C++ line() function not drawing anything

C++
1

cv::Mat annotated_frame = m_detector.annotate();
cv::line(annotated_frame, Point(0,0), Point(600,600), Scalar(255,0,0), 5);
cv::imshow(“window”, annotated_frame);

The above code example does not actually draw any line to my Mat. I’ve also tested this using different line colours, as well as initializing a simple black matrix and trying to draw on that - still nothing draws. I see the same behaviour using rectangle()

2

i do not think, your drawing code is the problem,
you only see an updated image from imshow(), if you add a ‘cv::waitKey(some_millis)’

alternatively, try to imwrite it to disk (just to check)

3

I’ve done it with waitKey as well, and same behaviour.
imwrite to disk also produced no drawn lines (plain black image in example below).

int main()
{
Mat image(500,500,CV_8UC3, Scalar(0,0,0));

if(!image.data)
{
cout << “Error no data in image” << endl;
return 0;
}

Point p1(0,0), p2(400,400);
int thickness = 2;
Scalar colour(255,0,0);

line(image, p1, p2, colour, thickness, LINE_8);
imwrite(“image.jpg”, image);
waitKey(1);

return 0;
}

4

the code seems to be correct and I can see a blue line on a black background if I replace

 imwrite(“image.jpg”, image);
 waitKey(1);

with

imshow("x",image);
waitKey(0); //wait until key press

you can also print the matrix to console with cout << image; to check if all elements are still zero or if some are set to 255. I suggest to reduce the image size before you do that.

1 Like
5

is your computer screen misadjusted so that it can’t show you blue?

do you have a blue light filter app?

can you see blue things with your eyes?

some monitor test pages: