how to detect middle circle in IOS OpenCV

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hii thx for your support..actually now i have square on my image and approxPolyDP method gave me that square..but my question is how can i get all circles who are i nside my square..?

If the square has sides parallel to the borders of the image you could extract a region of interest of the original image, using for example: using namespace cv;

Mat img_orginal; Rect region = rect(x,y,w,h); Mat sub_img(region);

and then use hough_circle to find the circles inside that region. x,y : Coordinates of the top-left corner w,h : Rectangle width and height

thx...how can i fit octagon to square... i means i detected my biggest Contours(octagon)..but how can i apply warpperspective on that contour?

If I understood correctly your question, you extract contours from an image and the contours correspond to a warped octagon, and you want to unwarp it. To unwrap you could see this tutorial:

http://docs.opencv.org/doc/tutorials/imgproc/imgtrans/warp_affine/warp_affine.html#warp-affine

I don't know if there is a specific function to fit octagon with a square, maybe you could try to see convex hull:

http://docs.opencv.org/trunk/doc/tutorials/imgproc/shapedescriptors/hull/hull.html

however I think that if you have the sorted vertexes of the octagon you can use three of them to unwrap it.