MATLAB Answers


detect square in image

Asked by Abdul
on 29 Dec 2011
Latest activity Edited by John Kelly
on 14 Nov 2013 i want to detect square shape only in image. how can i do this using matlab?



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3 Answers

Answer by Chandra Kurniawan
on 29 Dec 2011
 Accepted answer


Is your goal only to detect square in this picture?

If so, you don't need to do shape recognition, etc.

You just need to perform some morphologichal operations only.

I = imread('3617ca70.png');
Ibw = ~im2bw(I,graythresh(I));
Ifill = imfill(Ibw,'holes');
Iarea = bwareaopen(Ifill,100);
Ifinal = bwlabel(Iarea);
stat = regionprops(Ifinal,'boundingbox');
imshow(I); hold on;
for cnt = 1 : numel(stat)
    bb = stat(cnt).BoundingBox;

And the result :

Is this what you need?

  1 Comment

on 3 Jan 2012

yes, thanks a lot. how about detecting circle in image?

Answer by Sean de Wolski
on 29 Dec 2011

Those squares are very well defined.

  • Set a less than threshold to make the dark colors true and everything else false. ( BW = I<thresh )
  • Perform a connected components analysis ( doc bwconncomp)
  • Then regionprops with EulerNumber specified.
  • Apply a threshold to the regions matching the EulerNumber ( idx = [stats(:).EulerNumber] == x ) of a sample square.
  • Build a new matrix of false. ( BWnew = false(CC.ImageSize))
  • Set the indices in the connected components analysis corresponding to the matching EulerNumber to true. ( BWnew(vertcat(CC.PixelIdxList{idx})) = true)

In theory you'll have a mask of the boxes.


Answer by Naz
on 29 Dec 2011
Edited by John Kelly
on 14 Nov 2013

I just thought of an alternative idea. Instead of detecting a square, you should register the image (filled form) of interest with the raw image. Then, since you know the coordinates of each square, you can just check the pixels at those coordinates for the white/black values.


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