c++ Mat 转 h264

要将 OpenCV 的 Mat 类型转换成 H.264 格式的视频，需要使用 FFmpeg 库。以下是一个简单的示例代码，可以将 Mat 类型的视频转换成 H.264 格式的视频：

#include <iostream>
#include <opencv2/opencv.hpp>
#include <libavcodec/avcodec.h>
#include <libavutil/opt.h>
#include <libavutil/imgutils.h>
#include <libavutil/mathematics.h>
#include <libswscale/swscale.h>

using namespace std;
using namespace cv;

int main(int argc, char** argv)
{
    // OpenCV video capture
    VideoCapture cap("video.mp4");
    if (!cap.isOpened())
    {
        cerr << "Failed to open video file!" << endl;
        return -1;
    }

    // FFmpeg encoder
    avcodec_register_all();
    AVCodec* codec = avcodec_find_encoder_by_name("libx264");
    if (!codec)
    {
        cerr << "Codec not found!" << endl;
        return -1;
    }

    AVCodecContext* codecCtx = avcodec_alloc_context3(codec);
    if (!codecCtx)
    {
        cerr << "Failed to allocate codec context!" << endl;
        return -1;
    }

    codecCtx->bit_rate = 400000;
    codecCtx->width = cap.get(CAP_PROP_FRAME_WIDTH);
    codecCtx->height = cap.get(CAP_PROP_FRAME_HEIGHT);
    codecCtx->time_base = (AVRational){1, 25};
    codecCtx->gop_size = 10;
    codecCtx->max_b_frames = 1;
    codecCtx->pix_fmt = AV_PIX_FMT_YUV420P;

    int ret = avcodec_open2(codecCtx, codec, NULL);
    if (ret < 0)
    {
        cerr << "Failed to open codec: " << av_err2str(ret) << endl;
        return -1;
    }

    // FFmpeg video output
    AVFormatContext* fmtCtx = NULL;
    ret = avformat_alloc_output_context2(&fmtCtx, NULL, NULL, "output.mp4");
    if (ret < 0)
    {
        cerr << "Failed to allocate output context: " << av_err2str(ret) << endl;
        return -1;
    }

    AVOutputFormat* fmt = fmtCtx->oformat;
    if (fmt->video_codec == AV_CODEC_ID_NONE)
    {
        cerr << "No video codec found!" << endl;
        return -1;
    }

    AVStream* stream = avformat_new_stream(fmtCtx, NULL);
    if (!stream)
    {
        cerr << "Failed to allocate stream!" << endl;
        return -1;
    }

    AVCodecContext* streamCtx = avcodec_alloc_context3(codec);
    if (!streamCtx)
    {
        cerr << "Failed to allocate codec context!" << endl;
        return -1;
    }

    ret = avcodec_parameters_from_context(stream->codecpar, streamCtx);
    if (ret < 0)
    {
        cerr << "Failed to copy codec parameters: " << av_err2str(ret) << endl;
        return -1;
    }

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

    // Open output file
    ret = avio_open(&fmtCtx->pb, "output.mp4", AVIO_FLAG_WRITE);
    if (ret < 0)
    {
        cerr << "Failed to open output file: " << av_err2str(ret) << endl;
        return -1;
    }

    // Write header
    ret = avformat_write_header(fmtCtx, NULL);
    if (ret < 0)
    {
        cerr << "Failed to write header: " << av_err2str(ret) << endl;
        return -1;
    }

    SwsContext* swsCtx = sws_getContext(codecCtx->width, codecCtx->height, AV_PIX_FMT_BGR24,
        codecCtx->width, codecCtx->height, codecCtx->pix_fmt, SWS_BICUBIC, NULL, NULL, NULL);

    AVFrame* frame = av_frame_alloc();
    frame->format = codecCtx->pix_fmt;
    frame->width = codecCtx->width;
    frame->height = codecCtx->height;
    ret = av_frame_get_buffer(frame, 0);
    if (ret < 0)
    {
        cerr << "Failed to allocate frame buffer: " << av_err2str(ret) << endl;
        return -1;
    }

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

    while (true)
    {
        Mat frameMat;
        cap >> frameMat;
        if (frameMat.empty())
            break;

        // Convert OpenCV Mat to FFmpeg AVFrame
        AVFrame* inputFrame = av_frame_alloc();
        inputFrame->format = AV_PIX_FMT_BGR24;
        inputFrame->width = frameMat.cols;
        inputFrame->height = frameMat.rows;
        ret = av_frame_get_buffer(inputFrame, 0);
        if (ret < 0)
        {
            cerr << "Failed to allocate frame buffer: " << av_err2str(ret) << endl;
            return -1;
        }

        uint8_t* inputData[AV_NUM_DATA_POINTERS] = {0};
        inputData[0] = (uint8_t*)frameMat.data;
        int inputLinesize[AV_NUM_DATA_POINTERS] = {0};
        inputLinesize[0] = frameMat.step;
        sws_scale(swsCtx, inputData, inputLinesize, 0, frameMat.rows, inputFrame->data, inputFrame->linesize);

        // Encode frame
        ret = avcodec_send_frame(codecCtx, inputFrame);
        if (ret < 0)
        {
            cerr << "Failed to send frame: " << av_err2str(ret) << endl;
            return -1;
        }

        while (ret >= 0)
        {
            ret = avcodec_receive_packet(codecCtx, &packet);
            if (ret < 0 && ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)
            {
                cerr << "Failed to receive packet: " << av_err2str(ret) << endl;
                return -1;
            }

            if (ret >= 0)
            {
                av_packet_rescale_ts(&packet, codecCtx->time_base, stream->time_base);
                packet.stream_index = stream->index;

                ret = av_interleaved_write_frame(fmtCtx, &packet);
                if (ret < 0)
                {
                    cerr << "Failed to write packet: " << av_err2str(ret) << endl;
                    return -1;
                }

                av_packet_unref(&packet);
            }
        }

        av_frame_unref(inputFrame);
    }

    // Flush encoder
    ret = avcodec_send_frame(codecCtx, NULL);
    if (ret < 0)
    {
        cerr << "Failed to send frame: " << av_err2str(ret) << endl;
        return -1;
    }

    while (ret >= 0)
    {
        ret = avcodec_receive_packet(codecCtx, &packet);
        if (ret < 0 && ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)
        {
            cerr << "Failed to receive packet: " << av_err2str(ret) << endl;
            return -1;
        }

        if (ret >= 0)
        {
            av_packet_rescale_ts(&packet, codecCtx->time_base, stream->time_base);
            packet.stream_index = stream->index;

            ret = av_interleaved_write_frame(fmtCtx, &packet);
            if (ret < 0)
            {
                cerr << "Failed to write packet: " << av_err2str(ret) << endl;
                return -1;
            }

            av_packet_unref(&packet);
        }
    }

    // Write trailer
    av_write_trailer(fmtCtx);

    // Cleanup
    avcodec_free_context(&codecCtx);
    avcodec_free_context(&streamCtx);
    av_frame_free(&frame);
    avformat_free_context(fmtCtx);

    return 0;
}

这段代码使用 OpenCV 从视频文件中读取每一帧图像，然后使用 FFmpeg 编码器将其转换成 H.264 格式，并写入输出文件。要使用此代码，需要先安装 FFmpeg 库并将其头文件和库文件路径添加到编译器的 include 和 lib 路径中。