Suggested image preprocessing before applying threshold I am trying to vectorize the contours (skewed rectangle) of the TV in the following original image.



My first step is applying threshold. Tried selection of threshold values based on what skimage has to offer, but non of them managed (for example) to separate correctly the little fawn to the left, or the TV stand from the TV border. For example this is the result using yen's method:



Can you suggest any preprocessing that may help before applying a threshold ?
In your case, I don't think a pre-processing is necessary. You may find the Hough transform useful in your case.

Since your image is big, it can be difficult to have a straight line due to the noise and blur.

This is what I got using a sobel contour detection with a smaller version of your image.

This is maybe a good start to use a line detection algorithm.



A second solution would be to use color segmentation. If you assume that your screen would always have black border you can work something in the LAb or HSV color space.