Table 2.

Matching and quality indicators used for ranking

EFDIs Match with Template	Matching between elliptic Fourier descriptor invariants (EFDIs) of object and template shape [51,52].
Hu Match for EFDIs Template	Matching between the Hu invariants [55] of the object and the template which matches best according to EFDIs.
Optimization Method	Morphological Operator used to improve the object contour. If an optimization was applied to derive a shape, its ranking is degraded, because the resulting outline might be inaccurate.
Standard Deviation of inner 50%	Standard deviation of the gray level distribution within the object boundaries. Only the inner 50% of the area are analyzed. This way, diatom valves, normally containing striae/costae/areolae, can be distinguished from empty girdle bands which can produce good outline matching but have a homogenous interior.
Width/Height Ratio	Ratio between object width and height. Usually objects of a certain type have a ratio within a certain range.
Contour Smoothness	Estimation of the object contour smoothness. The actual object outline usually is quite smooth, especially for diatom valves, whilst contours distorted by segmentation inaccuracies or failures usually are rough. The ratio between the outline perimeter and that of the outline smoothed by a Gaussian filter provides information about the contour smoothness.
Formfactor	Heuristic descriptor "formfactor" [56]