How to choose the boundaries of the gray zone in the image?

I have an image and I would like to write a program in order to separate the gray part of the image which is a footprint of a tire. I think the algorithm should be similar to fingerprint detection, however, I don't have sufficient knowledge to do so. Can anyone suggest me a solution?

[KSSV]

where is the image?

[Kian Azami]

It is attached to the question

[Image Analyst]

Exactly what does separate mean to you?

[Kian Azami]

I mean to separate the zone which I determined by the red line.

Answer 1

[Akira Agata]

By combining imclose, imopen, and bwconvhull functions, you can determine the target area. Here is an example.

% Read the image and binarize
I = imread('Footprint.jpg');
I = rgb2gray(I);
BW = imbinarize(I);

% Remove noise
se = strel('disk',5);
BW = imclose(BW, se);
BW = imopen(BW, se);

% Remove areas connected to image border
BW = imclearborder(BW,4);

% Generate convex hull image
ROI = bwconvhull(BW);

% Show the result
imshowpair(I,ROI)

[Kian Azami]

Amazing answer, thank you. Now I understand why you are a STAFF.