c++中使用解码器将mp4文件转化为avi文件

要将MP4文件转换为AVI文件，您需要使用视频编解码器和音频编解码器将MP4文件解码为原始视频和音频流，然后使用AVI编码器将它们重新编码为AVI格式。以下是C++中使用FFmpeg解码器将MP4文件转换为AVI文件的示例代码：

#include <iostream>
#include <string>
#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <cmath>

extern "C"
{
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>
}

using namespace std;

int main(int argc, char *argv[])
{
    AVFormatContext *pFormatCtx = NULL;
    AVCodecContext *pCodecCtx = NULL;
    AVCodec *pCodec = NULL;
    AVFrame *pFrame = NULL;
    AVPacket packet;
    int videoStream, numBytes;
    uint8_t *buffer = NULL;
    struct SwsContext *sws_ctx = NULL;
    AVOutputFormat *pOutputFormat = NULL;
    AVFormatContext *pOutputFormatCtx = NULL;
    AVStream *pOutStream = NULL;
    AVCodecContext *pOutCodecCtx = NULL;
    AVCodec *pOutCodec = NULL;
    uint8_t *outBuffer = NULL;
    int outBufferSize;
    int frameFinished;
    int width, height, fps, bitrate;

    if (argc < 3)
    {
        cout << "Usage: " << argv[0] << " input.mp4 output.avi" << endl;
        return -1;
    }

    av_register_all();

    if (avformat_open_input(&pFormatCtx, argv[1], NULL, NULL) != 0)
    {
        cout << "Error: Couldn't open input file." << endl;
        return -1;
    }

    if (avformat_find_stream_info(pFormatCtx, NULL) < 0)
    {
        cout << "Error: Couldn't find stream information." << endl;
        return -1;
    }

    av_dump_format(pFormatCtx, 0, argv[1], 0);

    videoStream = -1;

    for (int i = 0; i < pFormatCtx->nb_streams; i++)
    {
        if (pFormatCtx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO)
        {
            videoStream = i;
            break;
        }
    }

    if (videoStream == -1)
    {
        cout << "Error: Couldn't find video stream." << endl;
        return -1;
    }

    pCodecCtx = avcodec_alloc_context3(NULL);

    if (pCodecCtx == NULL)
    {
        cout << "Error: Couldn't allocate codec context." << endl;
        return -1;
    }

    if (avcodec_parameters_to_context(pCodecCtx, pFormatCtx->streams[videoStream]->codecpar) < 0)
    {
        cout << "Error: Couldn't copy codec parameters to codec context." << endl;
        return -1;
    }

    pCodec = avcodec_find_decoder(pCodecCtx->codec_id);

    if (pCodec == NULL)
    {
        cout << "Error: Couldn't find codec." << endl;
        return -1;
    }

    if (avcodec_open2(pCodecCtx, pCodec, NULL) < 0)
    {
        cout << "Error: Couldn't open codec." << endl;
        return -1;
    }

    pFrame = av_frame_alloc();

    if (pFrame == NULL)
    {
        cout << "Error: Couldn't allocate frame." << endl;
        return -1;
    }

    numBytes = av_image_get_buffer_size(AV_PIX_FMT_BGR24, pCodecCtx->width, pCodecCtx->height, 1);
    buffer = (uint8_t *)av_malloc(numBytes * sizeof(uint8_t));
    av_image_fill_arrays(pFrame->data, pFrame->linesize, buffer, AV_PIX_FMT_BGR24, pCodecCtx->width, pCodecCtx->height, 1);

    pOutputFormat = av_guess_format("avi", NULL, NULL);

    if (pOutputFormat == NULL)
    {
        cout << "Error: Couldn't guess output format." << endl;
        return -1;
    }

    if (avformat_alloc_output_context2(&pOutputFormatCtx, pOutputFormat, NULL, argv[2]) < 0)
    {
        cout << "Error: Couldn't allocate output context." << endl;
        return -1;
    }

    pOutCodec = avcodec_find_encoder(pOutputFormat->video_codec);

    if (pOutCodec == NULL)
    {
        cout << "Error: Couldn't find output codec." << endl;
        return -1;
    }

    pOutStream = avformat_new_stream(pOutputFormatCtx, pOutCodec);

    if (pOutStream == NULL)
    {
        cout << "Error: Couldn't allocate output stream." << endl;
        return -1;
    }

    pOutCodecCtx = avcodec_alloc_context3(pOutCodec);

    if (pOutCodecCtx == NULL)
    {
        cout << "Error: Couldn't allocate output codec context." << endl;
        return -1;
    }

    width = pCodecCtx->width;
    height = pCodecCtx->height;
    fps = (int)round(av_q2d(pFormatCtx->streams[videoStream]->r_frame_rate));
    bitrate = pCodecCtx->bit_rate;

    pOutCodecCtx->codec_tag = 0;
    pOutCodecCtx->codec_id = pOutputFormat->video_codec;
    pOutCodecCtx->bit_rate = bitrate;
    pOutCodecCtx->width = width;
    pOutCodecCtx->height = height;
    pOutCodecCtx->time_base.num = 1;
    pOutCodecCtx->time_base.den = fps;
    pOutCodecCtx->gop_size = 10;
    pOutCodecCtx->max_b_frames = 1;
    pOutCodecCtx->pix_fmt = AV_PIX_FMT_YUV420P;

    if (pOutputFormatCtx->oformat->flags & AVFMT_GLOBALHEADER)
    {
        pOutCodecCtx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
    }

    if (avcodec_open2(pOutCodecCtx, pOutCodec, NULL) < 0)
    {
        cout << "Error: Couldn't open output codec." << endl;
        return -1;
    }

    if (avcodec_parameters_from_context(pOutStream->codecpar, pOutCodecCtx) < 0)
    {
        cout << "Error: Couldn't copy codec parameters to output stream." << endl;
        return -1;
    }

    av_dump_format(pOutputFormatCtx, 0, argv[2], 1);

    if (!(pOutputFormat->flags & AVFMT_NOFILE))
    {
        if (avio_open(&pOutputFormatCtx->pb, argv[2], AVIO_FLAG_WRITE) < 0)
        {
            cout << "Error: Couldn't open output file." << endl;
            return -1;
        }
    }

    if (avformat_write_header(pOutputFormatCtx, NULL) < 0)
    {
        cout << "Error: Couldn't write output file header." << endl;
        return -1;
    }

    sws_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt, pOutCodecCtx->width, pOutCodecCtx->height, pOutCodecCtx->pix_fmt, SWS_BILINEAR, NULL, NULL, NULL);

    while (av_read_frame(pFormatCtx, &packet) >= 0)
    {
        if (packet.stream_index == videoStream)
        {
            avcodec_send_packet(pCodecCtx, &packet);

            while (avcodec_receive_frame(pCodecCtx, pFrame) == 0)
            {
                sws_scale(sws_ctx, pFrame->data, pFrame->linesize, 0, pCodecCtx->height, pFrame->data, pFrame->linesize);
                pFrame->width = pOutCodecCtx->width;
                pFrame->height = pOutCodecCtx->height;
                pFrame->format = pOutCodecCtx->pix_fmt;

                av_init_packet(&packet);

                if (avcodec_send_frame(pOutCodecCtx, pFrame) >= 0)
                {
                    while (avcodec_receive_packet(pOutCodecCtx, &packet) == 0)
                    {
                        av_packet_rescale_ts(&packet, pOutCodecCtx->time_base, pOutStream->time_base);
                        packet.stream_index = pOutStream->index;
                        av_write_frame(pOutputFormatCtx, &packet);
                        av_packet_unref(&packet);
                    }
                }
            }
        }

        av_packet_unref(&packet);
    }

    av_write_trailer(pOutputFormatCtx);

    if (pOutCodecCtx != NULL)
    {
        avcodec_close(pOutCodecCtx);
        avcodec_free_context(&pOutCodecCtx);
    }

    if (pOutStream != NULL)
    {
        avcodec_parameters_free(&pOutStream->codecpar);
        avformat_free_context(pOutputFormatCtx);
    }

    if (pFrame != NULL)
    {
        av_frame_free(&pFrame);
        av_free(buffer);
    }

    avcodec_close(pCodecCtx);
    avcodec_free_context(&pCodecCtx);
    avformat_close_input(&pFormatCtx);
    avformat_free_context(pFormatCtx);

    return 0;
}

该代码使用FFmpeg库进行解码和编码，并将最终的AVI文件写入磁盘。请注意，该代码可能需要进行一些调整以适应您的输入视频文件。