Because of the lack of a reference point on the contour, it is necessary to resolve the rotation-variant issue in order to use the fast and accurate TADA approach for oyster shape grading. For a contour with 50 data points, it could be rotated to cause 50 possible shifts in turn angle function. In order to compare two turn angle functions to determine if they have similar shape characteristics and belong to the same shape category, two turn angle functions must be ‘‘aligned” as close as possible to calculate the difference between them as a similarity measure.