Would you advise me how to detect all the black lines in an image?
Look for threshold function.
We can help you better if you post an example image describing what you are looking for.
For example, I don’t know if you are looking for straight lines. There is a fast line detector for such purpose. And there also is canny for edge detection.
Well, i’m looking only for straight black (or almost black, i know there is an option to give a color tolerance) lines together with their angle of rotation. A simple example is attached as an image - i want to get the rotation angle of the the 4 black lines.
Fortunately, there are many ways to do this. I’ll only describe one.
There are two problems: detect lines and filter color. You can detect lines and then check their color, or apply a filter (like inRange()) ant then detect lines.
You can detect straight lines segments with fast line detector. With their vertices coordinates you can calculate angles.