opencv在图形上绘制三维物体

OpenCV可以用于绘制三维物体，可以使用以下步骤：

1. 使用cv::Mat构建3D场景，定义坐标系和3D物体的位置和大小。

2. 使用cv::projectPoints函数将3D物体投影到2D图像上，生成2D点集。

3. 使用cv::polylines函数将2D点集连成三维图形。

下面是一个简单的示例代码来绘制一个立方体：

#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>

int main()
{
    // 创建一个3D场景
    cv::Mat scene = cv::Mat::zeros(3, 8, CV_64FC1);
    scene.at<double>(0, 0) = -1; scene.at<double>(1, 0) = -1; scene.at<double>(2, 0) = -1;
    scene.at<double>(0, 1) = 1; scene.at<double>(1, 1) = -1; scene.at<double>(2, 1) = -1;
    scene.at<double>(0, 2) = 1; scene.at<double>(1, 2) = 1; scene.at<double>(2, 2) = -1;
    scene.at<double>(0, 3) = -1; scene.at<double>(1, 3) = 1; scene.at<double>(2, 3) = -1;
    scene.at<double>(0, 4) = -1; scene.at<double>(1, 4) = -1; scene.at<double>(2, 4) = 1;
    scene.at<double>(0, 5) = 1; scene.at<double>(1, 5) = -1; scene.at<double>(2, 5) = 1;
    scene.at<double>(0, 6) = 1; scene.at<double>(1, 6) = 1; scene.at<double>(2, 6) = 1;
    scene.at<double>(0, 7) = -1; scene.at<double>(1, 7) = 1; scene.at<double>(2, 7) = 1;

    // 定义内参矩阵
    cv::Mat K = cv::Mat::eye(3, 3, CV_64FC1);
    K.at<double>(0, 0) = 100;
    K.at<double>(1, 1) = 100;
    K.at<double>(0, 2) = 320;
    K.at<double>(1, 2) = 240;

    // 定义外参矩阵
    cv::Mat R = cv::Mat::eye(3, 3, CV_64FC1);
    cv::Mat t = cv::Mat::zeros(3, 1, CV_64FC1);

    // 投影到2D图像上
    std::vector<cv::Point2d> points;
    cv::projectPoints(scene, R, t, K, cv::Mat(), points);

    // 绘制三维图形
    cv::Mat img = cv::Mat::zeros(480, 640, CV_8UC3);
    std::vector<std::vector<cv::Point>> contours = {{points[0], points[1], points[2], points[3]},
                                                    {points[4], points[5], points[6], points[7]},
                                                    {points[0], points[4], points[7], points[3]},
                                                    {points[1], points[5], points[6], points[2]},
                                                    {points[0], points[1], points[5], points[4]},
                                                    {points[3], points[2], points[6], points[7]}};
    cv::polylines(img, contours, true, cv::Scalar(0, 255, 0), 2);

    // 显示图像
    cv::imshow("3D Object", img);
    cv::waitKey(0);

    return 0;
}

这段代码可以生成一个立方体的图像，并在窗口中显示。你可以根据需要修改场景以及内外参矩阵来绘制其他形状的三维物体。