How to overlay text on image when working with cv::Mat type

Question

I am using opencv 2.1. In my code I have a few images stored as Mat objects initialized like this:

Mat img1 = imread(\"img/stuff.pgm\", CV_LOAD_IMAGE_GRAYSCA         


        

Answer 1

For C++ basic use:

cv::putText(yourImageMat, 
            "Here is some text",
            cv::Point(5,5), // Coordinates
            cv::FONT_HERSHEY_COMPLEX_SMALL, // Font
            1.0, // Scale. 2.0 = 2x bigger
            cv::Scalar(255,255,255), // BGR Color
            1, // Line Thickness (Optional)
            cv::CV_AA); // Anti-alias (Optional)

See putText() in OpenCV docs.

Answer 2

If you are using OpenCV 3.x or 4.0, you have to replace CV_AA by LINE_AA. So, the complete call would be:

cv::putText(yourImageMat, 
            "Text to add",
            cv::Point(5,5), // Coordinates
            cv::FONT_HERSHEY_COMPLEX_SMALL, // Font
            1.0, // Scale. 2.0 = 2x bigger
            cv::Scalar(255,255,255), // BGR Color
            1, // Line Thickness (Optional)
            cv::LINE_AA); // Anti-alias (Optional)

This was posted in this forum.

Answer 3

I was looking at the wrong place. I found the answer in the newer OpenCV documentation for cpp. There is a new function called putText() that accepts cv::Mat objects. So I tried this line and it works:

putText(result, "Differencing the two images.", cvPoint(30,30), 
    FONT_HERSHEY_COMPLEX_SMALL, 0.8, cvScalar(200,200,250), 1, CV_AA);

Hope this helps someone.

Answer 4

putText(result, "Differencing the two images.", cvPoint(30,30), 
    FONT_HERSHEY_COMPLEX_SMALL, 0.8, cvScalar(200,200,250), 1, CV_AA);

In the above line "result" should be a cvArr* or an IplImage*. but from the code provided here, I guess you are passing a cv::Mat object. So, you either need to convert it using cvarrToMat() or pass &result instead of result.

Hope it helps

Answer 5

You can also do the following to print text and variables.

    std::ostringstream str;
    str << "Here is some text:" << myVariable;
    cv::putText(image, cv::Point(10,10), str.str(), CV_FONT_HERSHEY_PLAIN, CV_RGB(0,0,250));

Answer 6

putText(img1, "TextString123", cvPoint(50,200), FONT_HERSHEY_SCRIPT_SIMPLEX, 2.5, cvScalar(255,0,0,255), 3, CV_AA);

You can find more information here: http://docs.opencv.org/2.4.9/modules/core/doc/drawing_functions.html

The main diference between this answer and the answers from above is the value of the 7-th parameter, the thickness level. With thickness==1 this function have not worked for me.