（28） Shape matching based on contour features

1、 Chain code histogram

Chain code histograms classify objects that look similar to human eyes . Direction chain code is a typical chain code representation used to represent the outline of an object . A discrete curve can be defined as z2 A limited number of 8 Connected . therefore , A digitized binary curve can be represented by direction chain code, which is connected by two adjacent pixels 8 Possible direction values . A curve is discretized by the mesh to form n Chain code direction , Finally, this curve chain code can be expressed as , Each chain points to 8 One of the directions ,i Is the index value of pixels ,ai By pixels i Point to pixels i+1 Direction chain code .

  (a)  Direction of coding             (b) Simple object shape        (c) The chain code of the shape indicates        （d） Chain code histogram

2、 Paired geometric histograms

Pairwise geometric histograms describe the target contour through relative angle and relative position features , And use the relationship histogram statistics to index the shape of this pair of geometric features . Using this coding method requires certain preconditions , That is, an irregular object can be approximated as a geometric polygon . This coding method can well describe a polygonal object . Approximate irregular shapes to polygons , And define it as the set of edge points . In this way, the contour edges contained in it can be calculated by continuous edge points .

3、 Combination of simple shape descriptors

Concavity and convexity ： The ratio of the circumference of the convex hull of the contour to the circumference of the original contour .

Spindle ： The orthogonal axis passing through the centroid of the object , The proportion between the principal axes can be calculated from the covariance matrix of the object contour .

Compactness ： The ratio of the area of the object to the circumference of a square with equal area , It can also be a garden .

Difference ： Expressed as the proportional mean square error compared with the template .