获取顶部和底部白色像素的坐标以绘制一条线

Question

我在位图扩展中有以下二进制图像：

现在我想：

获取上下白色像素点坐标（蓝色标记）
画一条线（黄线）与这两个坐标相交（类似于下图）

我试过at函数，image.at<uchar>(i,j)但没有成功。如果有人能帮助我，我将不胜感激。提前致谢！

Mat image = imread("Before.bmp");
int i=1;
imshow("Before", image);

vector<Point> locations;   // output, locations of non-zero pixels
cv::findNonZero(image, locations);
Point pnt = locations[i]; /Assertion error

for (int i = 0; i < image.rows; i++) {
    for (int j = 0; j < image.cols; j++) {
        if (image.at<uchar>(i,j) == 255  ) {
            cout << i << ", " << j << endl; //give wrong coordinate
        }
    }
}

//imshow("black", image);
//imwrite("blackie.bmp", image);

waitKey(0);
return(0);

Answer 1

您可以使用 cv::findNonZero 来查找任何 non-zero（即非黑色）像素的坐标。根据文档，这个函数要求输入是单通道图像，所以需要先转成灰度。

在下面的示例中，我假设白线从顶行一直延伸到底行，并且在图像的顶部和底部边缘恰好有两条白线（就像你的形象）。您可能希望将其扩展为更灵活。

示例代码

#include <opencv2/opencv.hpp>

int main(int argc, char *argv[])
{
    cv::Mat image(cv::imread("foo.png"));

    cv::Mat gray;
    cv::cvtColor(image, gray, cv::COLOR_BGR2GRAY);

    std::vector<cv::Point> locations[2];

    // Top points
    cv::findNonZero(gray.row(0), locations[0]);

    // Bottom points (need to offset Y since we take a single row ROI)
    cv::findNonZero(gray.row(image.rows - 1), locations[1]);
    for (auto& p : locations[1]) {
        p.y += image.rows - 1;
    }

    // Validate our assumption of having exactly two points on both top as well as bottom row
    if ((locations[0].size() != 2) && (locations[0].size() != 2)) {
        std::cerr << "Unexpected input.\n";
        return -1;
    }

    for (int i(0); i < 2; ++i) {
        cv::line(image, locations[0][i], locations[1][i], cv::Scalar(0, 255, 255));
    }
    cv::imwrite("foo_out.png", image);
}

获取顶部和底部白色像素的坐标以绘制一条线

Obtaining coordinate of the top and bottom white pixels in order to draw a line

c++

opencv

bitmap

coordinate

示例代码

示例输出