Abstract: Shape context is a popular algorithm of image shape feature extraction and matching,
traditional shape context algorithm lacks symmetry invariance, which means it can not match two
similar but symmetrical shapes. To overcome this shortcoming, this article proposed an improved
shape context algorithm with symmetry invariance. The method defines the gradient direction of a
sample point as its 0 rad in log polar coordinates when computing the angle relations in shape context,
then compare the number of sample points in 0 rad to π rad and π rad to 2π rad, and adjust the angel
increasing direction in the coordinate according to the results. In the process of iterative shape
warping and shape contexts calculation, the symmetry invariance descriptors are only applied in the
first iteration to obtain a more stable matching result. Experimental results show that the proposed
algorithm can effectively establish match between two similar and symmetrical shapes, and therefor
improve the retrieval precision.

Key words: image matching, shape context, symmetry invariance, matching stability