edge corners are not sharp with canny filter

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Hi,

I am trying to detect edges from an image using Canny filter. I have been able to detect the images but the corners are not very sharp (highlighted in red circles). Surprisingly, the first corner is sharp and not burned out compared to other corners. Why this is happening and how it can be avoided ??

Please have a look at the attached image.

My code:

edges = cv2.Canny(image,0,255)

I have not applied any blurring as it makes the corners worse. Moreover, the change of min and max values for Canny also does not make any difference