difference between CV_RETR_LIST,CV_RETR_TREE,CV_RETR_EXTERNAL?

Question

I am using cvFindContour function of opencv and in it there is a parameter RETR_TYPE means retrivel type,hence I am not getting what is the difference between CV_RETR_LIST

Answer 1

From imgproc.cpp:

//! mode of the contour retrieval algorithm
enum RetrievalModes {
    /** retrieves only the extreme outer contours. It sets `hierarchy[i][2]=hierarchy[i][3]=-1` for
    all the contours. */
    RETR_EXTERNAL  = 0,
    /** retrieves all of the contours without establishing any hierarchical relationships. */
    RETR_LIST      = 1,
    /** retrieves all of the contours and organizes them into a two-level hierarchy. At the top
    level, there are external boundaries of the components. At the second level, there are
    boundaries of the holes. If there is another contour inside a hole of a connected component, it
    is still put at the top level. */
    RETR_CCOMP     = 2,
    /** retrieves all of the contours and reconstructs a full hierarchy of nested contours.*/
    RETR_TREE      = 3,
    RETR_FLOODFILL = 4 //!<
};

OpenCV 2.4.13