MATLAB: Hough transform is detecting strange lines.

crackshoughimage processingImage Processing Toolboxtile

Hi all, We are trying to find cracks on tiles. We could accomplish that on the expected tile. However, when we tried to do that on a tile that doesn’t have a crack, it detected unexpected lines. We couldn’t figure out what is going on. This is ok:
This is not ok:
Is there a way to erase that detected lines? Does it all depend on Canny value? Here is attached the code used:
J = histeq(B);
figure, imshow(J), title('Histogram'); 
bw = edge(J, 'canny', 0.8);
figure, imshow(bw), title('canny'); 
bw2 = imdilate(bw,strel('line',1,0));
[H,T,R] = hough(
figure;imshow(H,[],'XData',T,'YData',R,...
              'InitialMagnification','fit');
  xlabel('\theta'), ylabel('\rho');
axis on, axis normal, hold on; 
P  = houghpeaks(H,5,'threshold',ceil(0.3*max(H(:))));
x = T(P(:,2)); y = R(P(:,1));
plot(x,y,'s','color','white');  
lines = houghlines(bw2,T,R,P,'FillGap',30,'MinLength',15);
figure, imshow(A), hold on
max_len = 0;
for k = 1:length(lines)
   xy = [lines(k).point1; lines(k).point2];
   plot(xy(:,1),xy(:,2),'LineWidth',2,'Color','White');
   len = norm(lines(k).point1 - lines(k).point2);
   if ( len > max_len)
      max_len = len;
      xy_long = xy;
   end
end

Best Answer

Do the so-called cracks always occur in the same locations? It almost looks like they are some kind of smear/smudge caused by some kind of wheels that are supposed to move the tile along some assembly line but "skidded" on the wet tile causing slippage and a change in texture.
I don't think I'd even use hough at all. Try using a texture filter like stdfilt() and then threshold it and look for blobs that are horizontal.
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