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Fig. 2. Features (coefficient of variation, median of gray values) of calibration samples for various degrees of preprocessing. For (a) to (c) the features were extracted from a five by five square filter window, for (d) from the layer shaped window shown in Fig. 1a. (a) Uncorrected image; (b) image corrected for inhomogeneous illumination by flat fielding with a gray cardboard; (c) image (b) corrected also for inhomogeneous reflection by bootstrap method; (d) same as image (c), but the features were evaluated for the layer shaped filter window of Fig. 1a. The large symbols indicate means and the horizontal and vertical bars standard deviations; the contour lines encompass about 75, 50, and 25% of the data of each sand. The standard deviations and contour lines are drawn in white for coarse and in black for the other sands.
