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A Prototype Optical Tracking System Investigation and Development - page 120 / 170

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Results and Discussion

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Figure 8.5 There are two angles that could have an effect on the standard deviation of the centroids from a marker. The first is the angle between the camera’s principal axis and a line that extends from the camera’s principal point to the marker (θ ). The second is the angle between this line and the marker’s principal axis (θ2). The marker’s principal axis is shown by the top green arrow and the camera’s principal axis is shown using the bottom green arrow.

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Standard deviation Y (pixels)

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Figure 8.6 These graphs plot the centroid variation versus X position. a) shows the standard deviation in X and b) shows the standard deviation in Y.

marker and the number of visible marker pixels. For the analysis all measured marker i n t e n s i t i e s w e r e a b o v e 8 0 m e a n i n g t h a t t h e c h a n g e i n i n t e n s i t y d u e t o a c h a n g e i n θ 2 h a s negligible affect on the results (see Section 8.2.2). a

Figures 8.6a, b show plots of centroid standard deviations in X and Y versus X coordinate. The graph appears to have a flatter region in the middle of the FOV and a region at either side of the FOV where the standard deviation increases. In these plots, data were used from every marker in the FOV. Quadratic curves were fitted to the plots and vertical lines were added showing where the fitted standard deviation curve is 25% and 50% higher than the minimum.

The same data set was analysed but this time markers from approximately the same Y

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