ffmpeg mp4转h264 cpp代码

以下是使用FFmpeg库将MP4文件转换为H.264格式视频的C++代码示例：

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

int main(int argc, char *argv[]) {
    AVFormatContext *pFormatCtx = NULL;
    int videoIndex = -1;
    AVCodecContext *pCodecCtx = NULL;
    AVCodec *pCodec = NULL;
    AVFrame *pFrame = NULL;
    AVPacket packet;
    int frameFinished;
    int numBytes;
    uint8_t *buffer = NULL;
    struct SwsContext *sws_ctx = NULL;
    AVDictionary *optionsDict = NULL;

    if (argc < 3) {
        std::cout << "Usage: " << argv[0] << " <input file> <output file>" << std::endl;
        return -1;
    }

    av_register_all();

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

    if (avformat_find_stream_info(pFormatCtx, NULL) < 0) {
        std::cout << "Could not find stream information." << std::endl;
        return -1;
    }

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

    if (videoIndex == -1) {
        std::cout << "Could not find video stream." << std::endl;
        return -1;
    }

    pCodecCtx = avcodec_alloc_context3(NULL);
    avcodec_parameters_to_context(pCodecCtx, pFormatCtx->streams[videoIndex]->codecpar);
    pCodec = avcodec_find_decoder(pCodecCtx->codec_id);

    if (pCodec == NULL) {
        std::cout << "Unsupported codec." << std::endl;
        return -1;
    }

    if (avcodec_open2(pCodecCtx, pCodec, &optionsDict) < 0) {
        std::cout << "Could not open codec." << std::endl;
        return -1;
    }

    pFrame = av_frame_alloc();

    AVFormatContext *outFormatCtx = NULL;
    AVStream *outStream = NULL;
    AVCodecContext *outCodecCtx = NULL;
    AVCodec *outCodec = NULL;
    int outVideoIndex = -1;
    int outWidth = pCodecCtx->width / 2;
    int outHeight = pCodecCtx->height / 2;

    outCodec = avcodec_find_encoder(AV_CODEC_ID_H264);
    if (!outCodec) {
        std::cout << "Unsupported output codec." << std::endl;
        return -1;
    }

    outStream = avformat_new_stream(outFormatCtx, outCodec);
    if (!outStream) {
        std::cout << "Failed to allocate output stream." << std::endl;
        return -1;
    }

    outCodecCtx = avcodec_alloc_context3(outCodec);
    if (!outCodecCtx) {
        std::cout << "Failed to allocate output codec context." << std::endl;
        return -1;
    }

    outCodecCtx->codec_id = AV_CODEC_ID_H264;
    outCodecCtx->codec_type = AVMEDIA_TYPE_VIDEO;
    outCodecCtx->width = outWidth;
    outCodecCtx->height = outHeight;
    outCodecCtx->time_base = pCodecCtx->time_base;
    outCodecCtx->bit_rate = pCodecCtx->bit_rate;
    outCodecCtx->gop_size = 10;
    outCodecCtx->pix_fmt = AV_PIX_FMT_YUV420P;

    if (avcodec_open2(outCodecCtx, outCodec, NULL) < 0) {
        std::cout << "Failed to open output codec." << std::endl;
        return -1;
    }

    buffer = (uint8_t *) av_malloc(numBytes);

    avformat_alloc_output_context2(&outFormatCtx, NULL, NULL, argv[2]);
    if (!outFormatCtx) {
        std::cout << "Failed to allocate output format context." << std::endl;
        return -1;
    }

    if (avio_open(&outFormatCtx->pb, argv[2], AVIO_FLAG_WRITE) < 0) {
        std::cout << "Failed to open output file." << std::endl;
        return -1;
    }

    avformat_new_stream(outFormatCtx, NULL);
    avcodec_parameters_from_context(outFormatCtx->streams[0]->codecpar, outCodecCtx);
    outStream = outFormatCtx->streams[0];

    if (avformat_write_header(outFormatCtx, NULL) < 0) {
        std::cout << "Failed to write output file header." << std::endl;
        return -1;
    }

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

    while (av_read_frame(pFormatCtx, &packet) >= 0) {
        if (packet.stream_index == videoIndex) {
            avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);
            if (frameFinished) {
                sws_scale(sws_ctx, (const uint8_t *const *) pFrame->data, pFrame->linesize, 0,
                          pCodecCtx->height, outFrame->data, outFrame->linesize);

                outFrame->pts = av_rescale_q(pFrame->pts, pCodecCtx->time_base, outCodecCtx->time_base);
                outFrame->pkt_dts = av_rescale_q(pFrame->pkt_dts, pCodecCtx->time_base, outCodecCtx->time_base);
                outFrame->pkt_duration = av_rescale_q(pFrame->pkt_duration, pCodecCtx->time_base, outCodecCtx->time_base);

                av_init_packet(&packet);
                packet.data = NULL;
                packet.size = 0;

                if (avcodec_send_frame(outCodecCtx, outFrame) < 0) {
                    std::cout << "Error while sending a frame to the encoder." << std::endl;
                    return -1;
                }

                while (1) {
                    int ret = avcodec_receive_packet(outCodecCtx, &packet);
                    if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
                        break;
                    else if (ret < 0) {
                        std::cout << "Error during encoding." << std::endl;
                        return -1;
                    }

                    av_write_frame(outFormatCtx, &packet);
                    av_packet_unref(&packet);
                }
            }
        }

        av_packet_unref(&packet);
    }

    av_write_trailer(outFormatCtx);

    avcodec_free_context(&pCodecCtx);
    avformat_close_input(&pFormatCtx);
    av_frame_free(&pFrame);
    av_free(buffer);

    return 0;
}

请注意，这只是一个示例代码，并且可能需要进行定制和调整才能适应您的需求。另外，这个代码只是将MP4文件转换为H.264格式视频，如果您需要更高级的功能，例如转换其他格式的视频或更改视频的分辨率、比特率和帧速率等，请查阅FFmpeg文档以获取更多信息。