Editing Geometric hashing (section)

=== Finding mirrored pattern ===
It seems that this method is only capable of handling scaling, translation, and rotation.  However, the input image may contain the object in mirror transform.  Therefore, geometric hashing should be able to find the object, too.  There are two ways to detect mirrored objects.

# For the vector graph, make the left side positive, and the right side negative. Multiplying the x position by -1 will give the same result.
# Use 3 points for the basis. This allows detecting mirror images (or objects).  Actually, using 3 points for the basis is another approach for geometric hashing.