How to detect this curve in the photo?

Question

Image processing

It is not possible to detect this curve normally. in this image, it was not entirely successful to get the curve (some things could not be restored) using Image Segmentation Using Color Spaces in OpenCV + ... . On another image, all the color spaces parameters are already needed by others, so this method is not successful (since the finished program should automatically extract the curve and do further transformations).
I have more than 30 similar photos, and each one has a different color and clarity, unfortunately. Tried many things, but nothing worked. The main goal: to get this curve automatically, and then complete it (I have already done the completion algorithm and it works).