error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor'

This error message is related to the OpenCV library and occurs when the input image or matrix is empty. The assertion failed because the function cvtColor() expects a non-empty input image or matrix.

To fix this error, you should check if the input image or matrix is empty before passing it to the cvtColor() function. You can use the empty() function provided by OpenCV to check if the image or matrix is empty.

Here is an example of how to check if an image is empty before converting it to grayscale using cvtColor():

cv::Mat image = cv::imread("image.jpg");
if (!image.empty()) {
    cv::Mat grayImage;
    cv::cvtColor(image, grayImage, cv::COLOR_BGR2GRAY);
    // do something with the grayscale image
} else {
    std::cout << "Error: input image is empty!" << std::endl;
}

In this example, we first read an image from a file and check if it is empty using the empty() function. If the image is not empty, we convert it to grayscale using cvtColor(). If the image is empty, we print an error message.