Fig. 2. A schematic view of stereo-DPIV image processing and calculation methods using data from a trout 21.5 cm in total body length (BL) swimming at 1.2 BL s^-1 with a freestream velocity in the x direction, U, of 25.8 cm s^-1. Images of the three-dimensional calibration grid (A) recorded from the two cameras show distortion of the two-dimensional views caused by the camera arrangement and Scheimpflüg configuration (Fig. 1). The uniform, symmetrical calibration grid appears slanted, with a decreased vertical distance between rows on the inner sides of both images. Image alignment and distortion are corrected using polynomial equation mapping functions created using a least-squares method and the calibration grid points. Each of the two cameras (Fig. 1) collects two-dimensional images of the wake from which two-dimensional velocity vectors (dimensions scaled in pixels, B) are calculated. Note that freestream U has not been subtracted from these calculations; the high value of U results in a downstream trajectory for both sets of two-dimensional vectors (unlike the diagram in Fig. 1), and that vector density was decreased for viewing clarity. The orientation of the trout caudal fin relative to the light sheet is depicted as a small icon below B. The two-dimensional vectors are used to calculate three-dimensional flow fields (scaled in m s^-1; C,D). Vector color in C and D indicates the magnitude of W (the lateral or z velocity component); note that free-stream flow has not been subtracted. Orange boxes connected by broken lines represent the alignment between areas of the calibration images (A), two-dimensional velocity vectors calculated from DPIV images scaled with the calibration images (B) and the final three-dimensional velocity vectors (viewed in the parasagittal plane in C, and viewed in three dimensions in D by rotating the parasagittal plane counterclockwise). The vector calculated for the image area identified by the orange boxes in A, B and C is identified by a black arrow in D, with the magnitude of its U, V and W components given. Rotating C counterclockwise shows strong lateral jet flow (emphasized here by the small scale of the z axis for ease of viewing) that alternates to the right (blue vectors) and left (red vectors) sides of the fish.

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