Re: Finding a square pattern in an image

Hello Punit,

I studied a similar problem some times ago: the fast detection of
corners for a industrial quality control. Before resuming this
algorithm, I want to give you my way to construct image analysis
solutions.

At the start, you know the original image(s) and the required
result(s). The original image contains a lot of information: color,
intensity, other objects, noise, ... The result includes only a very
reduced information: is the quality of an object acceptable, what is
the size of an object, etc.

Thus, your algorithm should advance to the solution step by step. Every
step is justified by deleting not needed information (noise, color
information, ...). The result of each step must contain however all
information of your result, and should be, of course, more easy to
handle.

For your application, this could mean (and you did the first steps very
well): color and intensity information are not needed, because you
search a form. Calculation of a gray-level image representing the
intensity and the consecutive threshold are, if they are applicable and
stable, good steps.

At this point, your solution (and the solutions proposed by Steward)
begin to scan horizontal/vertical lines (with all the difficulties of
interpretation: in fact, all the proposed methods will fail if you
image is a little bit noisy...). I would go one step further on: detect
the contour points of your object. The result is a list of the
coordinates of all these points. ((x0 y0) (x1 y1) ...). There are very
fast methods to perform this step. The greatest advantage: an enormous
reduction of information, the rest of the algorithm will handle only
some hundreds of coordinates and will be very fast.

Last step: finding corners from the contour. Corners are points where
the contour changes his direction. For my application, I used a
hierarchical, noise stable approach. Other ways could be linear
filtering of the contour and finding points with high curvature, etc.
For further information about this step, you could have a look at the
demo "Contour points" of our image analysis system.

Best regards,

Tilman JOCHEMS
http://mfj.chez.tiscali.fr/html/index_en.html

.

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