Learning OpenCV

Matching Contours

Now that we have a pretty good idea of what a contour is and of how to work with contours as objects in OpenCV, we would like to take a moment to understand how to use them for some practical purposes. The most common task associated with contours is matching them in some way with one another. We may have two computed contours that we'd like to compare or a computed contour and some abstract template with which we'd like to compare our contour. We will discuss both of these cases.

Moments

One of the simplest ways to compare two contours is to compute contour moments. This is a good time for a short digression into precisely what a moment is. Loosely speaking, a moment is a gross characteristic of the contour computed by integrating (or summing, if you like) over all of the pixels of the contour. In general, we define the (p, q) moment of a contour as

Here p is the x-order and q is the y-order, whereby order means the power to which the corresponding component is taken in the sum just displayed. The summation is over all of the pixels of the contour boundary (denoted by n in the equation). It then follows immediately that if p and q are both equal to 0, then the m₀₀ moment is actually just the length in pixels of the contour.^[125]

The function that computes these moments for us is

void cvContourMoments(
    CvSeq*     contour,
    CvMoments* moments
)

The first argument is the contour ...