Edge detetction in iOS application

I have a requirement to implement a functionality to draw edge of a object in a given image. So, when the user taps on a object in the Image ,it should draw its edge. For Eg: if I have a Image which has a circle,a triangle and a rectangle in it, and I tap on circle it should draw edge of that circle.

I believe it requires some Edge detection algorithms to be implemented or it can be done using some CoreImage Filters. I have noticed a f开发者_JAVA技巧ilter "CIEdge"/ "CIEdgeWork" in CoreImage Framework which filters out the edges of the whole Image. This filter could help me to a certain extent.But,I have found out that there is no such filter available in iOS sdk(also taking in account the iOS 5.0 sdk).

Take a look at OpenCV project (e.g. this tutorial might be helpful).