Image Processing: How to detect a quadrialteral signboard in image?

Question

Hi guys,

How can I detect a quadrilateral signboard from an image captured using a mobile phone? How can i detect shapes like rectangle? rounded rectangle (rounded corners instead of shape corners)?

I am using opencv.wrapper but i am new to it.

Thanks.

Here is the sample:

Due to the noise and many lines there, i am not able to determine which is the boundary line of the signboard. Sometimes I can find the boundary of the lines after hough transform. I am trapped....in this kind of scenario...

This is the 2 raw pictures taken from mobile phone camera

I need ya advice to see how i can process the image to get the signboard out?

Thank you very much

Answer 1

Keywords: opencv, hough transform

Duplicates:

http://stackoverflow.com/questions/1817442/

http://stackoverflow.com/questions/2068013/

Answer 2

Another approach that you can take that is more robust to noises in this situation is to generate a curve of the mean gray-level of the image along the x-axis and along the y-axis. That is, compute the mean gray level for each line/column in the image.

If, for example, the signboard (or its borders) is lighter than its surroundings (which is the case for all the examples shown in the question) you will have two peaks in your x-axis curve (for the left and right and borders) and two peaks in you y-axis curve (for the top and bottom borders). Using a border detection technique for a 1-dimension signal (maybe a high-pass filter) you can deduce the corners coordinates of the signboard.

I have seen this approach being used to to detect license plates and also for face recognition (the nose tends to be the brighter part of the face, so it generates a peak in the x-axis and y-axis curves).