
        return -1;
    }

    av_packet_free(&packet);
    av_write_trailer(formatCtx);
    
    avcodec_close(codectx);
    avio_close(formatCtx->pb);
    avformat_free_context(formatCtx);
    return 0;
}