how can i find dent in structure?

I = imread('diodo.jpg');

I = rgb2gray(I);

% f=figure,imshow(I); % g=figure,imshow(J);

hy = fspecial('sobel'); hx = hy'; Iy = imfilter(double(I), hy, 'replicate'); Ix = imfilter(double(I), hx, 'replicate');

gradmag = sqrt(Ix.^2 + Iy.^2);

K=figure,imshow(gradmag,[]); set(K, 'visible','on'); filename = 'temp_file.jpg' saveas(K, filename)

i1 = imread(filename) delete(filename)

[x, y, rgb] = ind2sub([size(i1,1) size(i1,2) size(i1,3)], find(i1 ~= 255)); A = i1(min(x):max(x)-1,min(y):max(y)-1,:);

A = rgb2gray(A)

I = edge(A,'sobel')

%934 742 I = imcrop(I,[10 7 914 728]); I = imresize(I, [2000 2500])

f=figure,imshow(I); set(f, 'visible','on');

imwrite(I, filename) bw = imread(filename) delete(filename) bw = im2bw(bw, graythresh(bw)) bw = bwareaopen(bw,65); set(f, 'visible','on'); f, imshow(bw)

But i don't need to find depth of the image just i need to find dent in the image

[vijendra sn]

i have used this code for dent detection on structure and for images i can find dent but not for all

i have attached image which i cannot find dent (DSC_img0976.jpg)

but i find dent on these images (img18.jpg) pls help on this