# C++ line() function not drawing anything
**URL:** https://forum.opencv.org/t/c-line-function-not-drawing-anything/20668
**Category:** C++
**Tags:** programming
**Created:** [March 28, 2025, 3:22pm UTC](https://forum.opencv.org/t/c-line-function-not-drawing-anything/20668 "2025-03-28T15:22:00Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@lawcata](https://forum.opencv.org/u/lawcata)
#### Post date: [March 28, 2025, 3:22pm UTC](https://forum.opencv.org/t/c-line-function-not-drawing-anything/20668/1 "2025-03-28T15:22:00Z")
> 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()
---
### Author:  [@berak](https://forum.opencv.org/u/berak)
#### Post date: [March 28, 2025, 3:45pm UTC](https://forum.opencv.org/t/c-line-function-not-drawing-anything/20668/2 "2025-03-28T15:45:04Z")
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)
---
### Author:  [@lawcata](https://forum.opencv.org/u/lawcata)
#### Post date: [March 28, 2025, 6:43pm UTC](https://forum.opencv.org/t/c-line-function-not-drawing-anything/20668/3 "2025-03-28T18:43:28Z")
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;
> }
---
### Author:  [@BadTrip](https://forum.opencv.org/u/BadTrip)
#### Post date: [March 28, 2025, 7:24pm UTC](https://forum.opencv.org/t/c-line-function-not-drawing-anything/20668/4 "2025-03-28T19:24:42Z")
the code seems to be correct and I can see a blue line on a black background if I replace
```auto
imwrite(“image.jpg”, image);
waitKey(1);
```
with
```auto
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.
---
### Author:  [@crackwitz](https://forum.opencv.org/u/crackwitz)
#### Post date: [March 29, 2025, 11:32am UTC](https://forum.opencv.org/t/c-line-function-not-drawing-anything/20668/5 "2025-03-29T11:32:26Z")
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:
- [Online monitor test - monitest](https://monitest.ru/en/)
- [Colour test screen](https://www.ledr.com/colours/multi.htm)
- [EIZO monitors for offices, photos & design, medicine, gaming and industry](https://www.eizo.be/monitor-test/)
- [LCD monitor test images](http://www.lagom.nl/lcd-test/) (might need a minute to become available after first request)