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

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: alt text http://www.freeimagehosting.net/uploads/b03442fd36.png

alt text http://www.freeimagehosting.net/uploads/e6b36040e8.png

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

alt text http://www.freeimagehosting.net/uploads/6dbd613edf.jpg alt text http://www.freeimagehosting.net/uploads/720da20080.jpg

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

Thank you very much