To get actual velocity data, it is first necessary to calibrate the images, transforming pixel displacements into real distances, using some reference object. For that purpose, open the movie at the beginning (or the corresponding image 'movie_1.png'). We can use the cylinder as the reference object.
Push the button 'select' on the upper right, and click the top of the cylinder with the mouse. You can first zoom the image for a good precision. The pixel numbers of the selected point, ix and iy appear in the upper right text window, followed by the x and y coordinates, and the image intensity components of A. The default calibration is 1 pixel/cm so the pixel numbers coincide with the coordinates (wih an inversion of direction for y). The distance between the two edges of the cylinder, 50 cm in diameter, is 155 pixels. We can enter the corresponding calibration 3.1 px/cm in the windows 'pxcmx' and 'pxcmy' on the left of the interface. Then pressing again on 'RUN', the images are displayed with real coordinates.