Learning OpenCV

More to Do with Contours

When analyzing an image, there are many different things we might want to do with contours. After all, most contours are—or are candidates to be—things that we are interested in identifying or manipulating. The various relevant tasks include characterizing the contours in various ways, simplifying or approximating them, matching them to templates, and so on.

In this section we will examine some of these common tasks and visit the various functions built into OpenCV that will either do these things for us or at least make it easier for us to perform our own tasks.

Polygon Approximations

If we are drawing a contour or are engaged in shape analysis, it is common to approximate a contour representing a polygon with another contour having fewer vertices. There are many different ways to do this; OpenCV offers an implementation of one of them.^[119] The routine cvApproxPoly() is an implementation of this algorithm that will act on a sequence of contours:

CvSeq*  cvApproxPoly(
   const void*   src_seq,
   int           header_size,
   CvMemStorage* storage,
   int           method,
   double        parameter,
   int           recursive  = 0
);

We can pass a list or a tree sequence containing contours to cvApproxPoly(), which will then act on all of the contained contours. The return value of cvApproxPoly() is actually just the first contour, but you can move to the others by using the h_next (and v_next, as appropriate) elements of the returned sequence.

Because cvApproxPoly() needs to create the objects that it will return a ...