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Lamina
Suppose you are to analyze the image with something 3-dimensional in it, for example, particles. Since we can't see the actual thickness of these particles in this 2D image, we have to make the (mathematical) assumption that the outline of the particle in the image is its shape and its thickness is uniform. That's a lamina. We use then its geometric center (centroid) as the location of the particle. The coordinates of this point appear in the Pixcavator's output table: Location X and Location Y.