You can solve the issue by the following,
- Using hough circle detect the circle.
- Draw the detected circle in new Mat(single channel) using OpenCV circle, here you need to set thickness = -1 and Scalar=255, which fill the circle with your scalar value.
- Now scan your source image as well as the newly created Mat, and access the source image pixel only when the newly created Mat has the pixel value 255.
And you can see OpenCV Doc about how to scan image here
